Claire Doutrelant

Amélie Fargevieille

Arnaud Grégoire

Evolution of female coloration: what have we learned from birds in general and blue tits in particular

Keywords: Plumage coloration, Comparative analyses, Blue tit, Carotenoids, Badges of status, Sexual selection, Social selection, Competition, Female ornaments, Costs, Signals, Maternal effects

Female ornaments have long been considered non-functional, but a paradigm shift has occurred over the two last decades. The adaptive nature of female ornaments is now widely accepted. After a rapid overview of this shift, we present the results of comparative studies focused on identifying the forces involved in the evolution of female coloration in birds. We then discuss the results of intraspecific ornithological field studies and finish up by summarising the work done by our group and others on female coloration in blue tits (Cyanistes caeruleus). Overall, this review confirms that female coloration traits function as ornaments and/or badges of status in many bird species. It also identifies several mechanisms that can circumvent trade-offs in investment between coloration traits and egg production.

Based on this review, we call for further research on certain topics and specific changes in practices. More precisely, at the macroevolutionary level, we should avoid framing our questions around sexual dichromatism and male-centered proxies of sexual selection if we wish to elucidate the female-specific selective forces and constraints involved in the evolution of female coloration. At the microevolutionary level, we need to quantify social and sexual selection in both sexes, and to perform experimental studies to compare the selective forces acting on female and male coloration. In particular, it appears important to investigate how maternal effects and physiological drivers of aggressiveness relate to female coloration. Finally, our work on blue tits illustrates the importance of conducting long-term studies in tandem with replicated experiments within a given species.

1-Introduction-female ornaments: a paradigm shift

Animal armaments and ornaments are exaggerated signals that increase respectively access to resources through dominance and mating success. They evolve through intersexual selection (i.e., mate choice [START_REF] Fischer | The genetical theory of natural selection[END_REF], and/or social selection (Lyon & Montgomerie, 2012;[START_REF] West-Eberhard | Sexual Selection, Social Competition, and Speciation[END_REF]. Because they are often more extreme in males, these traits are predominantly portrayed as male traits, leading to an asymmetrical perspective, which views sexual and social forces as placing greater selective pressure on male traits. Proposed explanations for it can be traced back to the work of Bateman and Trivers (Bateman, 1948;[START_REF] Trivers | Parental investment and sexual selection In[END_REF] [START_REF] Bateman | Intra-Sexual Selection In Drosophila[END_REF][START_REF] Trivers | Parental investment and sexual selection In[END_REF].

Several landmark studies provide an explanation for why sexual traits are often viewed as less likely to evolve in females. First, in an experiment on Drosophila, Bateman found that male reproductive success was much more variable than female reproductive success. He also found a positive relationship between mate number and reproductive success (i.e., number of offspring sired) in males but not in females. He proposed that this result could be partly explained by anisogamy. While males produce a large number of small, inexpensive gametes, females produce a comparatively smaller number of large, costly gametes, resulting in a different initial investment in individual offspring. Although this Bateman principle is debatable [START_REF] Gowaty | No evidence of sexual selection in a repetition of Bateman's classic study of Drosophila melanogaster[END_REF][START_REF] Tang-Martínez | Rethinking Bateman's principles: Challenging persistent myths of sexually reluctant females and promiscuous males[END_REF], a recent meta-analysis found support for Bateman's hypotheses and indicated that, in many animal species, the relationship between mate number and reproductive success is more common in males than in females [START_REF] Janicke | Darwinian sex roles confirmed across the animal kingdom[END_REF]. Second, [START_REF] Trivers | Parental investment and sexual selection In[END_REF] showed that interspecific differences in parental care could explain conventional sex roles (i.e., sex-specific behavior during mate acquisition and/or parental involvement, parental care being defined as a costly parental investment that increased offspring survival). He showed that any sex-based differences in parental care should lead the sex that invests the most in offspring-generally females-to become a limited resource for which the other sex competes. Mate choice should therefore most benefit the sex with the greatest investment in offspring if this enhances offspring survival, thus inducing a positive feedback loop [START_REF] Henshaw | Sex roles and the evolution of parental care specialization[END_REF]. Last, several authors argued that a male bias in the operational sex ratio (i.e., the ratio of males and females available for reproduction in the population) could also explain weaker sexual selection on female traits because if females face less competition for mates then male mate choice should be more relaxed [START_REF] Edward | The evolution and significance of male mate choice[END_REF]. Given this background, it is clear why conspicuous traits in females have long been considered as no more than anecdotal.

In addition, alongside anisogamy, the cost of reproduction has frequently been cited to explain why ornaments and armaments have not evolved in females. This argument is based on the assumption that females are more sensitive than males to the possible signaling costs and posits that cost can manifest itself in two ways. First, the cost of producing certain exaggerated traits could be prohibitive if it affects a female's ability to invest in offspring and there is no compensating investment by males. In such a case, allocating large amounts of resources to ornament production could negatively affect egg quality and fecundity in females, leading to a trade-off [START_REF] Fitzpatrick | Ornaments or offspring -Costs to reproductive success restrict sexual selection processes[END_REF]. Second, the cost could be associated with ornament display. More specifically, Wallace predicted that females that invest more in reproduction (e.g., by incubating their eggs) than males face greater predation risks, favoring selection for more cryptic females [START_REF] Wallace | The colours of animals and plants[END_REF]. Support for Wallace's hypothesis has been found in comparative studies showing that females tend to be more cryptic in species with more visible nests [START_REF] Martin | Sexual dichromatism in birds: importance of nest predation and nest location for females versus males[END_REF][START_REF] Soler | Evolution of sexual dichromatism in relation to nesting habits in European passerines: a test of Wallace's hypothesis[END_REF]. Finally, males may harass highly attractive females, decreasing their fecundity and latter's survival and thus intensifying selection against more ornamented females [START_REF] Hosken | Why aren't signals of female quality more common?[END_REF].

However, several reasons have led to question the classical asymmetrical view exposed above and to question the origin and evolution of female ornaments. First, the biological reality is that there are many species where both females and males are conspicuously ornamented. Even in sexually dimorphic species, females are rarely completely drab, and many females bear at least one conspicuous trait. For example, in various duck species with strong sexual dimorphism, females display conspicuous wing bars. Also, research on reproductive roles revealed that differences in sex-specific investment can shift rapidly if ecological conditions change. For instance, in the two-spotted goby fish Gobiusculus flavescens", sexual selection is varying within the breeding season. This temporal variation is due to a complete reversal of sex roles across the breeding season, driven by a change in the operational sex ratio heavily male-biased at the start of the season then heavily female-biased towards the end of the season [START_REF] Amundsen | Sex roles and sexual selection: lessons from a dynamic model system[END_REF]. Last, the assumption that all ornaments need to be costly or are subjected to allocation trade-offs is the subject of debate [START_REF] Prum | The Lande-Kirkpatrick mechanism is the null model of evolution by intersexual selection: implications for meaning, honesty, and design in intersexual signals[END_REF][START_REF] Weaver | What maintains signal honesty in animal colour displays used in mate choice?[END_REF]. Consequently, understanding why female ornaments exist and how they vary among species is a topic that clearly merits interest.

Several results concur to lead for a call for a paradigm shift [START_REF] Amundsen | Why are female birds ornamented?[END_REF]. Among the first voices were Jones and Hunter, who conducted one of the earliest experiments to find evidence of male preference for female ornaments in a monogamous bird species, the crested auklet, Aethia cristatella (I. L. [START_REF] Jones | Mutual sexual selection in a monogamous seabird[END_REF]. Roulin et al. also produced several papers showing that female ornaments had a function in the barn owl, Tyto alba [START_REF] Roulin | Nonrandom pairing by male barn owls (Tyto alba) with respect to a female plumage trait[END_REF][START_REF] Roulin | Female barn owls (Tyto alba) advertise good genes[END_REF][START_REF] Roulin | Female plumage spottiness signals parasite resistance in the barn owl (Tyto alba)[END_REF]. A landmark study on the eclectus parrot (Eclectus roratus) suggested that red coloration in females resulted from fierce female-female competition [START_REF] Heinsohn | Extreme reversed sexual dichromatism in a bird without sex role reversal[END_REF]. Around this same period, several comparative studies revealed that female ornaments had evolved independently on numerous occasions: they found that gains and losses of ornamentation often occurred separately in males and females, and that female traits seemed to be more labile than male traits [START_REF] Burns | A phylogenetic perspective on the evolution of sexual dichromatism in tanagers (Thraupidae): The role of female versus male plumage[END_REF][START_REF] Irwin | The evolution of plumage dichromatism in the New-World Blackbirds -Social selection on female brightness[END_REF][START_REF] Ord | Ornament evolution in dragon lizards: multiple gains and widespread losses reveal a complex history of evolutionary change[END_REF][START_REF] Wiens | Phylogenetic evidence for multiple losses of a sexually selected character in phrynosomatid lizards[END_REF].

Although the paradigm for understanding the evolution of ornaments then shifted, the more classical, asymmetric perspective was still favored to understand the evolution of female ornaments. The lability of female ornaments was mainly thought to be the result of genetic correlations with male ornaments [START_REF] Lande | Sexual dimorphism, sexual selection, and adaptation in polygenic characters[END_REF] tempered by the strength of natural selection [START_REF] Wallace | The colours of animals and plants[END_REF] or driven by more balanced sex roles [START_REF] Trivers | Parental investment and sexual selection In[END_REF]. This genetic correlation hypothesis treats the evolution of female ornaments as a non-adaptive byproduct of sexual selection on male traits that is consequently shaped by the limited genetic variation available for sex-dependent expression [START_REF] Kraaijeveld | The evolution of mutual ornamentation[END_REF][START_REF] Price | Sexual selection, selection load and quantitative genetics of zebra finch bill colour[END_REF].

A classical study supporting this hypothesis involved a cross-fostering experiment in zebra finches (Taeniopygia guttata): red bill coloration was explained by a strong genetic correlation between fathers and daughters [START_REF] Price | Sexual selection, selection load and quantitative genetics of zebra finch bill colour[END_REF]. However, species displaying conventional sex roles highlight a major mathematical issue with this idea: if conspicuous female traits were solely byproducts, there should be no variation in sex-dependent trait expression; in other words, the genetic correlation between the sexes should be exactly 1 [START_REF] Lande | Sexual dimorphism, sexual selection, and adaptation in polygenic characters[END_REF], a relatively unlikely situation. Moreover, models have shown that male sexual traits that are pleiotropically expressed in females can only evolve if females express a very attenuated form of the trait [START_REF] Servedio | Population genetic models of male and mutual mate choice[END_REF]. Finally, recent work suggests that ornament expression can easily be labile if ornaments arise later in development [START_REF] Kraaijeveld | Reversible trait loss: the genetic architecture of female ornaments[END_REF]. Therefore, although genetic correlation clearly played a key role in the evolution of the female ornaments, it cannot fully explain the equally intense expression of traits by both females and males in many species It follows that female traits may be adaptive as a result of predation pressures but may also be under other selective forces. More specifically, female traits could function as (i) signals for attracting males and/or (ii) signals used during sexual or social competition among females.

(i) Male mate choice-In the late 1990', theoretical research identified some conditions under which the evolution of mutual mate choice should be favored [START_REF] Johnstone | Mutual mate choice and sex differences in choosiness[END_REF][START_REF] Kokko | Why is mutual mate choice not the norm? Operational sex ratios, sex roles and the evolution of sexually dimorphic and monomorphic signalling[END_REF]. These models suggested that male mate choice evolves when there is variation in female quality and there are limits on male reproductive potential, which means that under these conditions, choosier males will more likely increase their reproductive success than indiscriminating males. Variation in female quality, on one side, is seen in a wide variety of taxa across the animal kingdom. For instance, in several species, female size is positively correlated with fecundity, and males seem to prefer larger females [START_REF] Nordeide | Female ornaments revisited-are they correlated with offspring quality[END_REF]. Coloration patterns are also variable and sometimes linked to female reproductive potential: for instance, in two-spotted gobies (Gobiusculus flavescens), yellow-orange belly coloration is correlated with fecundity in females, and mate choice experiments have revealed that males prefer females with more brightly colored bellies [START_REF] Amundsen | Male mate choice selects for female coloration in a fish[END_REF]. On the other side, male reproductive potential is limited by (i) the risk associated with rejecting a potential mate (which depends on encounter rate) [START_REF] Barry | Male mate choice: why sequential choice can make its evolution difficult[END_REF] and (ii) time and energy budgets. Time spent competing for and/or attracting a mate as well as the time spent providing parental care reduces the time available for mating with other individuals and thus decreases an organism's total number of mates. Energy budgets are influenced by the cost of sperm production and quality, which may be particularly important in polygynous species in which there is no paternal care and males have relatively unlimited access to females [START_REF] Reinhold | Cryptic male choice: sperm allocation strategies when female quality varies[END_REF]. Sperm limitation is the main argument used to explain why, in polygynous feral red junglefowl (Gallus gallus) and stalk-eyed flies (Teleopsis dalmanni), males transfer more sperm to females with larger combs [START_REF] Cornwallis | Experimental evidence that female ornamentation increases the acquisition of sperm and signals fecundity[END_REF][START_REF] Pizzari | Sophisticated sperm allocation in male fowl[END_REF] and larger eyespan [START_REF] Cotton | Male mate preference for female eyespan and fecundity in the stalk-eyed fly, Teleopsis dalmanni[END_REF], respectively. Consequently, abundant theoretical and empirical evidence favor the view that male mate choice is more widespread than previously thought in monogamous or polygynous species displaying conventional sex roles [START_REF] Hare | Sexual selection and its evolutionary consequences in female animals[END_REF][START_REF] Kraaijeveld | The evolution of mutual ornamentation[END_REF][START_REF] Schlupp | Male mate choice in livebearing fishes: an overview[END_REF].

(ii) Female-female competition-The hypothesis that conspicuous female traits evolved as armaments signaling fighting ability, otherwise known as "badges of status," started to garner attention in the mid-2000s (Clutton-Brock, 2007[START_REF] Clutton-Brock | Sexual selection in females[END_REF][START_REF] Clutton-Brock | Intrasexual competition and sexual selection in cooperative mammals[END_REF][START_REF] Lebas | Female finery is not for males[END_REF], even if it was suggested much earlier [START_REF] West-Eberhard | Sexual Selection, Social Competition, and Speciation[END_REF]. The competitive functions of female ornaments have been illustrated by work in a number of taxa [START_REF] Clutton-Brock | Social competition and selection in males and females[END_REF][START_REF] Rosvall | Intrasexual competition in females: evidence for sexual selection?[END_REF][START_REF] Rubenstein | Sexual and social competition: broadening perspectives by defining female roles[END_REF][START_REF] Stockley | Female competition and its evolutionary consequences in mammals[END_REF][START_REF] Tobias | The evolution of female ornaments and weaponry: social selection, sexual selection and ecological competition[END_REF]. At present, many researchers consider that social selection (i.e., competition for food, dominance, and territories) has played a major part in the evolution of female ornaments. Indeed, female-female competition may generate dramatic differences in fecundity and reproductive success among individuals, beyond what would be expected based on intrinsic female quality only. Also, the resources gained, which can be invested in future fecundity, could outweigh signaling costs. Finally, it is currently hypothesized that competitive context differs for the two sexes: female traits might more often evolve to mediate competition for ecological resources, while male traits might more often evolve to mediate competition for mate acquisition and resources [START_REF] Stockley | Female competition and its evolutionary consequences in mammals[END_REF][START_REF] Tobias | The evolution of female ornaments and weaponry: social selection, sexual selection and ecological competition[END_REF].

So overall there are now many studies suggesting that female conspicuous traits should also evolve through sexual or social selection and may even sometimes fulfil dual functions (i.e. be sexual ornaments and badges of status, e.g., [START_REF] Berglund | An intimidating ornament in a female pipefish[END_REF].

Aim of this review

The overview above shows that the paradigm for understanding the evolution of ornaments has gradually shifted over the years and that it is now widely accepted that female ornaments may have an adaptive function. It is now also considered that differences in traits between the sexes may be qualitative, quantitative, or nonexistent. Given that several reviews have already been written on this subject (see the references above), our goal here is to focus on what is known about the evolution of female ornaments in a specific class of animal-birds, with the aim of better identifying ideas that remain overlooked or that are weakly supported. In addition we also discuss current gaps in knowledge and propose better methods for testing hypotheses.

The byproduct hypothesis will not be addressed in detail as it was explored in a review on the genetic architecture of female ornaments [START_REF] Kraaijeveld | Reversible trait loss: the genetic architecture of female ornaments[END_REF]. However, it is obviously one key driver of female-and male-ornamentation [START_REF] Amundsen | Why are female birds ornamented?[END_REF].

Birds represent interesting models for exploring the evolution of female ornaments because variation in their life-history traits can be used to study the complex interplay between evolutionary forces and constraints. For instance, theoretical models have identified biparental care and life-long social monogamy as factors that favor the evolution of male mate choice [START_REF] Johnstone | Mutual mate choice and sex differences in choosiness[END_REF][START_REF] Kokko | Why is mutual mate choice not the norm? Operational sex ratios, sex roles and the evolution of sexually dimorphic and monomorphic signalling[END_REF]. These characteristics are both common and highly variable across bird species [START_REF] Cockburn | Prevalence of different modes of parental care in birds[END_REF]. Furthermore, variability in sociality over the winter, male parental care, territory quality, and sperm quality can be used to study the function of female ornaments in the context of female-female competition.

Birds employ two main forms of communication: acoustic and visual communication. In most textbooks, song is presented as a male-specific signal. However, very recently, female bird song was found to occur in 64% of extant songbird species [START_REF] Odom | Female song is widespread and ancestral in songbirds[END_REF], illustrating the strength of bias in perspectives on research. Because several reviews have recently been published on female acoustic communication [START_REF] Amy | The Functions of Female Calls in Birds Advances in the Study of Behavior[END_REF][START_REF] Riebel | New insights from female bird song: towards an integrated approach to studying male and female communication roles[END_REF], we will focus here on female visual communication, namely coloration (see Box 1 for more details). While we will mostly review work on female plumage, we will also deal somewhat with female bare-part ornaments. Also, although we discuss female eggshell coloration in passing, we will not extensively review that vast subject [START_REF] Moreno | Avian egg colour and sexual selection: does eggshell pigmentation reflect female condition and genetic quality?[END_REF].

In the first part of this review, we assess advances in our understanding of the forces shaping female coloration in birds by evaluating the results of recent comparative studies. Because most of these comparative studies were conducted to understand the evolution of dichromatism, the conclusion is that there remains an acute need to examine the sex specific selective forces underlying evolution of female coloration in birds. Then, we look at long-term studies in relation to what they have revealed and could reveal about the influence of sexual and social selection on female coloration traits in birds, and we present what is currently known about the signaling content of female coloration. This focus paves the way to discuss (i) whether ornament cost impedes the evolution of female coloration in birds and (ii) to evaluate the benefits choosy males might obtain beyond simply mating with partners that are more fecund.

(iii) It also leads to assess the results of experimental and physiological studies that tested whether female coloration traits could serve as badges of status. The conclusions of this second & third parts of the review are that long-term studies are needed and that future research should focus specifically on the relationship between maternal effects and female coloration in birds; dig deeper into the physiological basis of female coloration and aggressiveness; and test the importance of inter and intrasexual selection on ornaments in both sexes simultaneously. In the third part of the review, we describe research focused on one single bird species, the blue tit (Cyanistes caeruleus), a well-studied biological model for which we have gathered data on coloration and life-history traits in different study populations over many years. We thoroughly examine the work done by our group and other groups throughout Europe. The conclusions for this third part of the review are that studying different populations of the same model species can help critically assess the generality of observed patterns and point limitations and dangers of using too low sample size. Research on a single model also show that it is important to independently replicate studies using the same or complementary methods; and to run longterm studies to determine the forces behind the evolution of female ornamentation.

Macroevolution of female coloration-insights from comparative studies

Historically, comparative studies provided indirect insights into the evolution of female coloration while trying to understand the evolution of sexual dichromatism. Essentially, sexual dichromatism was thought to have arisen from intense sexual selection acting on males. As expected, several studies found that polygynous species were more likely to be sexually dichromatic and that socially monogamous species were more likely to be sexually monochromatic [START_REF] Bennet | Evolutionary Ecology of Birds[END_REF]but see Dunn, Whittingham, & Pitcher, 2001;[START_REF] Price | Reconstruction the evolution of sexual dichromatism: current color diversity does not reflect past rates of male and female change[END_REF]. Some also found that levels of extrapair paternity (i.e., genetic mating patterns [START_REF] Møller | The evolution of plumage brightness in birds is related to extrapair paternity[END_REF] appeared to be tied to dichromatism. Yet other factors, either unrelated or only indirectly related to sexual selection in males, were also found to be associated with sexual dichromatism; they included latitude, paternal care, and parasitism [START_REF] Scott | Mating systems, parasites and plumage dimorphism in waterfowl[END_REF]) but also nest ecology [START_REF] Martin | Sexual dichromatism in birds: importance of nest predation and nest location for females versus males[END_REF]. Meanwhile, studies in Icteridae [START_REF] Irwin | The evolution of plumage dichromatism in the New-World Blackbirds -Social selection on female brightness[END_REF] and Thraupidae [START_REF] Burns | A phylogenetic perspective on the evolution of sexual dichromatism in tanagers (Thraupidae): The role of female versus male plumage[END_REF] revealed that variation in sexual dichromatism was better explained by variation in female rather than male coloration. Based on these studies, calls began for research to include variation in both female and male plumage to better understand the evolution of sexual dichromatism, and avoid using sexual dichromatism as a proxy for the intensity of sexual selection (see Box 2 for more details and Figure 1). However, most recent comparative studies still adopted a biased perspective and worked within the framework of sexual dichromatism. More specifically, they tested potential limitations placed on the evolution of female coloration by ecological factors such as migration [START_REF] Friedman | Correlated evolution of migration and sexual dichromatism in the New World Orioles (Icterus)[END_REF], nest characteristics, and habitat openness [START_REF] Soler | Evolution of sexual dichromatism in relation to nesting habits in European passerines: a test of Wallace's hypothesis[END_REF]. The first comparative study to explore evolutionary forces that might accentuate female coloration was not published until 2009. Still framing their questions from the perspective of sexual dichromatism, Rubenstein and Lovette tested how cooperative breeding could reduce the degree of sexual dimorphism by favoring the occurrence of conspicuous plumage in female African starlings (Sturnidae). Cooperative breeding is a mating system in which sexually mature individuals help raising the offspring of others [START_REF] Cockburn | Evolution of helping behaviour in cooperatively breeding birds[END_REF], meaning that only a few individuals have access to reproduction, which results in intense competition for mating in both sexes and favors monochromatism. In this study, cooperative breeding species were more likely to be sexually monochromatic than were non-cooperative ones, underscoring that competition may be an important factor in the evolution of female coloration [START_REF] Rubenstein | Reproductive skew and selection on female ornamentation in social species[END_REF]). Yet, more than a decade after studies showing that female coloration could be labile [START_REF] Irwin | The evolution of plumage dichromatism in the New-World Blackbirds -Social selection on female brightness[END_REF], comparative research still had not stepped outside the sexual dichromatism framework when examining evolution of female plumage coloration.

In 2015, two studies addressed the evolution of female plumage coloration together with male coloration and dichromatism [START_REF] Dale | The effects of life history and sexual selection on male and female plumage colouration[END_REF][START_REF] Dunn | Natural and sexual selection act on different axes of variation in avian plumage color[END_REF]. Although the studies differed in their methods for assessing plumage coloration (i.e., color estimated from book plates vs. spectrophotometry), they used a similar statistical approach: after testing how well certain indices predicted variability in both female and male coloration (i.e., within the framework of sexual dichromatism), they then looked at coloration in each sex separately (aiming at breaking down the composite nature of sexual dichromatism). Both studies found that most of the variation in plumage dichromatism was explained by factors acting on female coloration, not male coloration. [START_REF] Dale | The effects of life history and sexual selection on male and female plumage colouration[END_REF] found that indices associated with living in the tropics (i.e., lower latitude, yearlong nesting, smaller clutch size) or higher body mass tended to enhance the likelihood of female coloration, while migration tended to limit it [START_REF] Dale | The effects of life history and sexual selection on male and female plumage colouration[END_REF]. Interestingly, while this study found that indices of intense male sexual selection (social polygyny, lack of paternal care) were positively correlated with sexual dichromatism, this relationship largely arose from a negative correlation with female coloration rather than from a positive correlation with male coloration. Additionally, the method used by Dale and colleagues made it possible to suggest that the genetic correlation between female and male coloration is strong. [START_REF] Dunn | Natural and sexual selection act on different axes of variation in avian plumage color[END_REF] found that presence of sexual monochromatism was associated with indices of natural selection (i.e., migratory behavior, breeding in the tropics, paternal care, nesting ecology, body mass), while sexual dichromatism was associated with indices of both sexual selection in males (i.e., social mating system, ratio of testes size to body mass) and natural selection (nest height, paternal care) [START_REF] Dunn | Natural and sexual selection act on different axes of variation in avian plumage color[END_REF]. This study also confirmed that sexual dichromatism was more related to evolutionary changes in females than in males.

However, from the perspective of this review, these two important studies [START_REF] Dale | The effects of life history and sexual selection on male and female plumage colouration[END_REF][START_REF] Dunn | Natural and sexual selection act on different axes of variation in avian plumage color[END_REF] shared a significant limitation: they used proxies traditionally associated with sexual selection in males, which are poorly suited to examining the influence of femalebiased sexual selection. For instance, social mating system type-essentially monogamy or polygyny-mainly reflects the intensity of sexual selection in males. The presence/absence of paternal care also serves as a proxy for sexual selection in males, as well as for natural selection in females. This two-faceted nature of the latter variable may explain why, in contrast to Dale and colleagues, Dunn and colleagues treated presence/absence of paternal care as an ecological factor rather than as a proxy of sexual selection. Consequently, neither social mating system type nor the presence/absence of paternal care may be helpful indices if the goal is to understand how sexual selection in females shapes female coloration. It would be better to stop treating paternal care as a binary variable and to instead examine variation in male investment across different stages of parental care; the latter may better reflect dynamics of sexual selection in females, since males that invest more may be choosier about their mates [START_REF] Johnstone | Mutual mate choice and sex differences in choosiness[END_REF][START_REF] Kokko | Why is mutual mate choice not the norm? Operational sex ratios, sex roles and the evolution of sexually dimorphic and monomorphic signalling[END_REF], while females may compete for males that provide a greater degree of care. In line with this proposition, Soler and colleagues found that, in larger species, females were more conspicuous (i.e., displayed conspicuous monochromatism) when males participated in nest building [START_REF] Soler | Conspicuousness of passerine females is associated with the nest-building behaviour of males[END_REF].

The focus on sexual dichromatism instead of on sex-specific coloration has led to overlook the importance of patch location and size in the evolution of female coloration-females may use different body parts than males for signaling and may use smaller patches to remain more cryptic if they are more vulnerable to predation, for instance during incubation. Historically, comparative studies tested whether sexual selection was acting on body parts associated with visual communication in males-the crown, nape, throat, and breast-and, indeed, they found that these parts were more often dichromatic and/or conspicuous [START_REF] Delhey | Revealing The Colourful Side of Birds: Spatial Distribution of Conspicuous Plumage Colours on The Body of Australian Birds[END_REF][START_REF] Gomez | Influence of ambient light on the evolution of colour signals: comparative analysis of a Neotropical rainforest bird community[END_REF], 2007). However, visual communication in females might involve smaller and/or more concealed patches due to the antagonist pressures of natural selection. A handful of recent comparative studies on sexual dichromatism have looked at the location of color patches thought to serve as visual signals. They revealed that sexual dichromatism in patch coloration is explained by male-biased sexual selection in Thraupidae [START_REF] Shultz | The role of sexual and natural selection in shaping patterns of sexual dichromatism in the largest family of songbirds (Aves: Thraupidae)[END_REF] and Tyranninae [START_REF] Cooney | Sexual selection predicts the rate and direction of colour divergence in a large avian radiation[END_REF] and by female-biased natural selection in Maluridae [START_REF] Medina | Habitat structure is linked to the evolution of plumage colour in female, but not male, fairy-wrens[END_REF]; in the latter case, exposed female traits were more cryptically colored in open habitats than in closed habitats. Schultz and Burns (2017) also found that evolutionary changes in female coloration were reflected in wing primary feathers and tail feathers and were constrained by natural selection. Thus to better understand the evolution of female coloration, future studies should focus on characterizing the size and location of female-specific color patches and then examine how they are influenced by indices of sexual, social, and natural selection specific to females.

Finally, focusing on the coloration types displayed by birds should yield insights into whether costs might prevent the evolution of female coloration in species where females invest more in parental care than males. The costs associated with signaling may be related to the mortality risks associated with conspicuousness and more conspicuous colors may be avoided more frequently in females than males. Also, physiological costs might cause some colors to occur less frequently in females than males. For instance, carotenoid-based colors (i.e., yellow, orange, red) are sometime hypothesized to be more costly than melanin-based colors (e.g., black, gray, brown, rusty, orange-red: [START_REF] Galván | Color measurement of the animal integument predicts the content of specific melanin forms[END_REF] or psittacofulvin-based colors. This is because carotenoids are photosynthetic pigments that birds must obtain from dietary sources and because their incorporation into signals means that they are no longer available for other physiological processes, such as detoxification and immune function [START_REF] Hasselquist | Physiological mechanisms mediating costs of immune responses: what can we learn from studies of birds?[END_REF]; but see for a contrasting perspective: [START_REF] Koch | Do carotenoid-based ornaments entail resource trade-offs? An evaluation of theory and data[END_REF]; for review see: [START_REF] Olson | Costly sexual signals: are carotenoids rare, risky or required?[END_REF]; for a meta-analysis see: [START_REF] Simons | What does carotenoid-dependent coloration tell? Plasma carotenoid level signals immunocompetence and oxidative stress state in birds-a meta-analysis[END_REF][START_REF] Svensson | Carotenoid-based signals in behavioural ecology: a review[END_REF]. Furthermore, carotenoid processing is affected by certain essential cellular features, such as vitamin A metabolism and redox state [START_REF] Hill | The vitamin A-redox hypothesis: a biochemical basis for honest signaling via carotenoid pigmentation[END_REF]. The situation is similar for structural coloration that is also sometimes expected to be less costly to produce than carotenoid-based colors [START_REF] Prum | Anatomy, physics and evolution of avian structural colors[END_REF]. Structural coloration presents a broader diversity of colors than pigmentary colors [START_REF] Delhey | The colour of an avifauna: a quantitative analysis of the colour of Australian birds[END_REF][START_REF] Stoddard | How colorful are birds? Evolution of the avian plumage color gamut[END_REF] and is produced by the physical interaction between light and feather microstructure [START_REF] Prum | Anatomy, physics and evolution of avian structural colors[END_REF].

Potential differences in costs (linked to physiology or conspicuousness) for the different types of colorations may explain why the color gamut available for females is less extended than for males [START_REF] Delhey | The colour of an avifauna: a quantitative analysis of the colour of Australian birds[END_REF]. Too high contrast (which may decrease crypsis) has also been put forward to explain why females of Western Palearctic species use more pheomelanin (which is lighter brown) than eumelanin in their signals [START_REF] Negro | Melanin-based sexual dichromatism in the Western Palearctic avifauna implies darker males and lighter females[END_REF]. Based on the assumption that coloration types have different costs, researchers have also hypothesized that carotenoid-based colors should be more widespread than other types of colors in sexually dichromatic species. Species utilizing carotenoid-based colors have been found to be more sexually dichromatic in certain groups: North American passerines [START_REF] Gray | Carotenoids and sexual dichromatism in North American passerine birds[END_REF], Cardueline finches [START_REF] Badyaev | Evolution of sexual dichromatism: contribution of carotenoid-versus melanin-based coloration[END_REF], Australian passerines (Delhey & Peters, 2017), and pigeons [START_REF] Mahler | Dietary and sexual correlates of carotenoid pigment expression in dove plumage[END_REF][START_REF] Taysom | The contribution of structurall, psittacofulvin and melanin-based colouration to sexual dichromatism in Australasian parrots[END_REF]. Also, in males of the Tyrannidae family, carotenoid-based colors evolved faster in dichromatic species and within body regions commonly involved in intersexual or intrasexual displays (e.g., the crown, throat, breast, and/or rump). In females, however, no such pattern has been seen [START_REF] Cooney | Sexual selection predicts the rate and direction of colour divergence in a large avian radiation[END_REF].

We need more studies, which think beyond sexual dichromatism and investigate each sex separately. Characterizing the gamut of colors in females and exploring how it is influenced by female-biased sexual selection or social selection is thus another necessary step in the quest to identify the main evolutionary forces underlying female coloration.

Summary -To conclude this section, it is evident that several changes must be made if we wish to clarify the relative importance of the different factors shaping female coloration. First, we are still sorely lacking research that focuses on the evolution of female coloration. Second, studies looking at social and sexual selection in females should use appropriate proxies of these selection pressures (e.g. densities, skewed sex ratio, paternal investment, maternal investment).

The use of those that reflect sexual selection in males (e.g. polygyny, testes size, absence of paternal care) should be avoided if looking at the effect of social and sexual selection in females.

Last, one difficult aspect we faced in this review was the lack of methodological consistency between studies, especially in assessments of coloration. Methods ranged from using the human eye to discretely score coloration to using photographs that were assessed from the perspective of avian visual space to defining principal components from spectrophotometric data (Table 1).

Obviously, these differences made it difficult to properly interpret and compare the results. We strongly encourage future studies to use more precise methods for quantifying coloration traits (see the details in Box 1 and 2).

(Table 1 about here.)

Microevolution-insights from long-term studies

When Darwin proposed his theory of evolution [START_REF] Darwin | On the Origins of Species by Means of Natural Selection[END_REF], he defined three conditions that are needed for evolution to occur under selection: (i) variation in the focal trait, (ii) inheritance of trait value, and (iii) a relationship between the trait and fitness. In other words, when the value of an inherited trait allows an individual to reproduce more successfully, evolutionary changes follow.

(i) Variability: female coloration displays a notable level of phenotypic variation. For instance, a study conducted on six passerine birds, the Eurasian blackcap (Sylvia atricapilla), the European robin (Erithacus rubecula), the blue tit, the great tit (Parus major), the common blackbird (Turdus merula), and the European greenfinch (Carduelis chloris), found that female coloration varied as much as male coloration in both sexually monochromatic and dichromatic species (Delhey & Peters, 2008).

(ii) Heritability. The heritability of coloration traits in birds has still rarely been quantified but so far, most components of coloration appear to be heritable and more and more studies manage to identify the genetic basis of switches in pigment types, colour intensity and colour pattern (e.g. [START_REF] Lopes | Genetic basis for red coloration in birds[END_REF][START_REF] Mundy | Colouration genetics: pretty polymorphic parrots[END_REF][START_REF] Poelstra | Transcriptomics of colour patterning and coloration shifts in crows[END_REF][START_REF] Roulin | Genetics of colouration in birds[END_REF]. Studies have found that the heritability of color patch size is high (ranging from 0.28 to 0.90 for melanin and white patches, i.e. [START_REF] Hubbard | Quantitative genetics of plumage color: lifetime effects of early nest environment on a colorful sexual signal[END_REF][START_REF] Roulin | Sex-linked inheritance, genetic correlations and sexual dimorphism in three melanin-based colour traits in the barn owl[END_REF][START_REF] Saino | Sexual dimorphism in melanin pigmentation, feather coloration and its heritability in the barn swallow (Hirundo rustica)[END_REF]. Melanin is endogenously synthesized in specialized cells (melanocytes), and its production is thus strongly genetically determined, which can explain the high heritability of melanin-based coloration [START_REF] Ekblom | Gene expression divergence and nucleotide differentiation between males of different color morphs and mating strategies in the ruff[END_REF][START_REF] Mundy | A window on the genetics of evolution: MC1R and plumage colouration in birds[END_REF]. Studies found indices of color intensity and brightness of chromatic colorations (yellow, blue) to be heritable, even to a lesser extent [START_REF] Charmantier | Colour ornamentation in the blue tit: quantitative genetic (co) variances across sexes[END_REF][START_REF] Evans | Quantitative genetics of a carotenoid-based color: heritability and persistent natal environmental effects in the great tit[END_REF][START_REF] Hadfield | Direct versus indirect sexual selection: genetic basis of colour, size and recruitment in a wild bird[END_REF]; but see: [START_REF] Vergara | Genetic basis and fitness correlates of dynamic carotenoid-based ornamental coloration in male and female common kestrels Falco tinnunculus[END_REF]. Sex-specific estimates of heritability have rarely been computed. In blue tits, the chromatic component of structural coloration is heritable in both sexes, and the heritability of carotenoid chromaticity tends to be higher in males than in females [START_REF] Charmantier | Colour ornamentation in the blue tit: quantitative genetic (co) variances across sexes[END_REF]. Sex-specific heritability values can be calculated by assuming autosomal inheritance, as was done for blue tits in the study mentioned above, or by assuming there is also sex-linked inheritance. Indeed, if many genes underlying sexual dichromatism are not sex linked [START_REF] Badyaev | Growing apart: an ontogenetic perspective on the evolution of sexual size dimorphism[END_REF], sex linkage (i.e., the fact that the phenotypic expression of an allele is directly tied to the sex chromosomes) could also partially explain sexual dichromatism [START_REF] Husby | Sex chromosome linked genetic variance and the evolution of sexual dimorphism of quantitative traits[END_REF][START_REF] Larsen | On estimation and identifiability issues of sex-linked inheritance with a case study of pigmentation in Swiss barn owl (Tyto alba)[END_REF]. For instance, Z-linked genetic variance explained more of the total phenotypic variation in white wing patch size than did autosomal genetic variance (11% versus 40%) in the collared flycatcher, Ficedulla albicollis [START_REF] Husby | Sex chromosome linked genetic variance and the evolution of sexual dimorphism of quantitative traits[END_REF].

However, the contribution of such sex-linked variance to phenotypic variation is generally thought to be weak, although studies may have lacked sufficient power to distinguish autosomal genetic variance from sex-linked genetic variance [START_REF] Charmantier | Colour ornamentation in the blue tit: quantitative genetic (co) variances across sexes[END_REF][START_REF] Husby | Sex chromosome linked genetic variance and the evolution of sexual dimorphism of quantitative traits[END_REF]. Furthermore, very few studies have yet attempted to estimate the Z-linked or W-linked heritability of coloration [START_REF] Evans | Nonautosomal genetic variation in carotenoid coloration[END_REF].

(iii) Fitness-related traits: phenotypic traits like female colorations evolve under sexual, social, and/or natural selection if they affect proxies of fitness. We will review below the progress that has been made in understanding the strength of sexual, social and natural selection on coloration in female birds

Progress in understanding the strength of sexual selection on coloration in female birds

Under sexual selection, individuals compete for mates and/or the opportunity to fertilize their gametes, and differences in their relative success leads to variation in reproductive success [START_REF] Andersson | Sexual selection[END_REF][START_REF] Darwin | The descent of man, and selection in relation to sex[END_REF]. Sexual selection is expected to operate on female coloration traits if they affect an individual's chances of obtaining a mate(s) and producing offspring. In addition to mate number, mate quality may also play a role.

The strength of sexual selection is in consequence estimated by two steps: first it is needed to measure the correlation between variation in phenotype, (here female coloration) and variation in mating success and, second, the correlation between variation in the number of partners and variation in reproductive success [START_REF] Anthes | Measuring and interpreting sexual selection metrics: evaluation and guidelines[END_REF][START_REF] Henshaw | How to quantify (the response to) sexual selection on traits[END_REF]. To estimate the correlation between variation in phenotype and variation in mating success, the mating differential and gradients as well as opportunity for sexual selection are computed. Mating differential and gradients correspond to the covariance between trait values and mating success. Opportunity for sexual selection corresponds to the variance in mating success divided by the squared mean value of mating success for the population. This latter metric establishes an upper bound for the mating differential for a standardized trait. To estimate the correlation between variation in the number of partners and variation in reproductive success, a Bateman gradient is used. It quantifies the relationship between mate number and reproductive success (this approach thus does not provide information about the traits which are determinant for mating success).

What do we know about the strength of sexual selection on female coloration in birds? To answer this question, we must first make a detour reviewing what is known about the Bateman gradient and opportunity for sexual selection in birds. Janicke and colleagues (2016) performed a meta-analysis that included all animal studies in which the opportunity for sexual selection and the Bateman gradient had been calculated for both males and females. This [START_REF] Janicke | Darwinian sex roles confirmed across the animal kingdom[END_REF]'s data set included 12 species of birds that are presented in Table 2. Table 2 shows that for each of these 12 bird species, the two metrics are male-biased but also that in most bird species the effect size for the female Bateman gradient is significantly positive in female birds suggesting that females benefit from multiple mating in these cases. A meta-analysis run on this table 2 (i.e. specific to bird species and not on the whole data set as in [START_REF] Janicke | Darwinian sex roles confirmed across the animal kingdom[END_REF] shows the mean effect size (mean±SE) for the sex difference in opportunity for sexual selection (0.24±0.088); the sex difference in Bateman gradient (0.45±0.103) and the female Bateman gradient (0.39±0.089). Table 2 also shows that these 3 metrics are all statistically significant (Janicke et al. unpublished data). [START_REF] Hare | Sexual selection and its evolutionary consequences in female animals[END_REF] mentioned two additional bird species: the black-legged kittiwake, Rissa tridactyla (Coulson & Thomas, reported in Clutton-Brock, 1983) and the house wren, Troglodytes aedon [START_REF] Whittingham | Effects of extra-pair and within-pair reproductive success on the opportunity for selection in birds[END_REF]. For these two monogamous species, the opportunity for sexual selection was also low, but never absent, in females compared to males, especially in the colonial monogamous bird (the kittiwake).

(Table 2 about here) Could mating system explain some of the variability in Bateman gradient? In polygamous bird species, a steep and positive Bateman's gradient was found for the females of one brood parasite species: the great-spotted cuckoo, Clamator glandarius [START_REF] Bolopo | Flexible mating patterns in an obligate brood parasite[END_REF]. In the brown-headed cowbird, Molothrus ater, the results are opposite as gradient is greater in males [START_REF] Louder | Greater opportunities for sexual selection in male than in female obligate brood parasitic birds[END_REF]but see [START_REF] Woolfenden | High opportunity for sexual selection in both sexes of an obligate brood parasitic bird, the brown-headed cowbird (Molothrus ater)[END_REF]. In polygynandrous species, positive Bateman gradients were seen in female red junglefowl [START_REF] Collet | The measure and significance of Bateman's principles[END_REF] and female wild turkeys, Meleagris gallopavo [START_REF] Krakauer | Sexual selection and the genetic mating system of wild turkeys[END_REF]. However, the gradient was only statistically significant in the red junglefowl. In contrast, in territorial and socially monogamous species, the gradients were less pronounced and different between the sexes. They were slightly positive overall in females (even if the statistical support was weak). This finding was found in the blue tit [START_REF] García-Navas | Extrapair paternity in Mediterranean blue tits: socioecological factors and the opportunity for sexual selection[END_REF][START_REF] Schlicht | Effects of social and extra-pair mating on sexual selection in blue tits (Cyanistes caeruleus)[END_REF], the dark-eyed junco , Junco hyemalis [START_REF] Gerlach | Reinterpreting Bateman gradients: multiple mating and selection in both sexes of a songbird species[END_REF], the hihi, Notiomystis cincta [START_REF] Walker | Sexually selected dichromatism in the hihi N otiomystis cincta: multiple colours for multiple receivers[END_REF]), the mountain bluebird, Sialia currucoides [START_REF] Balenger | Extra-pair paternity in the socially monogamous mountain bluebird Sialia currucoides and its effect on the potential for sexual selection[END_REF], and the white-crowned sparrow, Zonotrichia leucophrys [START_REF] Poesel | Extrapair Fertilizations and the Potential for Sexual Selection in a Socially Monogamous Songbird[END_REF]. Interestingly in addition, in two species of Darwin finches (Geospiza fortis & G. scandens) for which mating patterns change when environmental fluctuations alter sex ratios, females of both species were more frequently polyandrous in male-biased populations, and fledged more offspring by changing mates [START_REF] Grant | Adult sex ratio influences mate choice in Darwin's finches[END_REF]. Lastly, in cooperative breeding species, a positive Bateman gradient was found in females but not in males for the superb starling, Lamprotornis superbus [START_REF] Apakupakul | Bateman's principle is reversed in a cooperatively breeding bird[END_REF]. Taken together these findings indicate that positive Bateman gradients have been found in female birds, so mating with more mates may increase reproductive success in some species. They also indicate that gradient occurrence and strength could change according to mating system and environmental conditions.

The suggestion that mating system affects sexual selection estimates in bird is supported by the work of [START_REF] Hauber | Bateman's principle in cooperatively breeding vertebrates: the effects of non-breeding alloparents on variability in female and male reproductive success[END_REF], who calculated sex-specific "measures of relative reproductive variability" between males and females for eight cooperative breeding species, and found that the value of this proxy of opportunity for selection (i.e., variance in relative reproductive success) was greater in females than males in five of these species. These five specie are the brown jay, Cyanocorax morio [START_REF] Williams | Female control of reproductive skew in cooperatively breeding brown jays (Cyanocorax morio)[END_REF], the white-browed scrubwren, Sericornis frontalis [START_REF] Whittingham | Relatedness, polyandry and extra-group paternity in the cooperatively-breeding white-browed scrubwren (Sericornis frontalis)[END_REF]), the Arabian babbler, Turdoides squamiceps [START_REF] Lundy | Reproduction by subordinates in cooperatively breeding Arabian babblers is uncommon but predictable[END_REF], the superb fairywren, Malurus cyaneus [START_REF] Double | Subordinate superb fairy-wrens (Malurus cyaneus) parasitize the reproductive success of attractive dominant males[END_REF], and the bicolored wren, Campylorhynchus griseus [START_REF] Haydock | Extra-pair paternity uncommon in the cooperatively breeding bicolored wren[END_REF]. It was identical for males and females in the red-cockaded woodpecker, Picoides borealis [START_REF] Haig | Genetic evidence for monogamy in the cooperatively breeding red-cockaded woodpecker[END_REF] and the Florida scrub jay, Aphelocoma coerulescens [START_REF] Fitzpatrick | Components of lifetime reproductive success in the Florida scrub jay[END_REF] and was lower in females than males in the Seychelles warbler, Acrocephalus sechellensis [START_REF] Richardson | Parentage assignment and extra-group paternity in a cooperative breeder: the Seychelles warbler (Acrocephalus sechellensis)[END_REF]. However, opportunity for selection is often considered to be the metric with the loosest link to sexual selection and few species are included (8), so more research is needed to confirm this bias in sexual selection for cooperative breeding species.

Several researchers (e. g. [START_REF] Anthes | Measuring and interpreting sexual selection metrics: evaluation and guidelines[END_REF][START_REF] Collet | The measure and significance of Bateman's principles[END_REF][START_REF] Gerlach | Reinterpreting Bateman gradients: multiple mating and selection in both sexes of a songbird species[END_REF] have pointed that positive Bateman gradients in females could be biased because more fecund females may be more attractive mates to males, such that many males choose to copulate with those fecund females. This would lead to a positive relationship between mating success and fecundity without any other causal relationship coming into play (i.e., females would not gain fitness benefits-i.e. no additional or better-quality nestlings-from having multiple mates). This argument is worth considering (and should also be systematically applied to sperm limited males who may have higher Bateman gradients when they produce more sperm). Recent papers have presented possible methods for dealing with this problem and for quantifying the strength of sexual selection [START_REF] Anthes | Measuring and interpreting sexual selection metrics: evaluation and guidelines[END_REF][START_REF] Henshaw | How to quantify (the response to) sexual selection on traits[END_REF]. Henshaw and colleagues (2018) propose for instance a method, based on a single path analysis model that includes how traits influence mating success and how mating success influences fitness.

In regard to the relationship between coloration and mating success in females (i.e. mating differential and gradients), we recommend referring to the recent review of [START_REF] Hare | Sexual selection and its evolutionary consequences in female animals[END_REF]. However, we need to stress out that if the existing research quantifying the link between female coloration and mating success is most often performed in short-term studies. Using longterm data, ideally lifetime reproductive success, is needed to address identified issues related to randomness created by variation in environmental conditions and sampling [START_REF] Clutton-Brock | Individuals and populations: the role of long-term, individual-based studies of animals in ecology and evolutionary biology[END_REF][START_REF] Cockburn | Behavioral ecology as big science: 25 years of asking the same questions[END_REF]. It is important to note that these issues are also a problem when characterizing how sexual selection affects male coloration in natural populations [START_REF] Chaine | Adaptive plasticity in female mate choice dampens sexual selection on male ornaments in the lark bunting[END_REF][START_REF] Robinson | Environmentdependent selection on mate choice in a natural population of birds[END_REF].

Lastly for birds, a particularity is that many species present short-or long-term monogamy [START_REF] Kvarnemo | Why do some animals mate with one partner rather than many? A review of causes and consequences of monogamy[END_REF]. A common expectation is that monogamy leads to little or no sexual selection. However, as pointed by [START_REF] Kvarnemo | Why do some animals mate with one partner rather than many? A review of causes and consequences of monogamy[END_REF], sexual selection can be substantial even under mutual monogamy, as mate quality is obviously more important than mate numbers, which in turn increases the strength of the pre-mating mate choice which can be associated with high variation in reproductive success [START_REF] Hooper | Mutual mate choice can drive costly signaling even under perfect monogamy[END_REF][START_REF] Jones | Mate choice and sexual selection: what have we learned since Darwin[END_REF]. More research is needed to quantify mating gradients and understand how sexual selection is working in those long-term monogamous species.

Overall, this section shows that in birds, males tend to have steeper indices of sexual selection (probably due to the taxon's high level of extrapair mating). However, there is also evidence that females have statistically significant positive estimates too. Furthermore, it seems that the occurrence and strength of estimates of sexual selection in males and females might vary according to mating system and degree of sociality. However, because we have still too few species for which sexual selection estimates has been computed, this conclusion requires confirmation. We therefore call for more research into defining mating gradient, opportunities for sexual selection and Bateman gradients for both sexes in birds and agree with the recent review of [START_REF] Kvarnemo | Why do some animals mate with one partner rather than many? A review of causes and consequences of monogamy[END_REF] that sexual selection in long-term monogamous birds need more studies. It is hard to find cases where the strength of social selection has been quantified in females and/or for female traits. The opportunity for social selection arises whenever reproductive success and/or survival vary among individuals as a direct result of interactions with conspecifics of the same or opposite sex [START_REF] Wolf | Interacting phenotypes and the evolutionary process. II. Selection resulting from social interactions[END_REF]. Traits affected by social selection are shaped by the beneficial (positive) or harmful (negative) effects of other individuals (McDonald, [START_REF] Mcdonald | Assortment and the analysis of natural selection on social traits[END_REF]. For instance, female coloration can play a determinant role in competition for resources (see section 4.2. below), which can be intense in females [START_REF] Tobias | The evolution of female ornaments and weaponry: social selection, sexual selection and ecological competition[END_REF]. Female coloration might also influence male behavior after pairing. A recent theoretical study showed that exaggerated mutual displays performed after mating could evolve via social selection if they increase parental investment, implying that mate stimulation could explain the presence of socially selected traits in females [START_REF] Servedio | Evolution of displays within the pair bond[END_REF]. This finding could provide an evolutionary explanation for the coloration traits displayed by both sexes or just by females after mating. Emblematic examples include the dance performed by the blue-capped cordon-bleu, Uraeginthus cyanocephalus [START_REF] Ota | Tap dancing birds: the multimodal mutual courtship display of males and females in a socially monogamous songbird[END_REF] and egg coloration [START_REF] Moreno | Avian egg colour and sexual selection: does eggshell pigmentation reflect female condition and genetic quality?[END_REF].

Progress in understanding the strength of social selection on female coloration in birds

Tools exist to measure social selection. The presence of social selection can be tested for using multilevel selection analysis, which builds on the classical Lande selection model [START_REF] Lande | The measurement of selection on correlated characters[END_REF]) by including the traits of social partners in addition to the traits of focal individuals [START_REF] Mcdonald | Assortment and the analysis of natural selection on social traits[END_REF]. This approach partitions the fitness contributions of each individual to determine how specific traits involved in interactions influence selection strength.

For instance, support for social selection has been found in great tits for arrival date: individuals that arrive late to the breeding site increase their probability of successfully acquiring a breeding territory if they associate with other late-arriving conspecifics [START_REF] Farine | Selection for territory acquisition is modulated by social network structure in a wild songbird[END_REF].

Indirect phenotypic effects (i.e. effect of the genotype of an individual on the phenotypic trait value of another individual) have been found to explain a large proportion of the variation in female breeding date in American red squirrels (Tamiasciurus hudsonicus), although the specific traits of female neighbors that mediate this indirect effect have not yet been identified [START_REF] Fisher | Social effects of territorial neighbours on the timing of spring breeding in North American red squirrels[END_REF]. These multilevel methods [START_REF] Fisher | Social effects of territorial neighbours on the timing of spring breeding in North American red squirrels[END_REF][START_REF] Mcdonald | Assortment and the analysis of natural selection on social traits[END_REF] could and should be used to determine whether social selection has influenced female (and/or male) coloration and constitute a promising line of future research. For instance, it would be important to test whether including color traits from neighboring females in addition to color traits of focal females in a classical Lande selection model would influence the reproductive success of focal females.

Progress in understanding the strength of natural selection on female coloration in birds

Natural selection sensu stricto could act on female coloration if coloration is linked to reproductive benefits and affects chances of survival; natural selection sensu lato encompasses both sexual and social selection.

Research measuring the strength of natural selection sensu lato acting on female coloration in birds is much more common than research measuring the strength of sexual and social selection; even if, unfortunately, such information on female coloration and fitness is not yet routinely obtained in most longitudinal studies. Generally, the relationship between phenotypes and fitness can be characterized using the classical Lande model mentioned above [START_REF] Lande | The measurement of selection on correlated characters[END_REF][START_REF] Morrissey | Unification of regression-based methods for the analysis of natural selection[END_REF], which estimates covariance between traits and fitness (reproductive success or survival). This method requires long-term data for both variables. A classical quantitative genetics approach that directly estimates genetic covariance between traits and fitness may also be useful in this context [START_REF] Charmantier | Quantitative genetics in the wild[END_REF]. Many studies could be mentioned but it is worth mentioning three recent studies conducted at the phenotypic level that used large sample sizes collected across several years. In the common kestrel (Falco tinnunculus), positive directional selection was found to operate on the yellow chromaticity of the eye ring when number of fledglings served as the proxy for female fitness. This relationship was not present in males [START_REF] Vergara | Genetic basis and fitness correlates of dynamic carotenoid-based ornamental coloration in male and female common kestrels Falco tinnunculus[END_REF]. In the prothonotary warbler (Protonotaria citrea), females with higher carotenoid levels in their crown feathers produced a greater number of fledglings [START_REF] Bulluck | Feather carotenoid content is correlated with reproductive success and provisioning rate in female Prothonotary Warblers[END_REF]. In the sociable weaver (Philetarius socius), viability selection (which operates on survival probabilities) was found to have an influence on female and male melanin bib size [START_REF] Acker | Disruptive viability selection on a black plumage trait associated with dominance[END_REF].

Summary -To conclude this section, it shows first that tools are available for measuring the influence of sexual, social and natural selection on female coloration in birds. It also points that these tools are still poorly used and that they require long-term data from natural populations. Such data set are scarce for females because their coloration traits are not systematically measured in long-term studies. Shorter-term experiments would also be of interest. For example, ecological factors such as density [START_REF] Aronsen | Sex ratio and density affect sexual selection in a sex-role reversed fish[END_REF], food availability [START_REF] Janicke | Environment-dependent sexual selection: Bateman's parameters under varying levels of food availability[END_REF] or sex ratio [START_REF] Grant | Adult sex ratio influences mate choice in Darwin's finches[END_REF]) could be modified to test how environmental conditions affect the opportunity for sexual and social selection in females.

Signaling content of female coloration traits in birds

Determining the signaling content of female ornaments is an important step in understanding how and why this information is used by conspecifics. This step will allow here to address the following key questions: (i) could signaling costs constrain the evolution of female coloration in birds? (ii) What direct benefits males may acquire by choosing more colored females? and (iii) do female coloration traits in birds represent badges of status and why? (i) Cost as a constraint in the evolution of female coloration?

Emitters and receivers may have different interests. In birds, same-sex individuals display signals during competition for food and/or mates, and males and females interact to choose or to be chosen as a mate. Under such circumstances, signals must be reliable to be used by the receivers [START_REF] Searcy | The Evolution of Animal Communication. Reliability and Deception in Signaling Systems[END_REF]. The condition-dependence/handicap hypothesis assumes that signals need to be costly to be reliable [START_REF] Grafen | Biological Signals As Handicaps[END_REF][START_REF] Zahavi | Mate selection -a selection for a handicap[END_REF]. It follows that only high-quality individuals (e.g., those who are strong or in good condition) should be able to bear the cost of maintaining the most elaborate ornaments.

Signaling costs are often viewed as constraints on the evolution of ornaments in the sex that pays the greater cost of offspring production (i.e., generally female: Chenoweth, Doughty, & [START_REF] Chenoweth | Can non-directional male mating preferences facilitate honest female ornamentation[END_REF][START_REF] Cuervo | Experimental manipulation of tail length in female barn swallows (Hirundo rustica) affects their future reproductive success[END_REF][START_REF] Fitzpatrick | Ornaments or offspring -Costs to reproductive success restrict sexual selection processes[END_REF]. Indeed, female reproductive fitness is likely to be more resource limited, and females investing in costly sexual traits may have decreased fecundity. In contrast, a similar investment by males is unlikely to impact their reproductive capacities. However, are signaling costs really constraining the evolution of color ornaments in females? What do we actually know about the cost of female color ornaments and its effect on reproductive investment in females? Could this trade-off be minimized or circumvented?

First, the trade-off could be minimized or circumvented if color production is temporally decoupled from reproductive investment. In the case of plumage coloration, a trade-off between coloration and investment in offspring in females could be avoided because plumage is inert after it has been produced (i.e., after molting), and so coloration is in place long before the breeding season starts in some species. For instance, in most European passerines, plumage is renewed after the breeding season, so it is unlikely that two events separated by more than nine months would be involved in a trade-off decreasing female future capacity to invest in reproduction. In the case of bare-part coloration and cosmetic coloration, costs might be also minimized if investment in coloration only takes place when an individual needs to signal (see below the example of the flamingo coloration). Bare body parts are caruncles, legs, eye rings, or bills, they are living tissues whose color can change within a few days following food deprivation or immune challenge [START_REF] Faivre | Immune activation rapidly mirrored in a secondary sexual trait[END_REF][START_REF] Iverson | The role of bare parts in avian signaling[END_REF][START_REF] Rosenthal | Ornamental bill color rapidly signals changing condition[END_REF][START_REF] Velando | Pigment-based skin colour in the bluefooted booby: an honest signal of current condition used by females to adjust reproductive investment[END_REF]. Cosmetic coloration occurs when an external substance is applied to plumage (Delhey, Peters, & Kempenaers, 2007). The greater flamingo (Phoenicopterus roseus) provides an interesting example of cosmetic coloration displayed only when needed. This monogamous species displays reversed sexual dichromatism even though it has conventional sex roles and male-biased size dimorphism (Perez-Rodriguez, [START_REF] Perez-Rodriguez | The effects of preen oils and soiling on the UV-visible reflectance of carotenoid-pigmented feathers[END_REF]. The degree of coloration depends on the concentrations of carotenoids in uropygial secretions, and carotenoid levels in uropygial secretions are higher during mate choice than during chick provisioning. Additionally, carotenoid concentrations in uropygial secretions do not reflect carotenoid concentrations in plasma. This example illustrates well that resources invested in signaling can be allocated rapidly based on need, a flexibility that makes cost-based constraints on the evolution of coloration less likely [START_REF] Amat | Dynamic signalling using cosmetics may explain the reversed sexual dichromatism in the monogamous greater flamingo[END_REF].

Second, different signals may have different costs, and females may preferentially

display ornaments that are less costly. In accordance with the conditiondependence/handicap hypothesis [START_REF] Grafen | Biological Signals As Handicaps[END_REF][START_REF] Zahavi | Mate selection -a selection for a handicap[END_REF], ornaments must have a cost to evolve into signals. However, according to the aesthetic evolution hypothesis [START_REF] Hill | Sexiness, individual condition, and species identity: the information signaled by ornaments and assessed by choosing females[END_REF][START_REF] Price | Understanding how neural responses contribute to the diversity of avian colour vision[END_REF][START_REF] Prum | Aesthetic evolution by mate choice: Darwin's really dangerous idea[END_REF][START_REF] Renoult | Beauty is in the efficient coding of the beholder[END_REF][START_REF] Renoult | Processing bias: extending sensory drive to include efficacy and efficiency in information processing[END_REF], cost is not a prerequisite. Traits can evolve simply as a result of sensory bias or because they allow quicker cognitive evaluation by receivers [START_REF] Renoult | Processing bias: extending sensory drive to include efficacy and efficiency in information processing[END_REF]. They might also evolve because of the runaway hypothesis [START_REF] Kirkpatrick | Sexual selection and the evolution of female choice[END_REF][START_REF] Lande | Models of speciation by sexual selection on polygenic traits[END_REF][START_REF] Rosenthal | Evaluation and hedonic value in mate choice[END_REF]. Consequently, secondary sexual ornaments can be sexy signals, entirely arbitrary traits, i.e characteristics that became successful just because they are preferred. Females may preferentially use this type of sexy signals.

To date, however, it remains unclear which color ornaments are more likely to be free of cost.

As mentioned earlier in this review, carotenoid-based colors may be more costly than other colors because they have tight metabolic links with important cellular processes [START_REF] Hill | The vitamin A-redox hypothesis: a biochemical basis for honest signaling via carotenoid pigmentation[END_REF]. A recent meta-analysis [START_REF] Weaver | Carotenoid metabolism strengthens the link between feather coloration and individual quality[END_REF] supported this idea and further showed that colors based on converted carotenoids are linked to proxies of individual quality, more than colors based on dietary carotenoids (dietary carotenoid such as lutein and zeaxanthin are present in food and are deposited unchanged in the feathers; converted carotenoids are derived from dietary carotenoids and are biochemically converted before deposition). By contrast the only meta-analysis that compared carotenoid-based plumage and melanin-based plumage colors found the same condition dependence for both type of coloration [START_REF] Griffith | Melanin-versus carotenoid-based sexual signals: is the difference really so black and red?[END_REF], but in this analysis the different types of carotenoids were not differentiated. In relation to structural coloration, although some authors have argued that structural coloration is not very costly [START_REF] Prum | Anatomy, physics and evolution of avian structural colors[END_REF], experiments have shown that structural coloration can be affected by an individual's condition [START_REF] Doutrelant | Female plumage coloration is sensitive to the cost of reproduction. An experiment in blue tits[END_REF][START_REF] Hill | The effect of coccidial infection on iridescent plumage coloration in wild turkeys[END_REF][START_REF] Mcgraw | Different colors reveal different information: how nutritional stress affects the expression of melanin-and structurally based ornamental plumage[END_REF]; but see: [START_REF] Peters | No evidence for general conditiondependence of structural plumage colour in blue tits: an experiment[END_REF][START_REF] Siefferman | Male eastern bluebirds trade future ornamentation for current reproductive investment[END_REF][START_REF] Siitari | Color signals in the black grouse (Tetrao tetrix): Signal properties and their condition dependency[END_REF]. Additional research is therefore needed to understand if coloration types differ in their costs and whether such costs are directly defined by production (e.g., as proposed for carotenoid-based colors) or indirectly defined by the risks associated with conspicuous display [START_REF] Delhey | Conspicuous plumage colours are highly variable[END_REF].

To date, only a few experiments have been performed to test the costs of coloration in females. They have found that female coloration is sensitive to the availability of food resources [START_REF] Morales | Fecundity compromises attractiveness when pigments are scarce[END_REF]Siefferman & Hill, 2005a) and greater reproductive costs [START_REF] Doutrelant | Female plumage coloration is sensitive to the cost of reproduction. An experiment in blue tits[END_REF]. In the common waxbill (Estrilda astrild), favorable environmental conditions (higher nighttime temperatures) were found to positively affect red bill coloration in females: both sexes had similar bill coloration when nighttime temperatures were high but only males had redder beaks under most conditions [START_REF] Funghi | Release from ecological constraint erases sex difference in social ornamentation[END_REF].

To determine which coloration types might be more or less costly, it is essential to conduct experiments on both sexes within a species [START_REF] Doutrelant | Female plumage coloration is sensitive to the cost of reproduction. An experiment in blue tits[END_REF]. This approach makes it possible to compare how different coloration traits respond to the same experimental treatments [START_REF] Hill | A multifactorial test of the effects of carotenoid access, food intake and parasite load on the production of ornamental feathers and bill coloration in American goldfinches[END_REF][START_REF] Peters | Conditiondependence of multiple carotenoid-based plumage traits: an experimental study[END_REF][START_REF] Siefferman | Male eastern bluebirds trade future ornamentation for current reproductive investment[END_REF]; ideally, both sexually selected and non-sexually selected traits would be included [START_REF] Cotton | Do sexual ornaments demonstrate heightened condition-dependent expression as predicted by the handicap hypothesis?[END_REF]). To our knowledge, this type of experiment has never been done. Additionally, data on different coloration traits from longterm studies are invaluable because environmental fluctuations result in somewhat variable conditions [START_REF] Cockburn | Swingin'in the rain: condition dependence and sexual selection in a capricious world[END_REF][START_REF] Vergara | The condition dependence of a secondary sexual trait is stronger under high parasite infection level[END_REF] and studies carried out over extended time periods are like natural experiments (or pseudo-experiments). Ideally, such long-term studies should examine both sexually and non-sexually selected traits.

Third, females may preferentially use badges of status which are viewed as ornaments with social costs but with no production costs. The social costs given by the receivers who attack emitters displaying similar signal levels are maintaining signal honesty [START_REF] Maynard-Smith | Animal signals[END_REF]. In this case, there would thus be no trade-off between coloration cost and future female fecundity, and females could be more unconstrained in their use of badges of status in competitive interactions. These badges of status should be quite frequent in females.

Research in American goldfinches (Spinus tristis) found evidence for the badge-of-status hypothesis: in females, bill coloration was not affected when flying capacity was experimentally impaired. However, it was impacted when social costs were experimentally increased (i.e., artificially creating winners and losers during social interactions leads to respectively increase bill coloration in winners and decrease coloration in loser: Tarvin et al., 2016a). A comparative study found a similar result: carotenoid-based bill coloration seemed to be more correlated with indices of sociality (sociality over the winter, coloniality) than with indices of sexual selection (Dey, Valcu, Kempenaers, & Dale, 2015).

Fourth, costs may be offset by benefits and costly female ornaments be present whenever benefits of displaying the signals are important. Costs do not exist in isolation [START_REF] Cain | Next steps for understanding the selective relevance of femalefemale competition[END_REF], so they must be associated with benefits. Males investing more in parental care are predicted to be choosier. As a result, if a female's investment in coloration is rewarded by a greater chance of acquiring a highly investing male or by enhancing parental care coordination between partners, then signaling costs may be offset. Support for this idea has been obtained in fishes (Méndez-Janovitz, Gonzalez-Voyer, & Macías Garcia, 2019): females were much more ornamented in a subfamily where males of the species invested heavily in reproduction (Goodeinae) than in a subfamily where they did not (Poeciliinae). In birds also, at the interspecific level, presence of parental care seems to be associated with more ornamented females [START_REF] Dale | The effects of life history and sexual selection on male and female plumage colouration[END_REF]. At the intraspecific level, the hypothesis above could be tested in birds by experimentally changing female coloration and determining whether highly ornamented females are chosen by better-quality males who have better territory or are better at taking care of the young. The differential allocation hypothesis which predicts that reproductive investment is influenced by mate attractiveness, also predicts such differences in investment in relation to female ornaments. The fact that more ornamented females have male capable of investing more have been observed in various species: for example, for sperm allocation in feral junglefowl [START_REF] Pizzari | Indirect partner choice through manipulation of male behaviour by female fowl, Gallus gallus domesticus[END_REF], for nest defense in rock sparrows (Petronia petronia [START_REF] Matessi | Male rock sparrows differentially allocate nest defence but not food provisioning to offspring[END_REF], or for feeding rate in tree swallow (Tachycineta bicolor [START_REF] Dakin | Plumage colour is associated with partner parental care in mutually ornamented tree swallows[END_REF] or mountain whitecrowned sparrow (Z. leucophrys oriantha [START_REF] Laubach | Determinants of parental care in Mountain White-crowned Sparrows (Zonotrichia leucophrys oriantha)[END_REF]. However not all studies found positive associations: (e.g. [START_REF] Berzins | Experimentally altered plumage brightness of female tree swallows: a test of the differential allocation hypothesis[END_REF]Limbourg, Mateman, & Lessells, 2013a), and a meta-analysis should be performed. In general, more experiments are needed to test this hypothesis. When mate choice is based on plumage coloration, experiments need to be performed before mate choice occurs because plumage coloration is not temporally dynamic. An experiment that manipulated plumage color after individuals had already paired would create unnatural conditions, which could explain the inconsistency in results found in several studies (e.g. Limbourg, Mateman, & Lessells, 2013b;[START_REF] Mahr | Female attractiveness affects paternal investment: experimental evidence for male differential allocation in blue tits[END_REF].

(ii) Benefits of male mate choice

To understand the evolution of female ornament though mate choice, it is needed to determine which benefits males may acquire when choosing a female based on their ornaments.

Direct benefits take place when mating with a high-quality partner increases male current reproductive success or survival. Indirect benefits occur when ornaments are heritable and when more ornamented females are linked to better quality genes. When direct benefits exist, male mate choice has been shown as an evolutionary stable strategy in both polygynous and monogamous species [START_REF] Courtiol | The evolution of mutual mate choice under direct benefits[END_REF][START_REF] Ihara | Sexual selection by male choice in monogamous and polygynous human populations[END_REF][START_REF] Servedio | Population genetic models of male and mutual mate choice[END_REF]. In contrast, in polygynous species when direct benefits are absent, and in species where there are pronounced sexual conflicts or sexual harassment on females, male mate choice is less likely to lead to the evolution of female-specific traits [START_REF] Fitzpatrick | Male mate choice, male quality, and the potential for sexual selection on female traits under polygyny[END_REF][START_REF] Long | A cost of sexual attractiveness to highfitness females[END_REF]. Additionally, papers suggest that, in polygynous species, male mate choice is less likely to evolve in the case of indirect benefits and arbitrary traits [START_REF] Fitzpatrick | The evolution of male mate choice and female ornamentation: a review of mathematical models[END_REF]. This review also indicates that when mating investment costs are low for males, indirect benefits are less likely to be maintained by selection and less likely to exceed the benefits of multiple mating. Clearly, understanding the overall benefits of male mate choice is key to understanding the evolution of female coloration.

The direct and indirect benefits obtained by females via mate choice have been thoroughly characterized. Thus, rather than simply listing similar benefits acquired by males who mate with high-quality females, we instead focus on direct benefits that are particularly exclusive to females and/or linked to female reproductive capacity.

A direct benefit exclusive to female ornaments: enhanced fecundity. In several bird species, different types of coloration have been shown to signal fecundity. For instance, in the sexually dichromatic upland goose (Chloephaga picta leucoptera), the reddish-brown coloration of the head and the yellow-orange coloration of the legs signal condition and fecundity [START_REF] Gladbach | Female-specific colouration, carotenoids and reproductive investment in a dichromatic species, the upland goose Chloephaga picta leucoptera[END_REF]. In the sexually monochromatic eastern bluebird (Sialia sialis), females with more pronounced chestnut plumage produce larger clutches [START_REF] Grindstaff | Structural coloration signals condition, parental investment, and circulating hormone levels in Eastern bluebirds (Sialia sialis)[END_REF]. Also, based on the results of a ten-year study in the common kestrel, the gray coloration of the rump appears to be correlated with clutch size in females [START_REF] Vergara | Inter-annual variation and information content of melanin-based coloration in female Eurasian kestrels[END_REF]. Experimental studies are less common, but they too suggest that female coloration is linked with egg production capacity. When feral pigeons (Columba livia) were subject to food restrictions, darker eumelanic females had higher egg production levels [START_REF] Jacquin | Melanin-based coloration reflects alternative strategies to cope with food limitation in pigeons[END_REF]; a similar result was found for yellow brightness in an experiment in blue tits where females were forced to lay a second clutch [START_REF] Doutrelant | Female coloration indicates female reproductive capacity in blue tits[END_REF]. However, not all studies have found a positive relationship between female coloration and fecundity. A meta-analysis focused on this question is needed if we wish to determine the strength of this relationship and to determine if some coloration types are more tightly linked to fecundity than others.

An important direct benefit linked to female reproductive capacity: stress resistance.

Resistance to stress is another quality that males may look for in females. Indeed, stresssensitive females are more likely to abandon their nestlings. Support for this idea was found in an experiment in tree swallows where wing feathers were bound together to reduce wing surface, hence birds were reversibly handicapped to mimic stressful conditions: females with brighter white breasts were less likely to abandon their nests [START_REF] Taff | Achromatic plumage brightness predicts stress resilience and social interactions in tree swallows (Tachycineta bicolor)[END_REF].

Coloration has been found to signal stress resistance in many species, an association that could be due to diverse mechanisms stress hormones might affect physiological trade-offs involving for instance immune responses, but also regulation of energy intake, feather quality due to change in moult speed induced by change in corticosterone level, and / or on contrary the effect of signals on stress level [START_REF] Desrochers | Exogenous and endogenous corticosterone alter feather quality[END_REF][START_REF] Kennedy | Feather coloration in museum specimens is related to feather corticosterone[END_REF][START_REF] Romero | Corticosterone inhibits feather growth: potential mechanism explaining seasonal down regulation of corticosterone during molt[END_REF]. So far, both negative and positive associations have been found for carotenoid-based coloration and levels of corticosterone in the blood or feathers. For instance, yellow coloration was negatively linked to feather corticosterone in females but not in old males yellow warblers (Setophaga petechia: Grunst, Grunst, Parker, [START_REF] Grunst | Pigment-specific relationships between feather corticosterone concentrations and sexual coloration[END_REF]. By contrast, the feather corticosterone of common redpolls (Acanthis flammea) was positively correlated with carotenoid pigmentation in adult males but not in females or younger males [START_REF] Fairhurst | Synchronizing feather-based measures of corticosterone and carotenoid-dependent signals: what relationships do we expect?[END_REF]. In studies that only include males, both negative and positive correlations have also been found (negative: [START_REF] Kennedy | Feather coloration in museum specimens is related to feather corticosterone[END_REF][START_REF] Mougeot | Physiological stress links parasites to carotenoid-based colour signals[END_REF] positive : Fairhurst, Damore, & Butler, 2015Lendvai, 2013 #4768;[START_REF] Fairhurst | Synchronizing feather-based measures of corticosterone and carotenoid-dependent signals: what relationships do we expect?[END_REF].

Mostly negative correlations have been reported for structural coloration, both in female and male eastern bluebirds [START_REF] Grindstaff | Structural coloration signals condition, parental investment, and circulating hormone levels in Eastern bluebirds (Sialia sialis)[END_REF] and in female blue tits [START_REF] Henderson | Ultraviolet crown coloration in female blue tits predicts reproductive success and baseline corticosterone[END_REF] but not in female tree swallows: [START_REF] Sarpong | Baseline corticosterone does not reflect iridescent plumage traits in female tree swallows[END_REF]. Clearly, to better characterize the relationship between stress resistance and coloration in females (and males), we need experimental studies like those performed in crimson rosella (Platycercus elegans) males(e. g. [START_REF] Berg | Do glucocorticoids or carotenoids mediate plumage coloration in parrots? An experiment in Platycercus elegans[END_REF] where stress-induced corticosterone predicted change in pigment-and structural-based coloration after molting.

A direct benefit exclusively to female ornaments: the advantages associated with maternal effects. Maternal effects are non-genetic mechanisms by which females can influence the phenotype and fitness of their offspring [START_REF] Mousseau | The adaptive significance of maternal effects[END_REF]. There are two categories: prenatal maternal effects and postnatal maternal effects. We will first address postnatal maternal effects, which arise from maternal care, namely providing food, clean nest conditions, and protection from predators. Postnatal maternal effects obviously play a determinant role in offspring survival and fitness and thus their relationship with maternal coloration is frequently examined. For instance, in the sexually dichromatic northern cardinal (Cardinalis cardinalis), female underwing color and face mask size were both found to be positively related to maternal care [START_REF] Jawor | Multiple ornaments correlate with aspects of condition and behaviour in female northern cardinals, Cardinalis cardinalis[END_REF]. In eastern bluebirds, maternal provisioning behavior was positively associated with female rump color (Siefferman & Hill, 2005a). By contrast, in the American redstart (Setophaga ruticilla), females with brighter yellow tails visited their nests less frequently [START_REF] Osmond | Relationships between carotenoid-based female plumage and age, reproduction, and mate colour in the American Redstart (Setophaga ruticilla)[END_REF].

Prenatal maternal effects involve the passive or differential allocation of nutrients and/or substances that affect embryonic development and/or offspring development [START_REF] Badyaev | Maternal Effects as Generators of Evolutionary Change A Reassessment Year in[END_REF][START_REF] Blount | Why egg yolk is yellow[END_REF][START_REF] Sheldon | Differential allocation: tests, mechanisms and implications[END_REF]. They are suspected to have a broad range of effects on offspring fitness. Although evidence for prenatal maternal effects has not been unequivocal in all studies [START_REF] Henriksen | Prenatal stress in birds: pathways, effects, function and perspectives[END_REF], we can nonetheless state that such effects can play a decisive role in offspring fitness in some species and/or in certain environments because they help offspring face harsh conditions. Egg size is an example of a prenatal maternal effect [START_REF] Krist | Egg size and offspring quality: a meta-analysis in birds[END_REF][START_REF] Williams | Intraspecific variation in egg size and egg composition in birds: effects on offspring fitness[END_REF] and was positively correlated with ornamentation in feral female junglefowl: females with larger combs laid heavier eggs [START_REF] Cornwallis | Experimental evidence that female ornamentation increases the acquisition of sperm and signals fecundity[END_REF].

Other examples of prenatal maternal effects include the compounds found in egg contents, such as carotenoids [START_REF] Biard | Effects of carotenoid availability during laying on reproduction in the blue tit[END_REF][START_REF] Mcgraw | Maternally derived carotenoid pigments affect offspring survival, sex ratio, and sexual attractiveness in a colorful songbird[END_REF].

Carotenoid levels in the yolk play a crucial role in offspring health, survival, growth, and fledging success [START_REF] Biard | Effects of carotenoid availability during laying on reproduction in the blue tit[END_REF][START_REF] Blount | Carotenoids and life-history evolution in animals[END_REF][START_REF] Mcgraw | Maternally derived carotenoid pigments affect offspring survival, sex ratio, and sexual attractiveness in a colorful songbird[END_REF]. For instance, higher levels of carotenoids in eggs enhance the immune response of offspring [START_REF] Biard | An analysis of pre-and post-hatching maternal effects mediated by carotenoids in the blue tit[END_REF][START_REF] Saino | Experimental manipulation of egg carotenoids affects immunity of barn swallow nestlings[END_REF] and can compensate for the negative effects of ectoparasitism [START_REF] Ewen | Maternally invested carotenoids compensate costly ectoparasitism in the hihi[END_REF]. It has been hypothesized that if coloration indicates female quality, then less colored females-but not highly colored females-might have to trade off between transmitting carotenoids to their eggs and keeping carotenoids for themselves. In agreement, a positive relationship between egg carotenoid levels and female coloration was observed in experimental studies on the black-backed gull, Larus fuscus [START_REF] Blount | Carotenoids and egg quality in the lesser black-backed gull Larus fuscus: a supplemental feeding study of maternal effects[END_REF], the zebra finch [START_REF] Mcgraw | Maternally derived carotenoid pigments affect offspring survival, sex ratio, and sexual attractiveness in a colorful songbird[END_REF], and the blue tit [START_REF] Midamegbe | Female blue tits with brighter yellow chests transfer more carotenoids to their eggs after an immune challenge[END_REF].

Results from correlative studies are more mixed. For instance, on the one hand, egg carotenoid content was correlated with the immaculateness of the white cheek patch in female great tits [START_REF] Remeš | Egg yolk antioxidant deposition as a function of parental ornamentation, age, and environment in great tits Parus major[END_REF]) and the number of spots in female diamond firetails, Stagonopleura guttata [START_REF] Zanollo | Maternal investment in Diamond Firetails Stagonopleura guttata: female spot numbers predict egg volume and yolk lutein content[END_REF], but on the other hand, there was no correlation between egg carotenoid content and carotenoid-based coloration in female blue tits or great tits [START_REF] Biard | Effects of carotenoid availability during laying on reproduction in the blue tit[END_REF][START_REF] Remeš | Egg yolk antioxidant deposition as a function of parental ornamentation, age, and environment in great tits Parus major[END_REF][START_REF] Szigeti | Egg quality and parental ornamentation in the blue tit Parus caeruleus[END_REF].

Females also transfer antibodies to their eggs, and these antibodies are essential in protecting nestlings during their first days of life [START_REF] Boulinier | Maternal transfer of antibodies: raising immuno-ecology issues[END_REF][START_REF] Hasselquist | Maternal transfer of antibodies in vertebrates: transgenerational effects on offspring immunity[END_REF]. There may even be a transgenerational impact; one experimental study in urban pigeons found that maternal effects carried all the way through to grandchildren [START_REF] Ismail | Transfer of humoural immunity over two generations in urban pigeons[END_REF]. To date, to our knowledge, only two experiments have examined the link between plumage coloration and antibody transfer capacity in females. In the first study, when feral female pigeons were injected with inactivated Chlamydia psittaci (a natural bacterial pathogen), darker mothers transferred more antibodies against the bacterium into their eggs than did paler ones, even though their levels of circulating antibodies were similar [START_REF] Jacquin | Darker female pigeons transmit more specific antibodies to their eggs than do paler ones[END_REF]. In barn swallows (Hirundo rustica), darker females allocated more antibodies to their eggs, but only when their offspring sex ratio was female-biased [START_REF] Saino | Melanic coloration differentially predicts transfer of immune factors to eggs with daughters or sons[END_REF]. Because melanogenesis and immunity are partly genetically controlled [START_REF] Ducrest | Pleiotropy in the melanocortin system, coloration and behavioural syndromes[END_REF], parental melanin-based pigmentation may covary with the levels of immune factors transferred to eggs. In addition, correlations between eggshell coloration and antibody levels have been found in the pied flycatcher (Ficedula hypoleuca) and the blue tit [START_REF] Holveck | Eggshell spottiness reflects maternally transferred antibodies in blue tits[END_REF][START_REF] Morales | Egg colour reflects the amount of yolk maternal antibodies and fledging success in a songbird[END_REF].

Lastly, maternally derived steroids (i.e., testosterone, 4-androstenedione) and corticosterone [START_REF] Gil | Hormones in avian eggs: Physiology, ecology and behavior Advances in the Study of Behavior[END_REF][START_REF] Groothuis | Maternal hormones as a tool to adjust offspring phenotype in avian species[END_REF][START_REF] Schwabl | Yolk is a source of maternal testosterone for developing birds[END_REF] are also present in the egg yolk and result in key maternal effects. The transfer of corticosterone is assumed to be passive [START_REF] Hayward | Maternal corticosterone is transferred to avian yolk and may alter offspring growth and adult phenotype[END_REF] and thus directly linked to female stress level. A recent meta-analysis of 117 studies that experimentally modified hormones in females before they laid eggs or in the eggs themselves prior to incubation, confirmed that maternal androgens affect offspring development and, more specifically, offspring dominance and competitive ability [START_REF] Podmokła | Chicken or egg? Outcomes of experimental manipulations of maternally transmitted hormones depend on administration method-a meta-analysis[END_REF]. Testosterone and androstenedione can increase begging behavior, growth, and survival during early life [START_REF] Groothuis | Maternal hormones as a tool to adjust offspring phenotype in avian species[END_REF]. However, they may also have negative effects, for instance yolk androgens reduce survival in American kestrels (Falco sparverius) [START_REF] Sockman | Yolk androgens reduce offspring survival[END_REF]. Similarly, excessively high corticosterone levels can reduce hatchling size and begging behavior [START_REF] Saino | Stressed mothers lay eggs with high corticosterone levels which produce low-quality offspring[END_REF]. Prenatal hormones can also have long-lasting effects on offspring phenotypes and fitness that impact dispersal behavior, survival [START_REF] Tschirren | Maternal modulation of natal dispersal in a passerine bird: An adaptive strategy to cope with parasitism?[END_REF], adult immunity [START_REF] Tobler | Short-and long-term consequences of prenatal testosterone for immune function: an experimental study in the zebra finch[END_REF], and metabolic rate [START_REF] Nilsson | Long-lasting consequences of elevated yolk testosterone for metabolism in the zebra finch[END_REF]. Transgenerational effects, stretching from females to their grandchildren, have also been observed [START_REF] Khan | Maternal corticosterone exposure has transgenerational effects on grand-offspring[END_REF]; (Podmokła, et al.). For instance, maternal hormones seem to affect the reproductive investments made by female offspring [START_REF] Hsu | No escape from mother's will: effects of maternal testosterone on offspring reproductive behaviour far into adulthood[END_REF][START_REF] Müller | Long-lasting consequences of elevated yolk testosterone levels on female reproduction[END_REF][START_REF] Podmokła | Chicken or egg? Outcomes of experimental manipulations of maternally transmitted hormones depend on administration method-a meta-analysis[END_REF].

Consequently, because maternal hormones and nutrients in eggs have complex effects that can only be detected in specific environmental contexts [START_REF] Groothuis | Introducing biological realism into the study of developmental plasticity in behaviour[END_REF][START_REF] Morosinotto | Predation risk affects the levels of maternal immune factors in avian eggs[END_REF], it is important to carry out more correlative and experimental studies that test whether maternal coloration can signal the ability to transfer maternal hormones, and how such maternal effects influence offspring development.

(iii) Female-female competition and female coloration

Experimental evidence for badges of status in birds. With badges of status, competitors can evaluate each other's competitive abilities from a distance, and conflicts can be resolved without paying high costs of fighting when signalers display badges of different sizes or intensities [START_REF] Rohwer | Social significance of avian winter plumage variability[END_REF][START_REF] Rohwer | Status signaling in harris sparrows: some experiments in deception[END_REF]. Badges of status have mainly been described in males [START_REF] Santos | Dominance and plumage traits: meta-analysis and metaregression analysis[END_REF][START_REF] Senar | Color Displays as Intrasexual Signals of Aggression and Dominance In[END_REF], but description of female badges of status is growing in literature. Female birds of many species compete for food and/or territories before or during the breeding season [START_REF] Stockley | Female competition and aggression: interdisciplinary perspectives[END_REF][START_REF] Tobias | The evolution of female ornaments and weaponry: social selection, sexual selection and ecological competition[END_REF]. Some authors have suggested that females may compete more between each other than males before the breeding season. This would explain why in American goldfinches, bill color seems to function as a badge of status in females but not in males [START_REF] Murphy | Same trait, different receiver response: unlike females, male American goldfinches do not signal status with bill colour[END_REF]. That said, we still have limited experimental evidence indicating that coloration plays a role in female-female competition. Our survey of the literature for birds found 16 species for which experimental studies had been performed (Table 3). For instance, in the female pied flycatcher, decreasing the size of the white wing patch reduced female incubation attendance, a result thought to be the results of their decreased status [START_REF] Plaza | Female incubation attendance and nest vigilance reflect social signaling capacity: a field experiment[END_REF], or in the lovely fairywren (Malurus amabilis), less colored females (and males) reacted more aggressively when shown images of themselves than when shown images of more colored birds [START_REF] Leitão | Female and male plumage colour signals aggression in a dichromatic tropical songbird[END_REF]. In prothonotary warblers, nest boxes were placed in high and low quality territories and it appeared that older females with purer yellow breast coloration occupied nest boxes in higher quality territories [START_REF] Beck | Nest-box acquisition is related to plumage coloration in male and female Prothonotary Warblers (Protonotaria citrea)[END_REF]. Most of the experimental studies listed in Table 3 yielded results that support the badge-of-status hypothesis. However, some are based on small sample sizes, and it is important to acknowledge that many negative results might have gone unpublished [START_REF] Sanchez-Tojar | Meta-analysis challenges a textbook example of status signalling and demonstrates publication bias[END_REF]. It is thus necessary to continue conducting experimental studies on a larger number of species, with adequate sample sizes and methods [START_REF] Chaine | Manipulating badges of status only fools strangers[END_REF]. More tests are particularly important because social and intersexual selection are thought to be the most powerful selective forces acting on female ornaments [START_REF] Stockley | Female competition and aggression: interdisciplinary perspectives[END_REF][START_REF] Tobias | The evolution of female ornaments and weaponry: social selection, sexual selection and ecological competition[END_REF].

(Table 3. about here) Is there a specific coloration type associated with badges of status and, if so, why? Based on Table 3, it appears that all types of coloration can act as badges of status in females, for example, five species had carotenoid-based badges of status, three had melanin-based badges of status, and seven had structural-coloration-based badges of status. For coloration traits to serve as badges of status, they need to reflect condition, dominance or aggressiveness.

Association between (i) condition, (ii) dominance or aggressiveness have been found for all type of colorations. (i) Condition -as stated earlier in the review, experiments have shown that iridescent and dark structural coloration is affected by bird condition [START_REF] Doutrelant | Female plumage coloration is sensitive to the cost of reproduction. An experiment in blue tits[END_REF][START_REF] Hill | The effect of coccidial infection on iridescent plumage coloration in wild turkeys[END_REF][START_REF] Mcgraw | Different colors reveal different information: how nutritional stress affects the expression of melanin-and structurally based ornamental plumage[END_REF]but see Peters, et al., 2011;[START_REF] Siefferman | Male eastern bluebirds trade future ornamentation for current reproductive investment[END_REF][START_REF] Siitari | Color signals in the black grouse (Tetrao tetrix): Signal properties and their condition dependency[END_REF]. White feathers could also convey condition at molting, since their lack of melanin may make them more sensitive to wear and more vulnerable to parasites and bacteria, especially when feather quality is low [START_REF] Ruiz-De-Castañeda | Bacterial degradability of white patches on primary feathers is associated with breeding date and parental effort in a migratory bird[END_REF][START_REF] Swaddle | Chest plumage, dominance and fluctuating asymmetry in female starlings[END_REF]. As also stated earlier in the text, carotenoid-based coloration (i.e., yellow, orange, red) reveals condition because carotenoids must be acquired from the environment, notably from food resources, and because they are used in other physiological processes, such as detoxification and immune function [START_REF] Simons | What does carotenoid-dependent coloration tell? Plasma carotenoid level signals immunocompetence and oxidative stress state in birds-a meta-analysis[END_REF]. For instance, in the red grouse (Lagopus lagopus scoticus), female comb brightness and size were inversely correlated with parasite load [START_REF] Martinez-Padilla | Condition-and parasite-dependent expression of a male-like trait in a female bird[END_REF]. ii) Aggressiveness and dominance-testosterone which is often linked to dominance and aggressiveness is linked to a wide range of coloration traits, as well as badge-of-status size, in females. In experimental studies, increasing testosterone has induced male-like patterns of carotenoid-based coloration [START_REF] Lahaye | Bare-part color in female budgerigars changes from brown to structural blue following testosterone treatment but is not strongly masculinized[END_REF][START_REF] Mcgraw | Sex steroid dependence of carotenoid-based coloration in female zebra finches[END_REF] and structural coloration in females [START_REF] Peters | Testosterone treatment of female Superb Fairy-wrens Malurus cyaneus induces a male-like prenuptial moult, but no coloured plumage[END_REF]. For melanin coloration this was not the case [START_REF] Anthes | Measuring and interpreting sexual selection metrics: evaluation and guidelines[END_REF][START_REF] Strasser | Yolk testosterone organizes behavior and male plumage coloration in house sparrows (Passer domesticus)[END_REF], however in the white-shouldered fairywren (Malurus alboscapulatus), it has been proposed that there is a link between testosterone and melanin-based coloration because in populations where females are more colored, females are also more aggressive and have higher levels of circulating testosterone [START_REF] Enbody | Female ornamentation is associated with elevated aggression and testosterone in a tropical songbird[END_REF]. Such a relationship with testosterone also seems to exist for melanin-based coloration within populations of North American barn swallows, Hirundo rustica erythrogaster [START_REF] Vitousek | Female plumage colour influences seasonal oxidative damage and testosterone profiles in a songbird[END_REF]; for carotenoid-based coloration in American goldfinches [START_REF] Pham | Honesty of a dynamic female aggressive status signal: baseline testosterone relates to bill color in female American goldfinches[END_REF], and for white structural coloration in pied flycatchers [START_REF] Cantarero | Testosterone levels in relation to size and UV reflectance of achromatic plumage traits of female pied flycatchers[END_REF][START_REF] Moreno | Extent of a white plumage patch covaries with testosterone levels in female pied flycatchers Ficedula hypoleuca[END_REF]. In the case of melaninbased coloration, this link may result because of pleiotropic genes that produce proteins involved in both melanin production and androgen production [START_REF] Ducrest | Pleiotropy in the melanocortin system, coloration and behavioural syndromes[END_REF]. For carotenoid-based coloration, testosterone has been found to increase carotenoid bioavailability [START_REF] Blas | Testosterone increases bioavailability of carotenoids: Insights into the honesty of sexual signaling[END_REF][START_REF] Mcgraw | Sex steroid dependence of carotenoid-based coloration in female zebra finches[END_REF]. In contrast, the proximate links between structural coloration and testosterone are less clear, but some research has found that manipulating testosterone levels in males can change their coloration [START_REF] Peters | Age-dependent association between testosterone and crown UV coloration in male blue tits (Parus caeruleus)[END_REF][START_REF] Roberts | Testosterone increases UV reflectance of sexually selected crown plumage in male blue tits[END_REF]. So the links between female coloration and badges of status can be explained because they have increased condition and/or testosterone level.

Historically, the byproduct hypothesis was used to explain the correlation between female ornamentation and testosterone [START_REF] Kimball | Evolution of avian plumage dichromatism from a proximate perspective[END_REF]. In the same time, it served to explain that evolution of female ornamentation was constrained. Like males, females with higher testosterone levels would invest less in offspring care (e. g. [START_REF] Clotfelter | Consequences of elevating plasma testosterone in females of a socially monogamous songbird: evidence of constraints on male evolution?[END_REF][START_REF] Gerlach | Experimental elevation of testosterone lowers fitness in female dark-eyed juncos[END_REF]. However, the effects of testosterone may be limited because testosterone levels often decline when the demands of offspring increase.

Finally, the fact that coloration traits can function as badges of status could be explained by their sensitivity to social context (i.e., the sum of the social interactions experienced by an individual). Social context influence the physiology, behavior, and signaling traits of individuals, which induces feedback loops between signals and hormones [START_REF] Vitousek | An integrative view of the signaling phenotype: Dynamic links between signals, physiology, behavior and social context[END_REF]. This has been shown in male white-crowned sparrows for which the size of the crown white patch signal an individual's ability to acquire resources, and experimental enhancement of the white patch increased baseline corticosterone levels [START_REF] Laubach | Are whitecrowned sparrow badges reliable signals?[END_REF]. Also in the North American barn swallow, males whose plumage had been darkened had higher levels of testosterone [START_REF] Safran | Sexual signal exaggeration affects physiological state in male barn swallows[END_REF]. This species is sexually monochromatic, and, intriguingly, females whose plumage had been darkened displayed the opposite effect: they had lower levels of both circulating testosterone and plasma oxidative damage [START_REF] Vitousek | Female plumage colour influences seasonal oxidative damage and testosterone profiles in a songbird[END_REF]. Clearly, more research is needed on both sexes to understand this opposite effect. Also clearly to better understand how coloration could help mediate aggressive interactions among females, we need a better grasp of the physiological mechanisms that underlie aggressiveness in females.

Physiological mechanisms underlying female aggressiveness.

As mentioned above, one hypothesis is that aggressiveness is driven by testosterone levels, like in males. In several species [START_REF] Rosvall | How research on female vertebrates contributes to an expanded challenge hypothesis[END_REF], such as the dark-eyed junco [START_REF] Cain | Competitive females are successful females; phenotype, mechanism, and selection in a common songbird[END_REF], females with experimentally heightened testosterone levels displayed more aggressive territory-defense behaviors. This relationship may arise because females have androgen receptors in brain regions associated with aggression [START_REF] Rosvall | Neural sensitivity to sex steroids predicts individual differences in aggression: implications for behavioural evolution[END_REF]. However, although long-term observational studies and natural experiments have shown that certain social conditions, such as higher breeding densities, are correlated with higher testosterone levels in females, the latter is not the systematic response to experimentally induced social challenges [START_REF] Rosvall | How research on female vertebrates contributes to an expanded challenge hypothesis[END_REF]. In addition it seems that testosterone is not responsible for all the similarities or differences between the sexes. For instance, a comparative study of 51 bird species found that high levels of testosterone in females were not related to variation in sexual size monomorphism, sexual plumage monochromatism, mating system, latitude, or the degree of coloniality [START_REF] Goymann | Male-to-female testosterone ratios, dimorphism, and life history-what does it really tell us[END_REF]. Several studies [START_REF] Goymann | Male-to-female testosterone ratios, dimorphism, and life history-what does it really tell us[END_REF][START_REF] Stockley | Female competition and aggression: interdisciplinary perspectives[END_REF] have thus suggested that other mechanisms should also be examined, such as reduced testosterone sensitivity at neural sites associated with maternal behavior or sex-specific genomic responses to testosterone [START_REF] Rosvall | Proximate perspectives on the evolution of female aggression: good for the gander, good for the goose?[END_REF][START_REF] Rosvall | Neural sensitivity to sex steroids predicts individual differences in aggression: implications for behavioural evolution[END_REF]. If aggression is regulated via sensitivity to sex-specific steroids, as opposed to via testosterone levels themselves, it could allow organisms to display aggressiveness while also avoiding the costs associated with systemically high testosterone levels. Interestingly, [START_REF] Rosvall | How research on female vertebrates contributes to an expanded challenge hypothesis[END_REF] advocates taking a further step: replacing classical indices of male-male competition and male-specific sexual selection with newly designed metrics that characterize the strength of female-female competition (such as competition for nesting sites, high quality males, or the benefits they provide). This recommendation to move beyond the field's current research framework, which is based on studies of males, is strikingly similar to the conclusion we reached in our section reviewing comparative studies of female coloration.

Summary -To conclude this section on the signaling content of female coloration traits, it appears that much remains to be done for most of the topics addressed. There is a need to study physiological mechanisms that are specific to females while also exploring relationships between coloration and other traits in both females and males. As mentioned, this is valid for condition, aggressiveness, testosterone, and this seems also true for other important functions such as immune function [START_REF] Kelly | Carotenoid-based ornaments of female and male American goldfinches (Spinus tristis) show sex-specific correlations with immune function and metabolic rate[END_REF] or oxidative stress [START_REF] Viblanc | Mutually honest? Physiological 'qualities' signalled by colour ornaments in monomorphic king penguins[END_REF]. In addition, questions should be more explored using experimental studies that include both females and males. Care should be taken to better represent complexity in the experiments. For example, a study could look at several traits at the same time in both sexes, and several types of experiments could be carried out in the same species. In the last part of this review, we will highlight why it is crucial to work extensively on a single study model and how such an approach will increase our scientific standards.

The blue tit as a model study system

The last part of this review is an overview of studies that have explored the evolution of female-and male-coloration traits in a single species, the blue tit (Table 4). Some of the studies are our own. Most research on female and male coloration in blue tits has focused on two coloration traits: the ultraviolet (UV)-blue crown and the yellow breast. The UV-blue coloration of the crown results from plumage macro-and microstructures [START_REF] Hegyi | Reflectance variation in the blue tit crown in relation to feather structure[END_REF][START_REF] Jacot | Effects of nestling condition on UV plumage traits in blue tits: an experimental approach[END_REF]. The yellow coloration of the breast is carotenoid-based [START_REF] Partali | Carotenoids in Food-Chain Studies .2. The Food-Chain of Parus Spp Monitored by Carotenoid Analysis[END_REF] and may also be due to microstructure [START_REF] Jacot | Dissecting carotenoid from structural components of carotenoid-based coloration: a field experiment with great tits (Parus major)[END_REF][START_REF] Shawkey | Carotenoids need structural colours to shine[END_REF]. In addition to working on these two traits (Figure 2), we have also studied eggshell coloration.

Initial studies in the late 1990s revealed the bird's sexual dichromatism [START_REF] Andersson | Ultraviolet sexual dimorphism and assortative mating in blue tits[END_REF][START_REF] Hunt | Blue tits are ultraviolet tits[END_REF]. Subsequently, a plethora of quality research was produced for males, and the results largely suggest that UV-blue crown coloration and yellow breast coloration are sexually selected (Alonso-Alvarez, [START_REF] Alonso-Alvarez | Ultraviolet reflectance affects male-male interactions in the blue tit (Parus caeruleus ultramarinus)[END_REF][START_REF] D'alba | Differential deposition of antimicrobial proteins in blue tit (Cyanistes caeruleus) clutches by laying order and male attractiveness[END_REF][START_REF] Delhey | Paternity analysis reveals opposing selection pressures on crown coloration in the blue tit (Parus caeruleus)[END_REF]Delhey & Kempenaers, 2006;[START_REF] Delhey | Seasonal changes in blue tit crown color: do they signal individual quality[END_REF], 2007;[START_REF] Dreiss | Sex ratio and male sexual characters in a population of blue tits, Parus caeruleus[END_REF][START_REF] Griffith | Correlations between ultraviolet coloration, overwinter survival and offspring sex ratio in the blue tit[END_REF][START_REF] Kingma | Manipulation of male attractiveness induces rapid changes in avian maternal yolk androgen deposition[END_REF][START_REF] Korsten | Primary sex ratio adjustment to experimentally reduced male UV attractiveness in blue tits[END_REF][START_REF] Korsten | Absence of status signalling by structurally based ultraviolet plumage in wintering blue tits (Cyanistes caeruleus)[END_REF][START_REF] Ornborg | Seasonal changes in a ultraviolet structural colour signal in blue tits, Parus caeruleus[END_REF][START_REF] Parker | Fecundity selection on ornamental plumage colour differs between ages and sexes and varies over small spatial scales[END_REF][START_REF] Peters | Age-dependent association between testosterone and crown UV coloration in male blue tits (Parus caeruleus)[END_REF][START_REF] Rémy | Mediating male-male interactions: the role of the UV blue crest coloration in blue tits[END_REF][START_REF] Roberts | Testosterone increases UV reflectance of sexually selected crown plumage in male blue tits[END_REF][START_REF] Senar | Brighter yellow blue tits make better parents[END_REF][START_REF] Sheldon | Ultraviolet colour variation influences blue tit sex ratios[END_REF][START_REF] Szigeti | Egg quality and parental ornamentation in the blue tit Parus caeruleus[END_REF][START_REF] Vedder | Ultraviolet plumage does not signal social status in free-living blue tits; an experimental test[END_REF][START_REF] Vedder | Ultraviolet crown colouration affects contest outcomes among male blue tits, but only in the absence of prior encounters[END_REF]. However, a meta-analysis that looked at all the studies together found support for just a single conclusion: that blue tits have a sexually dichromatic blue crown that displays greater UV reflectance in males than in females [START_REF] Parker | What do we really know about the signalling role of plumage colour in blue tits? A case study of impediments to progress in evolutionary biology[END_REF]. The equivocal nature of the results largely stems from small sample sizes and inflated type I error rate, but might also partly be explained by the fact that the results came from different blue tit populations across Europe.

We have put together a summary of the different studies that have looked at female coloration in the blue tit (Table 4). Below, we discuss what we have gleaned from them about the potential for selective pressures to act on female coloration in this species.

The heritability of coloration and the strength of inter-sex genetic correlations. In previous research, we estimated the heritability of female and male coloration and the strength of related genetic correlations using quantitative genetics tools [START_REF] Charmantier | Colour ornamentation in the blue tit: quantitative genetic (co) variances across sexes[END_REF]. We found a moderate level of heritability (around 15%) for the chromatic part of the coloration of both the UV-blue crown and the yellow breast, which is a bit higher than 0.07-0.10, the heritability found in a blue tit study population in the United Kingdom [START_REF] Hadfield | Direct versus indirect sexual selection: genetic basis of colour, size and recruitment in a wild bird[END_REF].

For the achromatic part (brightness) of both characteristics, we found little to no heritability, a result that was consistent with findings on yellow breast brightness in a great tit population in the United Kingdom [START_REF] Evans | Quantitative genetics of a carotenoid-based color: heritability and persistent natal environmental effects in the great tit[END_REF]. Our results also indicated that most of the genetic variation underlying coloration is shared by both sexes, a result that supports the existence of strong genetic correlations between sex-specific coloration traits (Charmantier, et al., 2017).

The link between female coloration and maternal quality. We have conducted experiments to better understand the relationship between female coloration and maternal investment in reproduction to determine whether maternal coloration could signal maternal quality. In a first experiment, we forced females to lay later in the season and found that, within this treatment group, there was a positive association between yellow breast brightness and the ability to lay a large clutch and recruit more offspring [START_REF] Doutrelant | Female coloration indicates female reproductive capacity in blue tits[END_REF]. No such relationship was found in the control group. We then conducted an experiment in which we provoked an immune response in females before they laid their eggs. In the treatment group, females with brighter yellow breasts transferred higher levels of carotenoids to their eggs [START_REF] Midamegbe | Female blue tits with brighter yellow chests transfer more carotenoids to their eggs after an immune challenge[END_REF]; this relationship was not seen in the control group. We also found female yellow breast brightness to be positively correlated with egg antibody concentrations [START_REF] Holveck | Eggshell spottiness reflects maternally transferred antibodies in blue tits[END_REF].

Taken together, these results led us to conclude that yellow breast brightness could be a signal of maternal quality in blue tits. They also suggested that relationships between signals and quality in females may be easier to detect under adverse conditions. Interestingly, results from our long-term data set (which includes more than 15 years of data on female and male coloration) also indicate that there is spatiotemporal variation in relationships between coloration traits and proxies of reproductive success. For example, the correlation between female coloration and clutch size fluctuates greatly across years and among our populations from southern France (Figure 3). Female ornaments were also found to be associated with proxies of maternal quality in certain other blue tit studies throughout Europe [START_REF] Garcia-Navas | Plumage yellowness predicts foraging ability in the blue tit Cyanistes caeruleus[END_REF][START_REF] Henderson | Ultraviolet crown coloration in female blue tits predicts reproductive success and baseline corticosterone[END_REF][START_REF] Szigeti | Egg quality and parental ornamentation in the blue tit Parus caeruleus[END_REF], but sometimes no such relationship was observed [START_REF] Hadfield | Direct versus indirect sexual selection: genetic basis of colour, size and recruitment in a wild bird[END_REF][START_REF] Lucass | Structural (UV) and carotenoid-based plumage coloration-signals for parental investment?[END_REF][START_REF] Parker | Fecundity selection on ornamental plumage colour differs between ages and sexes and varies over small spatial scales[END_REF]. If female coloration signals quality, one should expect to see positive associations between proxies of female quality and male investment in parental care. In one study, carried out within a single population, males invested more when paired with females whose UV-blue crowns had greater UV reflectance, i.e., lower hue [START_REF] Mahr | Female attractiveness affects paternal investment: experimental evidence for male differential allocation in blue tits[END_REF]. However, in two other studies performed in another population, the opposite was found: males invested less when paired with females whose UV-blue crowns had greater UV reflectance (Limbourg, et al., 2013a(Limbourg, et al., , 2013b)). Overall, this section shows that there is some support for the idea that female coloration signals maternal quality and a female's ability to invest in offspring, but it also supports the call by [START_REF] Parker | What do we really know about the signalling role of plumage colour in blue tits? A case study of impediments to progress in evolutionary biology[END_REF] and some others [START_REF] Sanchez-Tojar | Meta-analysis challenges a textbook example of status signalling and demonstrates publication bias[END_REF][START_REF] Wang | Irreproducible text-book "knowledge": The effects of color bands on zebra finch fitness[END_REF] for replicating studies in behavioral ecology, especially when relatively small sample sizes are used and studies are correlative.

Dealing with spatiotemporal variation. Based on long-term data from our study populations (Figure 3), it is difficult to determine if any of the five proxies estimating female blue tit coloration (blue and yellow brightness, blue hue, yellow and UV blue chroma) convey information about the ability to lay a large clutch. We faced a similar problem when trying to interpret the results of a study on assortative mating and coloration in these same populations [START_REF] Fargevieille | Assortative mating by colored ornaments in blue tits: space and time matter[END_REF]. Based on data from around 1,600 pairs of birds, we observed highly variable patterns of assortative mating across space and time. These patterns fluctuated from positive to negative without ever being clearly related to population, year, or coloration. We used a within-study meta-analysis [START_REF] Nakagawa | Methodological issues and advances in biological meta-analysis[END_REF] to determine if there was a mean overall assortative mating pattern over the study period. We found that there was positive assortative mating based on UV-blue crown and yellow breast coloration and that the phenomenon varied in strength among the study populations. We used another within-study meta-analysis to determine whether female and male coloration traits were correlated with three life-history traits related to reproduction: laying date, clutch size, and fledging success. Although there was a high degree of spatiotemporal variation, overall, there were relationships between female yellow and UV-blue coloration and two maternal traits: laying date and clutch size. In addition, both yellow and UV-blue female and male coloration were correlated with fledgling success (Fargevieille, Grégoire, Téplitsky, Del Rey & Doutrelant Unpublished). The next interesting step would be to explore the drivers of the inter-annual variability that we observed and to determine whether they are due to sampling error [START_REF] Siepielski | The spatial patterns of directional phenotypic selection[END_REF] and/or changes in environmental conditions, which may enhance, relax, or limit selection pressures on signals across time. It will be important to ascertain whether specific years and/or conditions have more of an influence than others. This task could be accomplished by examining variation in the opportunity for selection, which corresponds to the variance in fitness traits [START_REF] Björklund | The importance of selection at the level of the pair over 25 years in a natural population of birds[END_REF][START_REF] Wade | The intensity of sexual selection in relation to male sexual behaviour, female choice, and sperm precedence[END_REF]. We have started looking at fluctuations in the opportunity for selection in our study populations (Figure 4), and there are clearly interesting patterns that merit further study.

What do we know about male mate choice? Above, we mentioned finding evidence of positive assortative mating based on UV-blue crown and yellow breast coloration [START_REF] Fargevieille | Assortative mating by colored ornaments in blue tits: space and time matter[END_REF]. This finding concurs with the results of [START_REF] Hunt | Preferences for ultraviolet partners in the blue tit[END_REF], who found that, in another population of blue tits, males displayed preferences associated with female UV-blue crown coloration. Furthermore, when we modified female and male yellow breast coloration, we found that it affected mutual mate choice, and that both sexes preferentially select individuals with a pale chest plumage over colorful individuals (Caro, Doutrelant, Bonadonna et al. in prep). Consequently, sexual selection may be acting on both sexes in blue tits. Research in a Spanish blue tit population provided further support for this hypothesis when it found similar positive Bateman gradients for both females and males [START_REF] García-Navas | Extrapair paternity in Mediterranean blue tits: socioecological factors and the opportunity for sexual selection[END_REF]. That said, similar research in a more northern blue tit population found positive opportunity for sexual selection in females but no positive Bateman gradient [START_REF] Schlicht | Effects of social and extra-pair mating on sexual selection in blue tits (Cyanistes caeruleus)[END_REF]. It may be that the differences in the results of these studies were due to random effects and/or differences in sample size/length of the studies. However, it is worth considering that southern populations face harsher breeding conditions than do northern populations because of lower food availability. As a result, it might be easier to detect selection pressures in southern populations. Only carrying out replicated studies across Europe will allow us to test this hypothesis. In addition, it appeared in both Garcia-Navas et al (2013) and [START_REF] Schlicht | Effects of social and extra-pair mating on sexual selection in blue tits (Cyanistes caeruleus)[END_REF] that success with the social mate(s) contributed most to variation in male reproductive success, a result which could explain the interest for mate choice in this species.

Female blue tit plumage coloration and female competition. Blue tits breed in preexisting cavities, for which there is fierce competition [START_REF] Kempenaers | Polygyny in the blue tit: unbalanced sex ratio and female aggression restrict mate choice[END_REF]. We modified the hue of the female UV-blue crown to test if this trait could be used as a badge of status in female-female competition. In agreement with the prediction of the badge of status hypothesis, we found that females with a higher UV signal (i.e., a lower hue) reacted more aggressively to a decoy presenting a high UV signal [START_REF] Midamegbe | Female-female aggressiveness is influenced by female coloration in blue tits[END_REF]. To date, no other research of this type has been performed and the replicated studies conducted on this hypothesis on males show the importance of replicating this study [START_REF] Alonso-Alvarez | Ultraviolet reflectance affects male-male interactions in the blue tit (Parus caeruleus ultramarinus)[END_REF][START_REF] Korsten | Absence of status signalling by structurally based ultraviolet plumage in wintering blue tits (Cyanistes caeruleus)[END_REF][START_REF] Rémy | Mediating male-male interactions: the role of the UV blue crest coloration in blue tits[END_REF][START_REF] Vedder | Ultraviolet plumage does not signal social status in free-living blue tits; an experimental test[END_REF]. It is thus necessary to replicate this experiment and to conduct complementary experiments to determine whether this behavior is due to competition for territories or mates.

(Table 4. about here)

Evidence that blue tit plumage coloration is a condition-dependent signal. Several studies have been conducted to determine if blue tit coloration can be condition dependent and why.

Mild food restrictions during the molt did not affect UV-blue crown coloration [START_REF] Peters | No evidence for general conditiondependence of structural plumage colour in blue tits: an experiment[END_REF]. By contrast coloration seem to be sensitive to change in the reproductive costs. We conducted experiments in which we forced both sexes to reproduce twice and later in the season and measured whether female and male coloration changed after the next molt. We found similar results for females and males: relative to individuals in the control group, individuals that were forced to invest in a second clutch late in the season displayed decreased UV-blue crown and yellow breast coloration [START_REF] Doutrelant | Female plumage coloration is sensitive to the cost of reproduction. An experiment in blue tits[END_REF]. Mechanistically, this result could be explained by the treatment causing a reduction in molting time or an increase in stress levels at molt. Indeed, when the molt rate is experimentally increased, there is a decline in UV-blue crown reflectance, yellow breast brightness, and carotenoid chromaticity in both females and males [START_REF] Ferns | Carotenoid plumage hue and chroma signal different aspects of individual and habitat quality in tits[END_REF][START_REF] Griggio | Moult speed affects structural feather ornaments in the blue tit[END_REF]. Lastly, a link between condition and coloration is suggested by the negative relationship found between baseline corticosterone levels (when the nestling are five days old) and UV-blue crown chromaticity in female blue tits [START_REF] Henderson | Ultraviolet crown coloration in female blue tits predicts reproductive success and baseline corticosterone[END_REF].

Evidence that female blue tit coloration signals resistance to parasites.

As of yet, no clear pattern has been established between plumage coloration and parasitism. In a Spanish blue tit population, infection with avian malaria was negatively correlated with metrics of yellow breast coloration in both females and malesbirds infected by multiple genera of parasites were paler [START_REF] Del Cerro | Carotenoid-based plumage colouration is associated with blood parasite richness and stress protein levels in blue tits (Cyanistes caeruleus)[END_REF]. The opposite was true in a Swedish population -infected blue tits had higher levels of UV-blue crown coloration (greater brightness and UV chroma) and brighter yellow breast plumage [START_REF] Janas | Influence of haemosporidian infection status on structural and carotenoid-based colouration in the blue tit Cyanistes caeruleus[END_REF]. Experimental work is thus necessary to explore this question in greater depth.

Eggshell coloration-a potential female ornament in blue tits? Eggshell coloration has been proposed to be a signal of quality used by the male to fine tune its investment in reproduction [START_REF] Cherry | Avian eggshell coloration: new perspectives on adaptive explanations[END_REF][START_REF] Moreno | Avian egg colour and sexual selection: does eggshell pigmentation reflect female condition and genetic quality?[END_REF]. Blue tit eggs have a white eggshell that is speckled with brown spots containing protoporphyrin-IX pigments [START_REF] Kennedy | A survey of avian eggshell pigments[END_REF]. We first investigated the potential for eggshell coloration to be a female ornament. Using both experiments and visual models, we found that blue tits could distinguish colors under dim light and thus in the dark of the cavity [START_REF] Gomez | The intensity threshold of colour vision in a passerine bird, the blue tit (Cyanistes caeruleus)[END_REF][START_REF] Holveck | Can eggs in a cavity be a female secondary sexual signal? Male nest visits and modelling of egg visual discrimination in blue tits[END_REF]. We also established that mates and neighboring males visited nest cavities when eggs were present [START_REF] Holveck | Can eggs in a cavity be a female secondary sexual signal? Male nest visits and modelling of egg visual discrimination in blue tits[END_REF]; a result also found in great tits [START_REF] Firth | Spatial, temporal and individual-based differences in nest-site visits and subsequent reproductive success in wild great tits[END_REF]. We then invested whether eggshell coloration is related to egg and female quality. In our blue tit population, both correlative and experimental results suggest females of higher quality lay eggs with a larger brown-spotted surface area, more concentrated brown spots, higher white ground UV-chroma, and lower white ground brightness [START_REF] Holveck | Eggshell spottiness reflects maternally transferred antibodies in blue tits[END_REF][START_REF] Holveck | Eggshell coloration indicates female condition during egg-laying: a field experiment in blue tits[END_REF], a result that was found in another population [START_REF] Badás | Eggshell pigmentation in the blue tit: male quality matters[END_REF][START_REF] García-Navas | Experimental evidence for the role of calcium in eggshell pigmentation pattern and breeding performance in Blue Tits Cyanistes caeruleus[END_REF] but not by: Martinez-de la [START_REF] Martinez-De La Puente | Are eggshell spottiness and colour indicators of health and condition in blue tits Cyanistes caeruleus[END_REF][START_REF] Sanz | Eggshell pigmentation pattern in relation to breeding performance of blue tits Cyanistes caeruleus[END_REF]. In another study, we modified eggshell coloration to determine whether it affected male investment: we found that it influenced male parental care, independently of female incubation or parental care patterns (Holveck, Doutrelant & Grégoire unpublished data). These results suggest that eggshell coloration could potentially function as a signal. This finding is also compatible with a more proximate hypothesis. Eggshell thickness is linked to reduced incubation time and higher hatching success in blue tits [START_REF] García-Navas | Experimental evidence for the role of calcium in eggshell pigmentation pattern and breeding performance in Blue Tits Cyanistes caeruleus[END_REF][START_REF] Sanz | Eggshell pigmentation pattern in relation to breeding performance of blue tits Cyanistes caeruleus[END_REF]. It has been proposed that protoporphyrins have a structural function and could compensate for localized eggshell thinning (e.g., caused

In our review of the literature, we have identified some questions that still need more researches to be resolved and have call for specific changes in some of our research practices.

Such changes could greatly improve our understanding of the forces driving the evolution of female ornaments. Below, we summarize our main points.

1. For research on macroevolution, we suggest that to advance farther, we need to leave behind the sexual dichromatism framework and its composite perspective, which conceal many evolutionary aspects of female (and male) coloration. Once freed, we can more accurately test the selective forces and constraints involved in the evolution of female coloration while simultaneously integrating information on the specific locations and coloration types of female ornaments. Adopting a female perspective and using female-specific indices of social and sexual selection to test hypotheses about the evolution of female coloration is particularly important. If this recommendation arose from our review of the comparative studies, it is important to note that it is applicable at both interspecific and intraspecific scales. The information in the signals targeted by male mate choice may differ from the information in the signals targeted by female mate choice. For instance, maternal effects and the maternal capacity to invest in eggs are female-specific characteristics, and research should include them in interspecific and intraspecific analyses whenever possible.

2. We need more studies to determine the strength of selection on the microevolution of coloration in different environments. This type of research requires long-term data, but many groups are already collecting such data for their study organisms. Indeed, we could establish consortia of researchers that are centered on given model systems. The greatest challenge is likely to be measuring sexual selection gradients because characterizing individual mating success is difficult in most wild bird populations. In particular, it is hard to sample unmated individuals, which leads to biased estimates and which is particularly an issue in long-term monogamous species for which selection is on mate quality rather than mate quantity [START_REF] Kvarnemo | Why do some animals mate with one partner rather than many? A review of causes and consequences of monogamy[END_REF].

3. At the intraspecific level, we need to start performing studies where we compare female and male responses to the same experimental factors and where we modify several factors at the same time to compare for instance the effect of sexual and social contexts. Interestingly, a similar conclusion was recently reached for bird song. Riebel and colleagues (2019) argued that using such an approach could help eliminate biases: "the use of song for territorial defense is almost by default considered 'sexually selected' in males, but socially selected in females, whereas we cannot detect obvious contextual differences in many cases" [START_REF] Riebel | New insights from female bird song: towards an integrated approach to studying male and female communication roles[END_REF]. 4. Lastly, using our study model, the blue tit, we underscored the need to conduct replicated studies and complementary studies. Such studies should focus on a single study system, with a view to distinguishing statistical artefacts from robust patterns. Also, we pointed the interest of employing long-term studies and appropriate statistical tools (e.g., within-study meta-analyses) to contrast estimates obtained between years or sites and to compare short-versus medium-term microevolutionary trajectories. These recommendations should be applicable to all the species studied so far, and researchers have arrived at similar conclusions for other well-studied species models [START_REF] Sanchez-Tojar | Meta-analysis challenges a textbook example of status signalling and demonstrates publication bias[END_REF] and for male blue tit coloration [START_REF] Parker | What do we really know about the signalling role of plumage colour in blue tits? A case study of impediments to progress in evolutionary biology[END_REF]. They are also applicable to any species given that sexual and social traits are highly sensitive to environmental conditions, population densities, local sex ratios, and neighborhood characteristics [START_REF] Cornwallis | Towards an evolutionary ecology of sexual traits[END_REF].

Before concluding, we wish to mention two final perspectives that were not explored in this review but that appear important to tackle. First, using a multimodal approach to study female traits (i.e., sexually and non-sexually selected traits) can expand our understanding of their evolution. At least four different families of sexually selected traits have been described in female birds: song, body coloration, behavioral displays, and egg coloration. Some studies are starting to include both female song and coloration in their analyses [START_REF] Gomes | Multimodal signalling in estrildid finches: song, dance and colour are associated with different ecological and lifehistory traits[END_REF][START_REF] Hasegawa | Reproductive advantages of multiple female ornaments in the Asian Barn Swallow Hirundo rustica gutturalis[END_REF][START_REF] Soma | Evolution of patterned plumage as a sexual signal in estrildid finches[END_REF][START_REF] Webb | Female song occurs in songbirds with more elaborate female coloration and reduced sexual dichromatism[END_REF]. Second, sexual selection is presented as being intrinsically linked to individual success and the adaptability of populations. This perspective is rooted in the notion of good genes, in which selection is directed against deleterious mutations [START_REF] Agrawal | Sexual selection and the maintenance of sexual reproduction[END_REF][START_REF] Siller | Sexual selection and the maintenance of sex[END_REF][START_REF] Whitlock | Fixation of new alleles and the extinction of small populations: drift load, beneficial alleles, and sexual selection[END_REF]. It arose from the results of theoretical studies in which sex roles were assumed to be extremely conventional: in the models, males displayed and females chose. In this framework, many males are purged via sexual selection, allowing the population to rid itself of deleterious mutations without paying the full demographic price.

However, if females are also sexually selected, it is uncertain how the model output would change.

In closing, we hope this review has provided solid groundwork for future research on the vast subject of female ornaments.

Box 1: Quantifying coloration

Bird coloration was one of the first ornaments to be used to test hypotheses about sexual selection and ornament evolution, mainly because so much of the variation in bird coloration can be easily seen by humans. In the beginning, coloration traits were scored using the human eye, reference specimens, and reference color plates; studies made distinctions between "drab" and "bright" colorations [START_REF] Badyaev | Covariation between life history and sexually selected traits: an example with cardueline finches[END_REF][START_REF] Hamilton | Heritable true fitness and bright birds: a role for parasites?[END_REF] and similar versus different colorations [START_REF] Irwin | The evolution of plumage dichromatism in the New-World Blackbirds -Social selection on female brightness[END_REF]. Such methods are still common today [START_REF] Dale | The effects of life history and sexual selection on male and female plumage colouration[END_REF][START_REF] Mcqueen | Evolutionary drivers of seasonal plumage colours: colour change by moult correlates with sexual selection, predation risk and seasonality across passerines[END_REF][START_REF] Negro | Melanin-based sexual dichromatism in the Western Palearctic avifauna implies darker males and lighter females[END_REF]Juan José Soler, et al., 2019;[START_REF] Webb | Female song occurs in songbirds with more elaborate female coloration and reduced sexual dichromatism[END_REF]. However, the use of far more precise tools, namely spectrophotometry, has revealed something that is invisible to the human eye: in many species, there is variation in the ultraviolet (UV) part of the signal emitted by coloration traits. Neurophysiologists discovered that birds have four types of photoreceptors, including one that is sensitive to UV or violet radiation, depending on the species [START_REF] Bennett | Ultraviolet vision in birds: what is its function?[END_REF]. In contrast, humans have just three types of photoreceptors, and none of them can detect UV radiation. Furthermore, there are oil droplets on the bird retina that change cone sensitivity [START_REF] Hart | Modelling oil droplet absorption spectra and spectral sensitivities of bird cone photoreceptors[END_REF]. To analysis coloration, it is therefore important not only to take advantage of high-precision tools (e.g., spectrophotometry and calibrated cameras with UV filters), but also to account for other factors that can modify color perception, such as light conditions, background, and the visual abilities of the receiver [START_REF] Endler | Animal visual systems and the evolution of color patterns: Sensory processing illuminates signal evolution[END_REF], which can be done using multiple software [START_REF] Gomez | AVICOL v5. a program to analyse spectrometric data[END_REF][START_REF] Maia | pavo 2: new tools for the spectral and spatial analysis of colour in R[END_REF][START_REF] Stoddard | How colorful are birds? Evolution of the avian plumage color gamut[END_REF] Even though several studies have found some degree of correlation between human vision and characterizations of bird vision [START_REF] Armenta | Quantifying avian sexual dichromatism: a comparison of methods[END_REF][START_REF] Bergeron | Using human vision to detect variation in avian coloration: how bad is it?[END_REF][START_REF] Dale | The effects of life history and sexual selection on male and female plumage colouration[END_REF][START_REF] Seddon | Human Vision Can Provide A Valid Proxy For Avian Perception Of Sexual Dichromatism[END_REF], it seems important that we should strive to properly assess true variation in coloration traits, rather than simply relying on assessments using human vision [START_REF] Stoddard | I see your false colours: how artificial stimuli appear to different animal viewers[END_REF]. For instance, the overlap in photoreceptor sensitivity is different between birds and humans-birds and humans likely distinguish colors differently, which may result in differences in perception [START_REF] Håstad | Different ranking of avian colors predicted by modeling of retinal function in humans and birds[END_REF]. Indeed, [START_REF] Eaton | Human vision fails to distinguish widespread sexual dichromatism among sexually "monochromatic" birds[END_REF] found that more than 90% of the species classified as monochromatic based on human vision were actually dichromatic based on spectrophotometry (and: [START_REF] Armenta | Quantifying avian sexual dichromatism: a comparison of methods[END_REF]; see also: Kevin J [START_REF] Burns | Widespread cryptic dichromatism and ultraviolet reflectance in the largest radiation of Neotropical songbirds: implications of accounting for avian vision in the study of plumage evolution[END_REF][START_REF] Eaton | Human vision fails to distinguish widespread sexual dichromatism among sexually "monochromatic" birds[END_REF]. It therefore seems especially important to take advantage of quantitative tools when addressing questions related to intraspecific communication, especially when human vision may fail to pick up on subtle but essential variation in coloration [START_REF] Bergeron | Using human vision to detect variation in avian coloration: how bad is it?[END_REF].

Box 2: The limits of the sexual dichromatism framework Sexual dichromatism can be defined as the existence of differences in coloration between males and females of the same species. In contrast, sexual monochromatism describes the situation in which males and females show no differences in coloration. Variation in sexual dichromatism if often used as a proxy for the intensity of sexual selection (e.g., [START_REF] Cooney | Sexual selection predicts the rate and direction of colour divergence in a large avian radiation[END_REF] for a very recent reference) and to test hypotheses related to the evolution of coloration traits (Dustin R [START_REF] Rubenstein | Sexual and social competition: broadening perspectives by defining female roles[END_REF][START_REF] Simpson | Migration and the evolution of sexual dichromatism: evolutionary loss of female coloration with migration among wood-warblers[END_REF]Juan José Soler, et al., 2019). However, the terms dichromatism and monochromatism conceal a broad range of coloration scenarios. Species classified as sexually monochromatic can be fully drab, drab with small patches of color, slightly colored, or very conspicuous. Similarly, in dichromatic species, it is not necessarily the case that one sex is highly conspicuous, while the other is drab (Figure 1). In some species, both males and females can be conspicuous, but males may bear a contrasting and highly conspicuous patch that is absent from females. Some dichromatic species can also be "drab," with the sexes differing in their display of brown coloration [START_REF] Marcondes | Fifty shades of brown: Macroevolution of plumage brightness in the Furnariida, a large clade of drab Neotropical passerines[END_REF]. Additionally, sexual dichromatism is currently understood to be a composite trait that reflects different evolutionary forces acting independently on males and females [START_REF] Dale | The effects of life history and sexual selection on male and female plumage colouration[END_REF][START_REF] Irwin | The evolution of plumage dichromatism in the New-World Blackbirds -Social selection on female brightness[END_REF][START_REF] Price | Reconstruction the evolution of sexual dichromatism: current color diversity does not reflect past rates of male and female change[END_REF]. For these two reasons, using sexual dichromatism as a proxy for the intensity of sexual selection should not be done automatically.

Of course, when examining the evolution of plumage coloration, it is possible to test the association between sexual dichromatism and various factors, but the analysis should be coupled with separate analyses focused specifically on female and male coloration traits in order to tease apart the numerous forces shaping sexual dichromatism [START_REF] Dale | The effects of life history and sexual selection on male and female plumage colouration[END_REF][START_REF] Dunn | Natural and sexual selection act on different axes of variation in avian plumage color[END_REF][START_REF] Cooney | Sexual selection predicts the rate and direction of colour divergence in a large avian radiation[END_REF]. both the male (e) and female (f) are drab but have melanin based patches. The Martinique oriole (Icterus bonana) provides an example of mitigated monochromatism: the male (g) and female

Tables

(h) have similar black and rusty coloration. The blue tit (Cyanistes caeruleus) provides an example of conspicuous monochromatism: the male (i) and female (j) have similar conspicuous blue and yellow coloration. It should be noted here that the blue tit displays cryptic dichromatism in the UV reflectance of its blue crown that is invisible to the human eye. Photos:

Franck Théron (a-d, g, and h), Elise Blatti (e and f), and Claire Doutrelant (i and j).

  Under social selection sensus lato any trait involved in competitive social interactions among individuals(Lyon & Montgomerie, 2012) affects fitness, which means that social selection sensus lato encompasses sexual selection. Social selection on female coloration may occur if social interactions favor female coloration phenotypes that enhance success in acquiring or maintaining any reproductively valuable resource. Both positive and negative social interactions (i.e., cooperation versus competition) can lead to variation in reproductive success and survival and thus influence selection.

Figures captions Figure 1 .

 captions1 Figures captions

Figure 2 .

 2 Figure 2. A) Locations of the four populations that our research group in Montpellier (Southern

Figure 3 .

 3 Figure 3. Estimates of directional selection gradient relating clutch size in relation to female

Figure 4 .

 4 Figure 4. Variation in the opportunity for selection from 2005 and 2014, in our four blue tit

Table 1 .

 1 Comparative studies exploring the factors involved in the evolution of female coloration in birds

								Separ
								ate
								tests
	Family/C lade	Research focus	Proxy for sexual selection	Other proxies	Color assessment method	Body parts examined	Genetic on correlati	for and female	Sexual m present dichromatis	Association coloration with female	Reference
								male
								colora
								tion?
	African Sturnidae (45 species)	Is monochromatism more common in species where female access to reproduction is limited?	Cooperative breeding (intrasexual selection)		Human vision used to score degree of monochromatis m m/dichromatis	Whole body	Not applicable	No	Yes	Sexual social selection monochromatism is associated with female	Rubenstein & Lovette 2009
	Songbirds (~6000 species)	Role of sexual and natural selection in shaping female and male plumage coloration	Social mating system, sexual size dimorphism, paternal care	Breedin g latitude, clutch size, seasonal ity, n migratio	RGB from scanned reference plates	Nape, crown, forehead, throat, upper breast, lower breast	Yes	Yes	Yes, but also differences in male and plumage female	Positive relationship sexual selection. migration and between female body size and tropical life between relationship traits. Negative	Dale et al. 2015

Table 2 .

 2 Species for which Opportunity for selection, opportunity for sexual selection and/or Bateman gradient have been calculated for females and males. dI_lnCVR is the effect size for sex difference in opportunity for selection, dI_lnCVR_VAR is the variance of dI_lnCVR, dIs_lnCVR is the effect size for sex difference in opportunity for sexual selection, dIs_lnCVR_VAR is the variance of dIs_lnCVR, dbeta_g is the effect size for sex difference in Bateman gradient, dbeta_g_VAR is the variance of dbeta_g, f_beta_z is the female Bateman gradient, f_beta_z_VAR is the variance of f_beta_z. Data issued from[START_REF] Janicke | Darwinian sex roles confirmed across the animal kingdom[END_REF]. Positive value for the 3 metric estimating sex difference indicate that metrics are male biased (the larger the value, the stronger is the male biais, negative values indicate a female biais). Positve value for f_beta_z indicates an association between mating success and reproductive success in females.

									dIs_lnC
					Parental		dI_lnCVR		VR		dbeta_g		f_beta_
	Species	Authors	Year Journal	care	Dimorphism dI_lnCVR	VAR	dIs_lnCVR	VAR	dbeta_g	VAR	f_beta_z	z VAR
	Cyanistes_caeruleus	Garcia-Navas et al,	2014 Behav Ecol	shared	male-biased				0.05	0.03	0.36	0.02
		Schlicht &
	Cyanistes_caeruleus	Kempenears	2013 Evolution	shared	male-biased 0.34	0.0003	0.15	0.0004	0.45	0.0002	0.0002	0.0002
		Whittingham &
	Delichon_urbica	Lifjeld	1995 J Avian Biol	shared	male-biased 0.53	0.15	0.00	0.15	0.60	0.21	-0.07	0.13
		Whittingham &
	Delichon_urbica	Lifjeld	1995 J Avian Biol	shared	male-biased 0.96	0.14	0.58	0.14	1.01	0.19	0.16	0.17
	Dendroica_pensylvanica Byers et al,	2004 Auk	shared	male-biased 0.66	0.05	0.39	0.06	0.95	0.06	0.57	0.03
	Gallus_gallus	Collet et al,	2012 PNAS	only female	male-biased 0.31	0.07	0.72	0.04	0.46	0.15	0.68	0.10
		Whittingham &
	Geothlypis_trichas	Dunn	2005 Behav Ecol	shared	male-biased 0.57	0.08			0.50	0.10	0.31	0.06
						female-
	Jacana_jacana	Emlen & Wrege	2004 The Auk	only male	biased		-0.24	0.01
	Junco_hyemalis	Gerlach et al,	2012 Behav Ecol	shared	male-biased 0.19	0.01	0.17	0.0003	0.74	0.0004	0.31	0.0002
	Junco_hyemalis	Ketterson et al,	1997 Ornithol Monogr	shared	male-biased 0.17	0.05	0.27	0.04	-0.12	0.04	1.01	0.02
	Meleagris_gallopavo	Krakauer	2008 Condor	only female	male-biased 0.63	0.07	0.44	0.05	0.65	0.07	0.34	0.04
				Behav Ecol
	Molothrus_ater	Woolfenden et al,	2002	Sociobiol	none	male-biased 0,23	0,03	0,30	0,02	-0,02	0,01	0,86	0,01
	Notiomystis_cincta	Walker et al,	2014 J Evol Biol	shared	male-biased 0,41	0,03	0,60	0,02	0,12	0,02	0,46	0,01

Table 4 .

 4 Studies that examined whether female blue tit coloration was subject to social or sexual selection (the publications are ordered by main theme and year)

	Research focus	Main theme and presence of evidence (Yes/No)	Result	Experimental study?	Number of individuals	Number of years	Location population of study	Authors
	Relationship between UV-blue crown coloration and female and male mate choice	Mutual choice: Yes	UV-blue crown coloration is important in female and male mate choice	Yes	24 (50% females)		UK	Hunt et al. 1999
	Relationship between UV-blue crown coloration and resource allocation	Male choice: Yes	Males provide more food to the nestlings of more colorful females	Yes (UV-blue crown coloration reduced)	30 males (and 30 modified females)		Austria	Mahr et al. 2012
			Males provide less food to the
	Relationship between UV-blue crown coloration and resource allocation	Male choice: No	nestlings of less colorful females; females provide more food to the nestlings of more	No	33 females and 33 males		Netherlands	Limbourg et al. 2013
			colorful males
	Relationship between UV-blue crown coloration and resource allocation	Male choice: No	Males provide less food to the nestlings of less colorful females; females provide more colorful males food to the nestlings of more	Yes (UV-blue reduced) crown coloration	Not found		Netherlands	Limbourg et al. 2013
	Association between UV-blue crown and yellow breast coloration and assortative mating	Mutual Choice: Yes	Assortative mating based on UV-blue crown and yellow breast coloration	No	~1200 (about 50% females)	10	France	Fargevieille et al. 2017
			Females with a higher UV
			signal (i.e., lower hue) were
			more aggressive towards UV-
	Relationship between UV-blue crown and yellow breast coloration and competition in females	Social selection: Yes	enhanced intruders; females with a lower UV signal (i.e., higher hue) were more reduced intruders; females with aggressive towards UV-	Yes (decoys UV) with reduced vs. enhanced	48 females		France	Midamegbe et al. 2011
			darker yellow chests were
			more aggressive towards
			intruders

Acknowledgments ................................................................................................................. References ............................................................................................................................. Box 1: Quantifying coloration ............................................................................................... Box 2: The limits of the sexual dichromatism framework ....................................................

Acknowledgments

We are grateful to all the people involved in the long-term study of blue tits and in particular the ones who collected the long-term data and those with who we already collaborated on some papers. Their help in the field and/or constructive criticism improved our research. We give particular thanks to Philippe Perret, Afiwa Midamegbe, Marie Holveck, Doris Gomez, Maria del Rey Granado, Anne Charmantier, Céline Téplitsky, Raphaelle Mercier Gauthier, Denis Réale, Christophe de Franceschi, Annick Lucas, Pablo Giovaninni, Gabrielle Dubuc Messier, Samuel Caro, Samuel Perret, Marcel Lambrechts, and Jacques Blondel. We further thanks Tim Janicke, Céline Téplitsky, Marie Holveck for their comments on some specific sections of this review, Tim Janicke for sharing unpublished data, Franck Théron and Elise Blatti for sharing photos and Jessica Pierce for English language editing and comments. Thanks to Marc Naguib for the opportunity to write this review and synthetize these ideas and his comments. Thanks to

by calcium deficiencies), thereby strengthening the eggshell and reducing permeability and water loss during incubation [START_REF] Gosler | Why are birds' eggs speckled?[END_REF]. This results so far got mixed experimental support in blue tits [START_REF] García-Navas | Experimental evidence for the role of calcium in eggshell pigmentation pattern and breeding performance in Blue Tits Cyanistes caeruleus[END_REF][START_REF] Holveck | Eggshell coloration indicates female condition during egg-laying: a field experiment in blue tits[END_REF]. So overall, it seems that most findings are compatible with the fact that eggshell coloration in blue tits holds information on female and egg quality.

Summary -To conclude this section, we here specifically examined what the blue tit could tell us about the evolution of female coloration. The results of the research performed to date suggest that female coloration traits are condition-dependent and may function as sexual and social signals in this species. They also highlight the importance of carrying out (more) experimental studies as well as long-term studies. Indeed, the numerous studies we reviewed, with their similarities and differences, underscore that we need more research that will help clarify whether the differing conclusions could have arisen from spatiotemporal variation in environmental conditions and/or small sample sizes. Although this assessment may feel discouraging given the years of research on this subject, this is an important message for further studies and several research teams have made similar recommendations for other widely studied species, notably the house sparrow (Passer domesticus) and the zebra finch [START_REF] Sanchez-Tojar | Meta-analysis challenges a textbook example of status signalling and demonstrates publication bias[END_REF][START_REF] Wang | Irreproducible text-book "knowledge": The effects of color bands on zebra finch fitness[END_REF].

General conclusions

Based on the collective results of the many studies mentioned in this review, female coloration appears to often function as a sexual ornament and/or a badge of status in birds. Our findings also suggest that, while female coloration might be sensitive to costs, it can still serve as a signal because there are many ways for females to use colors that do not result in trade-offs with fecundity. the two reviewers who took time to read this manuscript. Research on blue tits coloration was funded by the ANR 09-JCJC-0050-0), the Regional Government of Languedoc-Roussillon (Chercheur d'Avenir grant to CD), and the OSU-OREME