F Pennec
email: fpennec@laas.fr

P Pons

A Coustou

P Calmon

R Plana

F Courtade

DC CONTACT RF MEMS SWITCHES WITH LOW ACTUATION VOLTAGE

Keywords:

⎯ Capacitive RF MEMS switches demonstrate that it is difficult to obtain a perfect capacitive contact for a low actuation voltage. Architectures of MEMS switches with resistive contact are then developed. The objective is to minimize the contact resistance to obtain the best RF performances while keeping a low actuation voltage. First set of results have shown that good RF performances, with contact resistance lower than 1.3Ω, can be obtained with a 30V actuation voltage and with a 6µm gap height. With cantilever height around 1.5µm it will be then possible to obtain enough contact force with 6V applied voltage.

I INTRODUCTION

Despite excellent RF characteristics (high isolation, low insertion, excellent signal linearity, low power consumption, high compactness) [START_REF] Rebeiz | RF MEMS switches and switch circuits[END_REF][START_REF] Yao | RF MEMS from a device perspective[END_REF][START_REF] Larson | Microwave MEMS technology for nextgeneration wireless communications[END_REF], electrostatic actuation RF MEMS switch shows limited performances like high switching time, costly packaging, and a high actuation voltage. In this way, four possible configurations of switch are investigated to reach the objective of low actuation voltage. Development of RF MEMS switches for 2-5 GHz frequency range applications needs to consider several issues : microwaves requirements (isolation, losses, frequency bandwidth), the electromechanical behavior (actuation voltage, switching time), the options and limitations offered by the technological process in terms of mechanical optimization and material properties, the deformation induced by initial stress relaxation, the ability for the switch to withstand temperature variation and RF power, and the available area. Thus the design of an electromechanical switch becomes rapidly complex. Our study is focused on the feasibility of low stiffness gold microstructure to reduce the actuation voltage. A great importance is attached to the effect of design parameters on RF MEMS switches performances. To overcome the problem related to capacitive contact and to optimize the metal/metal contact properties, new design of serial and shunt switch have been performed with DC contact. The overall goal is to decrease the contact resistance in order to increase the isolation or reduce insertion losses. The design parameters optimization is critical to achieve the desired contact performances. The four switch parameters (actuation voltage, displacement, restoring and contact forces) are tightly coupled to each other, which makes good switch design very difficult. On figure 1, it is shown how the contact material and the design parameters control the active forces and the contact performance of the switch, expressed in the contact resistance and in the contact reliability. A low contact resistance requires a high contact force, and a high restoring force is desired to improve the contact reliability in preventing contact stiction problems. A high restoring force can be achieved by a relatively stiff structure and a large displacement, resulting in an increased electrostatic force to pull down the structure. Also, increasing the restoring force demands an increased electrostatic force to maintain the desired contact force, requiring a higher actuation voltage or a larger actuation electrode area. Thus, it is difficult to create a switch with a low actuation voltage for the electronic compatibility, small actuation electrode to reduce the size and cost of devices, large distance between electrodes for a high RF isolation, a high restoring force for the contact reliability and a high contact force for a low contact resistance. The optimization of the switch behavior is then a trade-off between microwaves and electromechanical parameters, while considering the feasibility of the technological process. On the basis of analytical models and electromechanical simulations, two series and shunt DC contact switches composed of a stressed evaporated/electroplated gold bilayer [START_REF] Yacine | Biaxial initial stress characterization of bilayer gold RF-switches[END_REF] have been designed with a 1.5µm height air gap in order to achieve an actuation voltage close to 5V. These designs are consistent with good RF performances.

II DESIGN AND FABRICATION

II.2 DESIGN DESCRIPTION AND FABRICATION

The design of serial DC switch and shunt DC switch are shown respectively in Figure 2 and in Figure 3. In series configuration, a 30 µm gap is created on the signal line to allow a high isolation in the up state. When a voltage is applied between the actuation electrode and the cantilever, a short circuit is generated between ends of signal line. The electrical contact between the bump and the line is a direct gold/gold contact. Gold, according to the literature is the contact material of first choice [1, 2, 3], due to its low hardness and low resistivity, which imply a low contact resistance, its relatively high melting point temperature for a soft material and its resistance to absorption of surface contaminants.

III RESULTS AND DISCUSSIONS

III.1 INITIAL DEFLECTION ANALYSIS

An example of cantilever profile is given in figure 5 and in figure 6 after sacrificial layer etching and CO 2 drying respectively for a serial and shunt DC switch. The photoresist sacrificial layer is spun on 3µm instead of 1.5µm to reduce technological problems. The results show that it is possible to obtain quite flat cantilever. However, the deflection is not very reproducible from cell to cell and it seems that the low level of bridge planarization induces this problem. In this example the cantilever height is about 6µm but again the reproducibility is not very good as we obtained values ranging from 4µm to 7µm. This problem was unexpected because the sacrificial photoresist layer was planned to be close to 3µm. This effect can be related to a sacrificial layer problem (default during spinning, bad planarization) or if there is a large deflection close to the anchorage. In order to confirm this assumption, we can observe in figure 5 and in figure 6 that there is a disturbed area close to the anchorage which can perhaps explain this issue. Cantilever deflections are difficult to control, but they can be reduced by suppressing the non flat area with efficient planarization and by strengthening the anchorage to reduce the rotation.

III.2 RF RESULTS

To evaluate the losses in the active part of the switches, calibration kits have been used to obtain the results between the calibration plane showed in figure 2 and figure 3. The measured losses in this calibration kits are quite high (0.7dB/mm at 10GHz) and higher than previous results obtained on high resistivity silicon (0.06dB/mm at 10GHz). These losses can be attributed to bad RCA cleaning of silicon before silicon passivation with silicon oxide.

Concerning the serial DC switch, the actuation voltage has been evaluated for several cantilevers between 10V and 20V. These values are consistent with the cantilever height ranging from 4µm to 7µm. However, at 10 V and 20 V, the force was not strong enough to ensure a high quality contact. Therefore 30V have been applied to insured the best contact performance. Between 1GHz and 5GHz the measured isolation is better than -35dB. As for the measured losses they are lower than 1dB. These losses originate mainly from the transmission line (550µm length) because of cleaning problem. If we subtract theses dielectric losses, the electrical model gives constant losses around 0.2dB in the 1GHz/5GHz range. These losses can be modeled with 1.1Ω contact resistance for each DC contact.

Disturbed area

Disturbed area

Concerning the shunt DC switch, between 1GHz and 5GHz the measured losses are lower than 0.8dB and are related to surface states associated with a cleaning problem as mentioned before. Without this effect, the losses would be lower than 0.1dB as shown by simulation curve. In the down state, the measured isolation is better than -12dB. It has to be noticed that there is a significant difference with the value expected from simulation (-40dB). This effect can be related to the distance between the two cantilevers (280µm) which allows the propagation of a common mode. This effect could be drastically reduced by minimizing this distance or by using complete symmetric bridge. The electrical model shows that with symmetric cantilever it will be possible to obtain isolation better than -25dB for a contact resistance equal to 1.3Ω.

III.3 ANALYSIS OF ACTUATION VOLTAGE EFFECT ON RF PERFORMANCES

In order to evaluate the effect of applied voltage on RF performances we simulated the contact force versus the applied voltage for a typical cantilever used both in serial and shunt DC switch. The results are illustrated on figure 7 for three cantilever height (1.5, 3 and 5µm). The graph shows that it is desired to use a voltage higher than pull down voltage to increase the contact forces. It will be possible with 1.5µm cantilever height to obtain 10µN contact forces for 6V. This contact force will be enough if we see the literature to obtain contact resistance lower than 2Ω.

IV CONCLUSIONS

Capacitive switch development is limited, due to the difficulty to control and reproduce high capacitance value in the down state. Therefore, new shunt and series DC contact switches have been designed and fabricated. First results show that good RF performances, with contact resistance lower than 1.3Ω, can be obtained with contact forces evaluated to 30µN. This contact force has been generated by 30V actuation voltage related to higher cantilever height than expected. The cantilever heights are between 4µm and 7µm instead of 3µm planned that shows that it will be possible to get device featuring actuation voltage in the 10 Volts range or even lower. With cantilever height around 1.5µm it will be then possible to obtain enough contact force with 6V applied voltage. Simulations show that it is possible to obtain quite flat microstructure using a stable industrial process with well control stress in the layers. This reproducibility is very difficult to reach in an academic research center but some structures show a good behavior and the feasibility has been demonstrated. These new designs of DC MEMS switch require for validation an evaluation of reliability, packaging and reproducibility.

Figure 1 .

 1 Figure 1. Illustration of the connection between the design parameters and the active forces in the switch model and how they affect its performance

Figure 2 .Figure 3 .

 23 Figure 2. Design of a DC series switch

Figure 4 .

 4 Figure 4. Low actuation voltage switch process flow description

Figure 5 .

 5 Figure 5. Optical profilometer measurement for a DC serie switch after release

Figure 6 .

 6 Figure 6. Optical profilometer measurement for a DC shunt switch after release

Figure 7 .

 7 Figure 7. Contact resistance as a function of applied voltage for different gap height


Charlotte Récapet
email: charlotte.recapet@normalesup.org

Lise Dauphin

Lisa Jacquin

Julien Gasparini

Anne-Caroline Prevot-Julliard

Anne-Caroline Prévot-Julliard

Eumelanin-based colouration reflects local survival of juvenile feral pigeons in an urban pigeon house

Keywords: Columba livia, feral pigeon, melanin-based colouration, juvenile survival, natal dispersal, colour polymorphism, urban ecosystems

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The generalized urban sprawl in the 20 th century is responsible for the replacement of rural or natural habitats by new urban ecosystems [START_REF] Berry | Urbanization[END_REF], which are highly dependent on human-driven processes [START_REF] Pickett | A Conceptual framework for the study of human ecosystems in urban areas[END_REF][START_REF] Grimm | Integrated approaches to long-term studies of urban ecological systems[END_REF][START_REF] Shochat | From Patterns to emerging processes in mechanistic urban ecology[END_REF]. As a consequence, many species have declined in urban habitats, whereas others now successfully exploit these ecosystems [START_REF] Kark | Living in the city: Can anyone become an 'urban exploiter'?[END_REF]). Consequently, species richness and assemblages are highly variable along urbanisation gradients [START_REF] Clergeau | Bird abundance and diversity along an urban-rural gradient: a comparative study between two cities on different continents[END_REF][START_REF] Germaine | Relationships among breeding birds, habitat and residential development in greater Tucson, Arizona[END_REF][START_REF] Blair | The Effects of urban sprawl on birds at multiple levels of biological organization[END_REF]). Moreover, animal communities exploiting urban habitats exhibit different characteristics compared to their rural counterparts, both at the species [START_REF] Møller | Successful city dwellers: a comparative study of the ecological characteristics of urban birds in the western Palearctic[END_REF] and at the population level [START_REF] Luniak | Synurbization -Adaptation of animal wildlife to urban development[END_REF][START_REF] Evans | Effects of urbanisation on disease prevalence and age structure in Blackbird Turdus merula populations[END_REF]. For instance, urban populations usually have a longer reproductive season [START_REF] Blair | The Effects of urban sprawl on birds at multiple levels of biological organization[END_REF][START_REF] Partecke | Differences in the timing of reproduction between urban and forest European Blackbirds (Turdus merula): Result of phenotypic flexibility or genetic differences?[END_REF]) and higher immune capacity [START_REF] French | Variation in stress and innate immunity in the Tree Lizard (Urosaurus ornatus) across an urban-rural gradient[END_REF], Fokidis et al. 2008), but lower anti-predatory defences [START_REF] Blair | The Effects of urban sprawl on birds at multiple levels of biological organization[END_REF][START_REF] Møller | Successful city dwellers: a comparative study of the ecological characteristics of urban birds in the western Palearctic[END_REF]) compared to rural populations of the same species (reviewed in [START_REF] Evans | Effects of urbanisation on disease prevalence and age structure in Blackbird Turdus merula populations[END_REF]). In addition, urban and rural populations show strong divergence in behavioural traits. For instance, individuals living in urban areas are more tolerant to human presence [START_REF] Møller | Interspecific variation in fear responses predicts urbanization in birds[END_REF], have higher innovation capacity or live in higher density populations compared to rural ones [START_REF] Erz | Populationsökologische Untersuchungen an der Avifauna zweier norddeutscher Großstädte[END_REF][START_REF] Luniak | Synurbization -Adaptation of animal wildlife to urban development[END_REF][START_REF] Møller | Successful city dwellers: a comparative study of the ecological characteristics of urban birds in the western Palearctic[END_REF]. [START_REF] Luniak | Synurbization -Adaptation of animal wildlife to urban development[END_REF] suggested that directional selection in urban areas could explain such phenotypic divergence on several physiological, morphological and behavioural traits between rural and urban individuals, resulting in a socalled "urbanization syndrome" and in local adaptation to urban areas [START_REF] Evans | Effects of urbanisation on disease prevalence and age structure in Blackbird Turdus merula populations[END_REF].

However, empirical studies investigating the evolutionary processes underpinning phenotypic divergence between urban and rural animal populations are still scarce [START_REF] Shochat | From Patterns to emerging processes in mechanistic urban ecology[END_REF][START_REF] Evans | Effects of urbanisation on disease prevalence and age structure in Blackbird Turdus merula populations[END_REF].

Melanin-based colouration is a good candidate trait to investigate urbanization syndrome, as it has a strong genetic basis [START_REF] Roulin | The Evolution, maintenance and adaptive function of genetic colour polymorphism in birds[END_REF]), and a number of studies have reported differences in animal melanin-based colouration between urban and rural areas [START_REF] Obukhova | Geographic variation of colour in the synanthropic Blue Rock Pigeon[END_REF][START_REF] Yeh | Rapid evolution of a sexually selected trait following population establishment in a novel area[END_REF]). The degree of melanin-based colouration in vertebrates has been frequently reported to co-vary with other phenotypic traits such as immunity, behaviour or reproduction (reviewed in [START_REF] Ducrest | Pleiotropy in the melanocortin system, colouration and behavioural syndromes[END_REF], suggesting that melanin-based colouration reflects adaptations to different environmental conditions [START_REF] Roulin | The Evolution, maintenance and adaptive function of genetic colour polymorphism in birds[END_REF]. For instance, a darker eumelanic colouration is often associated with higher aggressiveness (Reyer et al. 1998, Quesada and[START_REF] Quesada | The role of melanin-and carotenoid-based plumage coloration in nest defence in the great tit[END_REF], a lower response to stress (Johnston andJaniga 1995, Almasi et al. 2010), a higher parasite resistance [START_REF] Roulin | Female Barn owls (Tyto alba) advertise good genes[END_REF][START_REF] Jacquin | Melanin-based colouration is related to parasite intensity and cellular immune response in an urban free-living bird: the feral Pigeon Columba livia[END_REF], and a higher reproductive rate under food limitation compared to paler ones [START_REF] Roulin | Melanin-based colorations signal strategies to cope with poor and rich environments[END_REF][START_REF] Jacquin | Melanin-based colouration reflects alternative strategies to cope with food limitation in Pigeons[END_REF]). Such patterns have been suggested to be due to pleiotropic effects of genes coding for melanocortin ligands or receptors [START_REF] Ducrest | Pleiotropy in the melanocortin system, colouration and behavioural syndromes[END_REF]. As traits associated with a darker colouration match those found in individuals exploiting urban and/or stressful areas, melaninbased colouration might reflect an adaptation to the urban environment (Johnston andJaniga 1995, Jacquin 2011). As a consequence, one could expect directional selection to act on this trait and fitness of dark individuals should be higher than that of pale individuals in urban areas, a difference that could stem from differences in survival, reproductive success and habitat choice. Such differences in survival and/or dispersal according to colouration have already been described in birds [START_REF] Roulin | Breeding rate is associated with pheomelanism in male and with eumelanism in female barn owls[END_REF], Roulin et al. 2010[START_REF] Van Den Brink | Melanin-based coloration predicts natal dispersal in the barn owl, Tyto alba[END_REF][START_REF] Saino | Viability is associated with melanin-based coloration in the Barn Swallow (Hirundo rustica)[END_REF].

In this study, we tested whether survival and/or dispersal differed according to melanin-based colouration in the urban feral pigeon Columba livia. Feral pigeons, originating from domesticated forms of the wild Rock Pigeon, have successfully colonized most cities of the world and have become one of the most common urban species (Haag-Wackernagel 1998, [START_REF] Kark | Living in the city: Can anyone become an 'urban exploiter'?[END_REF]. Interestingly, part of the colour variation selected for aesthetic purposes during the domestication process is maintained today in feral populations. This variation is strongly heritable (Jacquin et al. 2013a). Several explanations have been proposed such as differential adaptation to urban conditions [START_REF] Johnston | Feral Pigeons[END_REF], parasites [START_REF] Jacquin | Coloration mélanique et stratégies d'histoire de vie chez le pigeon biset urbain[END_REF], Jacquin et al. 2013, Jacquin et al. in press) or food availability [START_REF] Jacquin | Melanin-based colouration reflects alternative strategies to cope with food limitation in Pigeons[END_REF].

However the link between colouration and fitness remains elusive [START_REF] Leiss | Gefiederfärbungen bei der Stragentaube (Columba livia)[END_REF]. A survey conducted at 192 European locations showed a positive correlation between the urbanization level and the proportion of dark eumelanic phenotypes [START_REF] Obukhova | Polymorphism and phene geography of the Blue Rock Pigeon in Europe[END_REF]. The same correlation was found within the urban areas of Moscow [START_REF] Obukhova | Dynamics of balanced polymorphism morphs in Blue Rock Pigeon Columbia livia[END_REF], Paris [START_REF] Jacquin | Coloration mélanique et stratégies d'histoire de vie chez le pigeon biset urbain[END_REF], Jacquin et al. 2013b) and Manchester (S. L. Johnson, unpublished results). Such results suggest differences in fitness and/or habitat choice between dark and pale pigeons in urban areas. They are thus in agreement with our hypothesis that melanin-based colouration reflects an adaptation to the urban environment or to stressful habitats in general [START_REF] Roulin | Melanin-based colorations signal strategies to cope with poor and rich environments[END_REF]).

Local survival probability, i.e., the probability to stay both alive and present in the sampled population, is a proxy of individual survival and/or dispersal. In this paper, we estimated local survival of feral pigeons roosting in two managed pigeon houses in urban areas near Paris, France, through a mark-recapture survey. We captured differently coloured pigeons at different ages (juveniles and adults) and estimated monthly local survival probabilities according to age, colour and number of chicks born in the pigeon houses. If a darker melanin-based colouration reflects adaptation to the urban environment, we predict that local survival would be higher in dark than in pale individuals.

Material and methods

Study sites

The study was conducted between 2007 and 2009 in two pigeon houses near Paris, in Pantin (Seine-Saint-Denis, France) and Fontenay (Val-de-Marne, France). The pigeon houses were built in 2003 and 1999, respectively, and had been freely colonized by feral pigeons within one year. Microsatellite data have shown that there is no genetic differentiation between pigeon flocks sampled within and around Paris (G. Jacob, A.C.Prévot-Julliard and E. Baudry, unpublished data). The sampled populations were about 7 km apart and should thus be genetically similar. These locations are both highly urbanized areas, measured as the proportion of built surfaces within 1 km of the pigeon houses (92.4% in Fontenay and 85.2% in Pantin; Jacquin et al. 2013b). In these public pigeon houses, pigeons were fed and eggs were removed in order to control reproduction [START_REF] Jacquin | Reproduction management affects breeding ecology and reproduction costs in feral urban pigeons (Columba livia). -Can[END_REF]. The two pigeon houses displayed the same density and size of available nest boxes and the same amount of food was provided in both pigeon houses (i.e. 50 kg of seeds once a week) during the study period.

Feral pigeons lay 1 to 6 clutches of two eggs per year [START_REF] Johnston | Feral Pigeons[END_REF]. In Pantin, all newly laid eggs were removed in 2007, then one egg per clutch was removed in 2008 and 2009 (Table 1). In Fontenay, no eggs were removed in 2007 and 2008, then one egg per clutch was removed in 2009 (Table 1). These management procedures have been shown to cause changes in the reproduction phenology in these populations [START_REF] Jacquin | Reproduction management affects breeding ecology and reproduction costs in feral urban pigeons (Columba livia). -Can[END_REF]). In addition, they resulted in strong differences in the number of chicks born (Table 1) and juveniles fledged (Table 2). We thus included the number of chicks born in our analyses to take these differences into account. no data were available for some months, at different times in the two sites (Table 1). We therefore analysed the data from the two sites separately. We let every individual's capturerecapture history start with the first transponder record of the individual. Therefore, juveniles were only surveyed from the first time that they left the pigeon house (i.e. when approximately one-month old), and survival during their first month was therefore not modelled.

Colouration measurement

Melanin-based colourations in feral pigeons are due to two types of pigment: pheomelanic pigments which are responsible for red colouration and eumelanic pigments, which are responsible for black and grey colouration [START_REF] Haase | Melanin concentrations in feathers from wild and domestic Pigeons[END_REF]. In this study we focused on eumelanin-based colouration, which is the most widespread colouration in feral pigeon populations, and thus excluded reddish and white individuals from the analyses (N = 22 and N = 6 respectively). Eumelanin deposition in feathers can vary in intensity, from total absence of pigmentation to fully dark melanic colouration [START_REF] Haase | Melanin concentrations in feathers from wild and domestic Pigeons[END_REF]). Although pheomelanin is present in "greyish" pigeons, no correlation with the intensity of eumelanin deposition could be found [START_REF] Haase | Melanin concentrations in feathers from wild and domestic Pigeons[END_REF]. We divided the continuous variation in four groups, following the classification used by [START_REF] Johnston | Feral Pigeons[END_REF] and [START_REF] Jacquin | Melanin-based colouration is related to parasite intensity and cellular immune response in an urban free-living bird: the feral Pigeon Columba livia[END_REF]: (1) Blue bar (grey mantle with two black wing bars i.e. wild type); (2) Checker (a checked mantle with moderate black spots); (3) T-pattern (a black mantle with small grey marks); and (4) Spread (a completely black plumage). This classification reflects genetic differences at two multiallelic loci (Johnston andJaniga 1995, Haag-Wackernagel et al. 2006), thus defining colour morphs [START_REF] Roulin | The Evolution, maintenance and adaptive function of genetic colour polymorphism in birds[END_REF]). Although juveniles are slightly darker before their first moult, the morph of an individual remains the same throughout life [START_REF] Johnston | Feral Pigeons[END_REF], an observation that was confirmed by photographs taken at different ages in captivity (L. Jacquin, pers. obs.). Previous studies have shown that eye scoring is a reliable method to estimate the colouration intensity of adult feral pigeons: it is highly repeatable and correlated with the percentage of dark feathers on the wing measured on standardized photographs [START_REF] Jacquin | Melanin-based colouration is related to parasite intensity and cellular immune response in an urban free-living bird: the feral Pigeon Columba livia[END_REF].

Local survival analyses

To analyse recapture histories, we used Mark-Recapture statistical methodology to estimate two kinds of parameters: local survival probabilities Φ and recording probabilities p [START_REF] Lebreton | Modeling survival and testing biological hypotheses using marked animals: a unified approach with case studies[END_REF]). Recording probability is the probability for an individual alive and present to be recorded at a particular time point. Local survival probability is the probability to be still alive and present on the study site between two time points (thus referring to both survival and dispersal). In our study, these parameters were estimated on a monthly scale.

Adult and juvenile recording histories within each site were analysed separately, as a function of age (in months for juveniles) and plumage colouration score. To get a sufficient sample size within each colouration score, we pooled morphs 3 and 4 in adults and morphs 2, 3 and 4 in juveniles (Table 2). Separating these groups did not improve the models indicating that there was either no difference or not enough power to detect a difference between these groups. It should also be noted that pooling morphs 2, 3 and 4 in adults did not change our results. Juvenile mortality occurs mostly before the age of six months [START_REF] Johnston | Feral Pigeons[END_REF], and preliminary studies showed that juveniles older than five months have similar survival rate as adults (unpublished results). We thus tested six different age structures for each juvenile local survival model: age was described by a number of age-classes varying from one (no effect of age) to six (age effect up to six months). For example, five-month old chicks could either be considered as a distinct age class (six age-classes model) or be pooled with older individuals (one to five age-classes models).

We modelled the time-dependence in local survival by testing the effects of month, year and the interaction between the two. Since only one chick was born in 2007 in Pantin, it was not possible to estimate local survival rates and that year was excluded from the analysis for juveniles. Based on the results in [START_REF] Hetmański | Dispersion asymmetry within a feral Pigeon Columba livia population[END_REF], we included in our modelling the effect of juvenile density on local survival. We modelled the month by year effect on survival either as a categorical factor or as a numerical variable dependent on the number of chicks born during each month. In Fontenay where three years of data were available, we similarly modelled the year effect either as a categorical factor or as the total number of chicks born during each year.

The time-dependent model Φt pt for each colour-group was first tested for goodnessof-fit [START_REF] Pollock | Goodness-of-fit tests for open capture-recapture models[END_REF] in U-CARE 2.3.2 (Choquet et al. 2009a). A transience effect was detected in adults from both sites (Pantin: z = 2.53, P = 0.01; Fontenay: z = 3.80, P = 0.001), which means that local survival rate of newly marked individuals differed from that of previously marked ones. It could be explained either by the occurrence of 'transient' individuals that are only present during a short time in the study population, by an effect of marking on survival or dispersal, or by a heterogeneity in individual recording probabilities.

We added one virtual age-class when modelling local survival probabilities to take this effect into account [START_REF] Brownie | Estimation of time-specific survival rates from tag-resighting samples: a generalization of the Jolly-Seber model[END_REF]. Trap-dependence was significant for all site-bystage categories except Pantin's juveniles (Fontenay: adults z = -10.28, P < 0.001; juveniles z = -8.24, P < 0.001; Pantin: adults z = -11.58, P < 0.001; juveniles z = -0.97, P = 0.33). To take this effect into account, we modelled different recording probabilities according to the previous recording event for each individual in a multi-events model [START_REF] Pradel | Flexibility in survival analysis from recapture data: Handling trap-dependence[END_REF]).

We therefore defined three states (alive and recorded, alive but not recorded, dead), and two events (recorded, not recorded). Two transition matrices were defined: the first one allowed us to estimate local survival probabilities, independently from the previous recording event, and the second allowed us to estimate recording probabilities, as a function of the previous recording event for each individual. We were not able to estimate separately the probability of using the pigeon house when alive probability of being recorded when using the pigeon house. The event matrix E was therefore set to constant.

We conducted model selection to investigate the factors explaining local survival. We tested for the effect of age (for juvenile), time (year and month), colour score, and trapdependence effect on recording probability. All interactions (up to four-way interactions) were tested alongside the corresponding main effects. Model selection was conducted according to the procedure used in [START_REF] Lebreton | Modeling survival and testing biological hypotheses using marked animals: a unified approach with case studies[END_REF] using E-SURGE 1.7.1 (Choquet et al. 2009b). Local survival and recording probabilities were estimated using the maximum likelihood criterion. We first modelled recording probabilities (p), keeping the complete model for local survival probabilities (Φ). Once the best model was obtained for p, we adjusted the model for Φ. Models were compared using Akaike information criterion (AIC, Akaike 1981): models with lower AIC values are better supported by the data. When the difference between two AIC was less than two, the model with fewer parameters was preferred. AIC weights were calculated as they express the likelihood of the models relative to all the alternative models [START_REF] Burnham | Model selection and multimodel inference: a practical informationtheoretic approach[END_REF]. The beta parameters, and their significance levels following a Gaussian distribution, were estimated for each linear covariate and each level of categorical variables (Choquet et al. 2009b).

Results

Colour scores repartition

The proportion of darker individuals was higher in adults than in juveniles in both pigeon houses (Pearson Chi-squared test; Fontenay: χ 2 = 17.2, p = 0.0002; Pantin: χ 2 = 5.92, p = 0.05): 1.5% of the juveniles in Fontenay and 8.5% in Pantin belonged to the darker morphs "T-pattern" and "Spread", compared to respectively 16.1% and 15.5% of the adults. There was a decrease over age in the proportion of intermediate "Checker" individuals in both pigeon houses, but no consistent trend for the paler morph "Blue Bar" (Table 2). The distribution of colour groups was similar between houses in adults (χ 2 = 1.49, p = 0.47) but not in juveniles: more dark ("T-pattern" and "Spread") and less pale ("Blue Bar") juveniles were observed in Pantin than in Fontenay (Table 2, Pearson Chi-squared test with p-values computed by Monte Carlo simulations: χ 2 = 7.78, p = 0.022).

Local survival of adults

Estimates of monthly adult local survival were similar between Fontenay (mean = 0.984; range = 0.937 -1) and Pantin (mean = 0.990; range = 0.937 -1). They only depended on month (in Pantin and Fontenay) and on year (in Fontenay), with no noticeable trend. The monthly or yearly number of chicks did not affect these parameters (Tables 3 and6). In addition, adult local survival was not explained by plumage colouration (Tables 3 and6).

Local survival of juveniles in Fontenay

Juvenile local survival in Fontenay differed between months and increased with age until five months old (best model with five age-classes; Table 4). It was also negatively correlated with the yearly number of chicks (Tables 4 and6, Fig. 1). Moreover, juvenile local survival in Fontenay was linked to colouration, with pale juveniles ("Blue Bar") having a lower local survival than darker juveniles ("Checker", "T-pattern" and "Spread"; Tables 4 and6, Fig. 1).

Local survival of juveniles in Pantin

In Pantin, juvenile local survival was month-dependent and increased with age until three months old (best model with three age-classes; Table 4). However, there was no difference between colour morphs (Tables 4 and6). We did not test for the effect of yearly number of chicks, since we only had data for two years.

Discussion

The aim of the study was to compare local survival between differently coloured free-living feral pigeons. An increase in the proportion of darker individuals between juveniles and adults was noticed in both pigeon houses. Such differences have already been observed in Vienna [START_REF] Haag-Wackernagel | Phenotype-dependent selection of juvenile urban feral Pigeons Columba livia[END_REF]) and in different populations in and around Paris (Jacquin et al. 2013b), using a similar visual scoring protocol. These observations suggest directional selection at the juvenile stage, but could be flawed if detectability varies according to colouration [START_REF] Gimenez | The Risk of flawed inference in evolutionary studies when detectability is less than one[END_REF].

Therefore, we estimated local survival in two pigeon houses using a mark-recapture protocol. Contrary to what we predicted, colouration was not related to local survival in adults. However, we found a relationship for juveniles: pale eumelanic juveniles had a lower monthly local survival probability than darker eumelanic ones. This relationship however was restricted to only one pigeon house (Fontenay). Thus, darker eumelanic juveniles could have a selective advantage compared to paler juveniles in some environmental contexts, which would explain the higher proportion of dark adult individuals in urban populations of feral pigeons compared to more rural ones. Interestingly, a long term study of barn owls Tyto alba also found a higher local survival of dark individuals in juveniles but not in adults (Roulin et al. 2010). Although other mechanisms (e.g. annual reproductive success) could generate fitness differences later in life, it thus seems that differences of survival and/or dispersal according to colouration are mainly expressed at the juvenile stage.

Adult local survival was similar between the two studied populations and was not linked to the number of chicks born. In contrast, local survival of juveniles was negatively correlated to the yearly number of chicks born in Fontenay, which varied according to egg removal management. Despite not being able to test this effect for juveniles in Pantin (due to having only two years of data), the observed decrease in local survival between 2008 and 2009 (Table 5) still coincided with an increase in the number of chicks. The observed variation in juvenile local survival between years may thus have resulted directly from variation in reproductive output due to chicks removal rather than from variation in environmental conditions. This contrasted response between adult and juveniles is consistent with previous studies that found juveniles to be more sensitive to environmental variation, in terms of survival [START_REF] Gaillard | Temporal variation in fitness components and population dynamics of large herbivores[END_REF] or dispersal [START_REF] Schreiber | Dispersal and survival rates of adult and juvenile Red-tailed tropicbirds (Phaethon rubricauda) exposed to potential contaminants[END_REF]). Taken together with the difference in colouration-dependence between adults and juveniles, this suggest that juvenile and adult local survival vary quite independently in feral pigeons, similarly to what was observed for other bird species [START_REF] Altwegg | Age-specific fitness components and their temporal variation in the Barn Owl[END_REF]).

The human-driven variation in chick number is likely to have induced changes in competition for food, which may impact juvenile mortality. Indeed, juveniles are less efficient foragers than adults, which makes them more sensitive to competition for food [START_REF] Sol | Competition for food in urban Pigeons: The Cost of being juvenile[END_REF]. A possible scenario is that when egg removal rate is high (i.e. before 2009 in Pantin, in 2009 in Fontenay), the number of chicks is lower, and juveniles are likely to experience a lower competition with other juveniles and/or with adults feeding them. Reduced competition could then act positively on local survival through lower mortality. Such effect could also be driven by differential natal dispersal rate. Indeed, juveniles tend to disperse towards less dense populations when densities on their natal site are high [START_REF] Hetmański | Dispersion asymmetry within a feral Pigeon Columba livia population[END_REF].

Several underlying mechanisms could explain the difference in survival and/or dispersal between differently coloured juveniles. Indeed, melanin-based colouration intensity has been shown to be associated with various life-history traits such as behaviour, reproduction and parasite resistance in pigeons [START_REF] Johnson | A multifactorial study of variation in interclutch interval and annual reproductive success in the feral Pigeon, Columba livia[END_REF][START_REF] Johnston | Feral Pigeons[END_REF], Jacquin et al. 2011, 2012) and in other species [START_REF] Moreno | Are melanin ornaments signals of anti-oxidant and immune capacity in birds?[END_REF][START_REF] Ducrest | Pleiotropy in the melanocortin system, colouration and behavioural syndromes[END_REF], McKinnon and Pierrotti 2010). Different factors could thus act advantageously for dark coloured juvenile pigeons in one of our population. For instance, differential selection on pale and dark juveniles could be due to differences in competitive abilities. Darker individuals are known to be more aggressive and to have better competitive abilities for access to food resources [START_REF] Senar | Bird colours as intrasexual signals of aggression and dominance[END_REF]. Parasitism could also play a role in linking survival and colouration [START_REF] Piault | Pheomelanin-based coloration and the ability to cope with variation in food supply and parasitism[END_REF], because dark feral pigeons are known to develop stronger immune responses, to be more resistant to blood parasites [START_REF] Jacquin | Melanin-based colouration is related to parasite intensity and cellular immune response in an urban free-living bird: the feral Pigeon Columba livia[END_REF]) and to transmit more antibodies to their eggs (Jacquin et al. 2013a). As parasite transmission is generally higher in denser host populations [START_REF] Arneberg | An ecological law and its macroecological consequences as revealed by studies of relationships between host densities and parasite prevalence[END_REF], darker eumelanic juveniles could have a selective advantage in dense populations. Selection could thus act directly through a higher survival and/or a lower dispersal of darker juveniles in dense populations. In agreement with this hypothesis, juvenile local survival differed according to colouration in Fontenay but not in Pantin, and competition is likely to be higher in Fontenay compared to Pantin due to lower reproductive control and a higher number of chicks born (Table 1). Moreover, differences in local survival between pale and dark juveniles in Fontenay were more pronounced in 2007 and 2008, when the number of chicks born was high, compared to 2009, where the number of chicks was lower (Table 3, Fig 1). An alternative explanation could be the higher urbanisation rate in Fontenay compared to Pantin, resp. 92.4% and 85.2% (Jacquin et al. 2013b). However the effect of urbanisation could only be tested by studying several sites in a wide range of urbanisation rates. In birds, colouration-by-environment interactions have already been found on chicks' condition and growth, taken as proxies for survival prospect [START_REF] Roulin | Melanin-based colorations signal strategies to cope with poor and rich environments[END_REF][START_REF] Piault | Pheomelanin-based coloration and the ability to cope with variation in food supply and parasitism[END_REF]. However a colouration-byenvironment interaction on dispersal has only been described in insects [START_REF] Ahnesjö | Differential habitat selection by Pygmy Grasshopper colour morphs, interactive effects of temperature and predator avoidance[END_REF].

Our results suggest that eumelanin-based colouration is under directional survival selection in dense populations of highly urbanized areas, and that such selection occurs through differential survival or dispersal rate of juveniles. Interactions between melanin-based colouration and population densities could explain the observed differences in local survival.

Consistently, [START_REF] Obukhova | Dynamics of balanced polymorphism morphs in Blue Rock Pigeon Columbia livia[END_REF] found that proportions of darker morphs were positively correlated with pigeon densities both in space and time. Darker eumelanic colouration could thus reflect an adaptation to high population density, which could entail a selective advantage in more urbanized areas. Indeed, densely urbanized areas are associated with higher densities of pigeon populations compared to more rural ones [START_REF] Johnston | Feral Pigeons[END_REF], and stronger competition for food and changes in parasite exposure has often been invoked as a central selective factor in such densely colonized urban areas (Shochat et al. 2004, Bradley andAltizer 2004).

To conclude, this study provides the first demonstration of colouration-dependent selection in juvenile feral pigeons taking into account detection probabilities. It contributes to the growing body of evidence that darker eumelanic individuals have a higher survival and/or a lower dispersal than paler ones, especially at the juvenile stage and in stressful environments [START_REF] Roulin | Melanin-based colorations signal strategies to cope with poor and rich environments[END_REF][START_REF] Piault | Pheomelanin-based coloration and the ability to cope with variation in food supply and parasitism[END_REF]. The observed differences in local survival according to colouration intensity were indeed context-dependent, linked potentially to the degree of competition and/or parasite exposure in more or less dense populations.

Although our results are consistent with such density-dependent effects, the present study does not provide enough data to test them in full. Moreover, it was conducted in populations exploiting pigeon houses, so conditions might be quite different from those experienced by free-living populations. A current survey of colour-ringed pigeons from free-living populations and pigeon houses in various sites of Paris area may yield interesting information on the environmental correlates of colouration-dependent local survival and on the relative contributions of mortality and dispersal.

J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O

  Capture and survey procedureAdult pigeons, which feed or sleep in the pigeon houses, were captured twice a year with a net tunnel placed in front of the entrance of the structure. Between 2007 and 2009, reproduction in both pigeon houses was monitored every week and juveniles born in the pigeon houses were ringed when 15-day old, about two weeks before leaving the nest. All individuals were equipped with a transponder included in one of the plastic rings. Individuals were detected by a receptor placed at the entrance of the pigeon house. Recording was conducted from August 2007 to December 2009 in Pantin (269 individuals surveyed) and from February 2007 to December 2009 in Fontenay (288 individuals surveyed). These recordings were then pooled to obtain monthly presence data. Due to the failure of receptors,

Figure 1 :

 1 Figure 1: Monthly mortality rate of one-month old juveniles in Fontenay as a function of year and colouration ("Pale": Blue Bar, "Dark": 25

Table 1 :

 1 Description of the two pigeon houses surveyed: egg-removal protocol (0, 1 or 2 eggs removed per clutch), total number of chicks born 1

	2	per year, and months during which recording took place (in grey).
		Year	2007	2008	2009
		Month

Table 2 :

 2 Number of birds included in the survey. All were recorded at least once after ringing. The percentage of each colour morph relative to 4 the number of individuals of the same age and site are given in brackets. 5

	N D

Table 3 :

 3 Model selection for adult local survival (selected models in bold). All models with an AIC weight higher than 1% are reported. In 6 Pantin, recapture probabilities are set as year-dependent and include trap-dependence. In Fontenay, recapture probabilities are set as fully time-7 dependent (month x year) and include trap-dependence. Survival probabilities all include a transience-effect (see methods).

	Models with

Table 4 :

 4 Model selection for juvenile local survival (selected models in bold). For each model, only the model with the best age structure is 11 reported. Only models with an AIC weight higher than 2% are reported. In Pantin, recapture probability is modelled as age-dependent up to 4 12 months. In Fontenay, recapture probability is dependent on year and age (up to 4 months), with trap-dependence. Models with interactions 13 include the corresponding main effects. 14

	Site	Modelled effects on juvenile local survival	Parameters Deviance	AIC	AIC weight
	Pantin	Month + Age up to 3 months	20	291.0	331.0	0.095
		Year + Month + Age up to 3 months	21	289.9	331.9	0.061
		Colour + Month + Age to 3 months	21	290.3	332.3	0.051
		Colour × Age up to 3 months + Year + Month	24	284.1	332.1	0.055
		Colour + Year + Month + Age up to 3 months	22	289.3	333.3	0.031
		(Year + Age up to 3 months) × Colour + Month	25	283.5	333.5	0.027
	Fontenay Colour + Year + Month + Age up to 5 months	28	648.5	704.5	0.256

Colour + Yearly number of chicks + Month + Age up to 5 months 27 651.3 705.3 0.166

	Colour + Yearly number of chicks × Age up to 5 months + Month	27	653.9	707.9	0.045
	Colour + Yearly number of chicks + Age up to 5 months	15	678.0	708.0	0.043
	Colour + Month + Age up to 5 months	26	656.2	708.2	0.040
	Colour × Age up to 6 months + Year + Month	34	640.3	708.3	0.038
	Colour × Age up to 6 months + Yearly number of chicks + Month	33	643.5	709.5	0.021

Acknowledgments -All experiments and protocols were approved and authorized by the veterinary department of Seine-et-Marne (authorization N°77-05). We are grateful to the AERHO association, and especially to Catherine Dehay and Pascale Beauvois, for conducting the captures and collecting data, and to Maryline Berthaud and Romain Lorrillière for computing the data before analysis. We would like to thank three anonymous reviewers for their comments on a previous version of this manuscript. This work was supported by grants from the Region Ile-de-France (Sustainable Development Network R2DS N° 2008-07). C. Récapet was supported by grants from the French Ministry of Research. L. Jacquin was supported by the French Ministry of Research and the Fyssen foundation. L. Dauphin was supported by an ATM program in the French National Museum of Natural History.