Mylène Dutour
email: mylene.dutour@hotmail.com

Marion Cordonnier

Jean-Paul Léna

Thierry Lengagne

Seasonal variation in mobbing behaviour of passerine birds

Keywords: Bird calls, Interspecific recognition, Interspecific communication, Mobbing, Passerines, Paridae

When they detect a predator, many birds exhibit mobbing behaviour and produce mobbing calls that quickly draw other prey against the predator. Such antipredator strategy often involves several species and therefore implies heterospecific communication. As fledging and nestling stages could be particularly targeted by predators, a high mobbing intensity is to be expected during the breeding season. While recognizing other species' mobbing calls is critical to setting up this behaviour, to date, we have no information about the perception of these calls with regard to t season. Here, we used playbacks of mobbing calls to study the variation in response of the Great Tit (Parus major) and the Blue Tit (Cyanistes caeruleus) exposed to the mobbing calls of two heterospecific species, the Eurasian Nuthatch (Sitta europaea), and the Eurasian Wren (Troglodytes troglodytes). To investigate mobbing response seasonality, we conducted playback experiments during spring (breeding season) and autumn (non-breeding season). Contrary to most previous studies, we found that mobbing intensity was greater in autumn than in spring. Additionally, although neither Nuthatch nor Wren is related to the Tit family, we found that both Tit species responded more to the former than the latter species. At the heterospecific communication level, this study demonstrates a previously unsuspected level of complexity in the use of mobbing calls.

Introduction

The key role of anti-predation defences in predator-prey relationships has been studied for many years, particularly in birds. When birds encounter predators, many species are likely to emit alarm calls. These alarm calls may be used to alert conspecifics of potential danger [START_REF] Weary | Response of eastern chipmunks to conspecific alarm calls[END_REF] or recruit nearby individuals for mobbing defence against the predator [START_REF] Curio | The adaptive significance of avian mobbing[END_REF]. Mobbing calls typically incite other potential prey to fly towards the caller and the predator. This mobbing behaviour may have several different functions and one of the most important is probably to drive the predator away from the vicinity [START_REF] Pettifor | The effects of avian mobbing on a potential predator, the European kestrel, Falco tinnunculus[END_REF][START_REF] Flasskamp | The adaptive significance of avian mobbing. V. An experimental test of the 'move on' hypothesis[END_REF]. The greater the number of mobbers, the greater the chances of driving away a predator [START_REF] Picman | Antipredation role of clumped nesting by marsh-nesting red-winged blackbirds[END_REF][START_REF] Krams | Group effect in nest defence behaviour of breeding pied flycatchers, Ficedula hypoleuca[END_REF]). However, mobbing entails a real risk to the prey involved, owing to its proximity to the predator [START_REF] Denson | Owl predation on a mobbing crow[END_REF][START_REF] Curio | Predator harassment implies a real deadly risk: a reply to Hennessy[END_REF]. Preys adjust the strength of their mobbing behaviour according to the perceived risk associated with the predator's threat level [START_REF] Billings | Are chickadees good listeners? Antipredator responses to raptor vocalizations[END_REF][START_REF] Dutour | Mobbing behaviour varies according to predator dangerousness and occurrence[END_REF]Dutour et al. 2017a). Other factors, including the proximity of the predator [START_REF] Creswell | Escape responses by redshanks, Tringa totanus, on attack by avian predators[END_REF][START_REF] Kleindorfer | Avian nest defence behaviour: assessment in relation to predator distance and type, and nest height[END_REF], its posture [START_REF] Hamerstrom | The influence of a hawk's appetite on mobbing[END_REF][START_REF] Coss | Perceptual aspects of leopard recognition by wild bonnet macaques (Macaca radiata)[END_REF] and its behaviour [START_REF] Lind | Information, predation risk and foraging decisions during mobbing in Great Tits Parus major[END_REF][START_REF] Nolen | Asymmetries in mobbing behaviour and correlated intensity during predator mobbing by nuthatches, chickadees and titmice[END_REF] are also involved in explaining the strength of the mobbing. Among these, seasonality is a probably an important factor to explain mobbing behaviour intensity.

Although mobbing can occur in the autumn and winter months, many studies have shown that the mobbing response to predators is higher during the breeding season [START_REF] Altmann | Avian mobbing behavior and predator recognition[END_REF][START_REF] Shedd | Seasonal variation and function of mobbing and related antipredator behaviors of the American Robin (Turdus migratorius)[END_REF][START_REF] Shedd | Seasonal variation in mobbing intensity in the Black-capped Chickadee[END_REF][START_REF] Krams | Interspecific reciprocity explains mobbing behaviour of the breeding chaffinches, Fringilla coelebs[END_REF]. Authors have suggested that the willingness of birds to mob during the breeding season probably stems from at least two factors: (1) birds are territorial at this time and lack the option of easy relocation away from predators on their territories, and (2) birds are likely to have eggs or young more vulnerable to the predator [START_REF] Shedd | Seasonal variation and function of mobbing and related antipredator behaviors of the American Robin (Turdus migratorius)[END_REF]. Some studies have also documented a temporal intensification in mobbing behaviour during the breeding cycle. For example, parents increase the strength of this mobbing during the breeding cycle, whereas mobbing is rarely performed during the nest building or egg-laying phase [START_REF] Montgomerie | Risk and rewards of nest defence by parent birds[END_REF][START_REF] Redondo | Avian nest defence: theoretical models and evidence[END_REF]). Finally, an individual's response intensity to the mobbing playback is a significant predictor of reproductive success [START_REF] Doran | A test of the mobbing playback method for estimating bird reproductive success[END_REF], although one study showed that mobbing calls can increase the rates of nest predation and lower breeding success [START_REF] Krams | Long-lasting mobbing of the pied flycatcher increases the risk of nest predation[END_REF]). However, Dutour et al. (2017a) observed a reverse pattern when studying responses in birds communities to the Eurasian Pygmy Owl (Glaucidium passerinum), with a higher response rate in autumn than during the breeding season. Because the study of Dutour et al. (2017a) focused on a global response from all the species predated by the Pygmy Owl, any comparison between seasons on a species scale would be impossible. In addition, with such an experimental design it is not possible to discriminate between responses emitted towards predators and responses to interspecific mobbing calls. To our knowledge, the mobbing behaviour set up in response to interspecific calls has never been studied across seasons. Hence, there is a need for a pairwise comparison of caller-receiver species to explain their behavioural responses during different seasons.

Like other members of the Paridae family, the great tit (Parus major) and the blue tit (Cyanistes caeruleus) are highly vigilant and aggressive during mobbing events: they form mixed-species flocks and mob together (Dutour et al. 2017a). These interspecific flocks can constitute facilitators of mobbing [START_REF] Goodale | Alarm calling in Sri Lankan mixed-species bird flocks[END_REF][START_REF] Nolen | Asymmetries in mobbing behaviour and correlated intensity during predator mobbing by nuthatches, chickadees and titmice[END_REF]).

Here, we address one previously unexamined aspect of the mobbing behaviour of passerine birds. We examine to what extent responses to interspecific mobbing calls vary according to the season. We investigate the variation in response of the Great Tit and the Blue Tit exposed to the mobbing calls of the Eurasian Nuthatch (Sitta europaea) and the Eurasian Wren (Troglodytes troglodytes). We predict that the mobbing response should be more intense during the autumn than during spring.

Methods

Species and sites studied

The study was conducted in large mixed deciduous-coniferous forests near Lyon in the Rhône-Alpes region (France; 45°80'N, 4°52'E). To ensure the independence of experimental testing (no bird was tested twice during our study), playback sites were separated by more than 100 m and we never came back a second time to the same place. In addition, we avoided any temporal effect during our experiments by evenly distributing the playbacks of the different species across the study period. All tests were conducted in the breeding season (April-Jun 2016) and in the autumn (September-October 2016). Tests involved 140 different individuals (the detailed number of tests conducted for each combination is presented in Table 1).

Playback Experiments

Once a target bird was identified, we placed the loudspeaker used to broadcast the acoustic signal 30 m away from the bird at the base of a tree. We performed our playback experiments only when no other passerines were observed near the targeted individual, to be sure that the response of the tested bird was due to our playback rather than to the behaviour of other passerines. All tests, where an untargeted bird started to mob before the targeted bird, were discarded from the dataset. Once the loudspeaker was positioned, two observers with binoculars stood opposite each other at vantage points 15 m away from the loudspeaker to observe bird response during the test. All tests were divided into a 1 min baseline of silence, followed by 1 min of signal playback. We found no evidence that our presence disturbed the behaviour of the target bird before the playback emission. During the playback, we considered that the target bird responded positively to our test if it approached within a 15m radius of the loudspeaker (see Dutour et al. 2017b for more details).

Choice of experimental stimuli and playback materials

The mobbing calls produced by Wren and Nuthatch in response to a pygmy owl or uploaded from Xeno Canto (XC file hereafter) online database were played with a Shopinnov 20W loudspeaker (frequency response 100Hz-15kHz). To limit pseudo replication [START_REF] Hurlbert | Pseudoreplication and the design of ecological field experiments[END_REF][START_REF] Kroodsma | Pseudoreplication in playback experiments, revisited a decade later[END_REF], we used mobbing calls from 5 different individuals for the Wren (XC252499) and we used two soundtracks recorded in two populations for the Nuthatch (Germany XC252502 and Sweden XC28224).

Statistical analysis

Analyses were done using R v.2.15.1 software (R Development Core Team 2012). We used generalised linear mixed models (GLMM) to investigate the variation in mobbing propensity (i.e. speaker approach) among the receiver species (i.e. Great Tit or Blue Tit) and according to the mobbing stimuli broadcast (i.e. mobbing calls of Nuthatch and Wren) and the season (breeding season and autumn). More specifically, the individual binary response (mobbing response = 1; no mobbing response = 0) was introduced as dependent variable using a logit link and a binomial distribution for the error term, and the season, the receiver species, the mobbing stimuli and their interactive effects were introduced as explanatory terms in the fixed part of the model. As different playback soundtracks were used for each receiver species, soundtracks were introduced in the model as random effects. We also performed a model for each receiver species to investigate the variation in mobbing propensity according to the season and the mobbing stimuli.

Results

Our analyses showed that the season, the receiver species and mobbing stimuli have a significant additive effect on the proportion of individuals which exhibited mobbing behaviour (Table 2; Fig. 1). Blue Tits responded more strongly than Great Tits (p = 0.033). In Great Tits, both mobbing stimuli and season had a significant additive effects on the proportion of individuals that exhibited mobbing behaviour (season effect: 𝜒 1 2 = 3.527, p = 0.060; mobbing stimuli effect: 𝜒 1 2 = 5.10; p = 0.024). Great Tits mobbed 1.8 times more during autumn than they did during the breeding season. In response to Nuthatch calls, Great Tits were more likely to approach within 15 m of the loudspeaker than in response to Wren calls. Concerning the Blue Tits, we obtained the same result (season effect: 𝜒 1 2 = 5.36, p = 0.021; mobbing stimuli effect: 𝜒 1 2 = 5.12, p = 0.023): individuals mobbed 1.7 times more during the autumn than they did during the breeding season and were more prone to respond to Nuthatch calls than to Wren calls.

Discussion

Our playback experiments indicated that mobbing intensity was significantly greater in autumn than in the breeding season, whatever the receiver species or mobbing stimuli used and despite the fact that we may have tested juveniles (3-4 months old) with lesser experience on mixed-flocks and potentially showing weaker response. This result does not agree with most of the previous studies, which suggested increased mobbing activity during the breeding season (e.g., [START_REF] Altmann | Avian mobbing behavior and predator recognition[END_REF][START_REF] Shedd | Seasonal variation and function of mobbing and related antipredator behaviors of the American Robin (Turdus migratorius)[END_REF][START_REF] Shedd | Seasonal variation in mobbing intensity in the Black-capped Chickadee[END_REF][START_REF] Krams | Interspecific reciprocity explains mobbing behaviour of the breeding chaffinches, Fringilla coelebs[END_REF]. However, those studies examined mobbing behaviour in response to conspecific mobbing calls or to predator playbacks but not against interspecific mobbing calls (but see [START_REF] Tremblay | Factors affecting the permeability of transportation and riparian corridors to the movements of songbirds in an urban landscape[END_REF]. Nonetheless, these results confirm, on the species scale, the results obtained in a previous study conducted on the community scale (Dutour et al. 2017a). Increased mobbing activity during autumn could be explained by a seasonal variation in predator diet, as predation pressure on passerine birds is probably different across seasons (Dutour et al. 2017a). In the case where birds make up a large share of the predator's diet year round, vigorously attacking them at any time may make evolutionary sense [START_REF] Cully | Seasonality of mobbing intensity in the Pinyon Jay[END_REF][START_REF] Nijman | Seasonal variation in naturally occurring mobbing behaviour of Drongos (Dicruridae) towards two avian predators[END_REF][START_REF] Chiver | Mobbing behavior and fatal attacks on snakes by Fasciated Antshrikes (Cymbilaimus lineatus)[END_REF]Dutour et al. 2017a). In our case, a higher predation pressure in autumn could then explain more intense mobbing behaviours at this time. In addition, contrary to the breeding season, where mobbing behaviour is constrained by territorial boundaries [START_REF] Betts | Avian mobbing response is restricted by territory boundaries: experimental evidence from two species of forest warblers[END_REF], during the non-breeding season mobbing could occur in areas outside the territory, increasing opportunities to respond to mobbing calls. Alternatively, higher mobbing intensity during autumn could be explained by migratory patterns. If the birds tested are migratory birds, they have less opportunity to meet local predators, possibly leading Tits to respond to the mobbing calls of resident species in order to gather information [START_REF] Nocera | Inspection of mob-calls as sources of predator information: response of migrant and resident birds in the Neotropics[END_REF]. Ultimately, increased mobbing activity during autumn could be related to variations in sensitivity of the receiver [START_REF] Lucas | A comparative study of avian auditory brainstem responses: correlations with phylogeny and vocal complexity, and seasonal effects[END_REF][START_REF] Lucas | Seasonal variation in avian auditory evoked responses to tones: a comparative analysis of Carolina chickadees, tufted titmice, and white-breasted nuthatches[END_REF]). During the breeding season, Tits spend time with their conspecifics (mate and nestlings), whereas they form mixed-species flocks during the non-breeding season. We could suggest that species are more sensitive (i) to conspecific mobbing calls during the breeding season and (ii) to heterospecific mobbing calls during the non breeding season. During the breeding season, a targeted individual has an interest in responding to conspecific mobbing calls because the probability that mobbing calls are emitted by its mate are high, even though this strategy presumes individual recognition [START_REF] Kennedy | Individual distinctiveness in the mobbing call of a cooperative bird, the noisy miner Manorina melanocephala[END_REF][START_REF] Wheatcroft | Reciprocal cooperation in avian mobbing: playing nice pays[END_REF][START_REF] Mcdonald | Cooperative bird differentiates between the calls of different individuals, even when vocalizations were from completely unfamiliar individuals[END_REF]. Furthermore, for conspecifics, mobbing could play important indirect roles during the breeding season. For example, da Cunha et al. ( 2017) suggest that males may use mobbing to display their phenotypic quality to females. In this case, predator mobbing could be seen as a way to influence sexual selection. Conversely, during the autumn and winter months, some passerine birds conduct the majority of their daily activities with mixed-species flocks [START_REF] Ekman | Ecology of non-breeding social-systems of Parus[END_REF]). As birds often respond "by contagion" to the reactions of other birds, it is possible that during this period a higher number of prey species increase the propensity of birds to react and thus increase the strength of the mobbing, as shown by [START_REF] Sieving | Heterospecific facilitation of forest-boundary crossing by mobbing understory birds in north-central Florida[END_REF].

Thus, in the non-breeding season, all these factors together can trigger a strong response from passerines to the mobbing calls of other species.

Our results indicated that Great and Blue Tits responded more strongly to the calls of Nuthatch and much less to those of Wren. The similarity in response of Blue Tits and Great Tits to both Nuthatch and Wren calls suggests that they may use these signals in an analogous manner. Nuthatch is a passerine that may occur in mixed flocks with Tits during the nonbreeding season [START_REF] Hinde | The behaviour of the great tit (Parus major) and some other related species[END_REF], whereas the Eurasian Wren is a species with which they rarely co-exist. Thus, Tits may have opportunities to learn to associate the heterospecific mobbing calls of Nuthatches with predatory threats, and this may contribute to the rapid spread of antipredator behaviour within a bird community [START_REF] Wheatcroft | Learning and signal copying facilitate communication among bird species[END_REF][START_REF] Magrath | Wild birds learn to eavesdrop on heterospecific alarm calls[END_REF][START_REF] Suzuki | Referential calls coordinate multi-species mobbing in a forest bird community[END_REF]. Our results go beyond simple learning, as they suggest that previous exposure and learning maintain heterospecific responses [START_REF] Wheatcroft | Learning and signal copying facilitate communication among bird species[END_REF], in addition to innate processes [START_REF] Randler | A possible phylogenetically conserved urgency response of great tits (Parus major) towards allopatric mobbing calls[END_REF]Dutour et al. 2017b). To test the role of learning between species, we could have tested juveniles with less experience of mixed-flocks in Autumn.

Juveniles should respond less than adults. Other hypotheses could be advanced to explain an increasing mobbing response to Nuthatch: Tits are very similar in size to Nuthatches, occupy many of the same habitats, and are therefore attacked by most of the same predators.

Furthermore, it has been found that the White-Breasted Nuthatch (Sitta carolinensis), a close relative of the Eurasian Nuthatch, has (i) a greater ability to detect the predator playback [START_REF] Lucas | A comparative study of avian auditory brainstem responses: correlations with phylogeny and vocal complexity, and seasonal effects[END_REF][START_REF] Lucas | Seasonal variation in avian auditory evoked responses to tones: a comparative analysis of Carolina chickadees, tufted titmice, and white-breasted nuthatches[END_REF]) and (ii) a greater tendency to mob before other species [START_REF] Nolen | Asymmetries in mobbing behaviour and correlated intensity during predator mobbing by nuthatches, chickadees and titmice[END_REF]. In our case, Eurasian Nuthatch may have played a key role in the initiation of mobbing behaviour and that can be beneficial in responding to its mobbing calls.

Tits make complex antipredator responses depending on the mobber's identity and their ecological relations. Recognizing other species' mobbing calls is critical for the efficiency of the antipredator behaviour. Previous studies suggest that Red-Breasted Nuthatches (Sitta canadensis) discriminate between subtle differences in Black-Capped Chickadees (Poecile atricapillus) alarm calls that contain information about the size of potential predators [START_REF] Templeton | Allometry of alarm calls: black-capped chickadees encode information about predator size[END_REF][START_REF] Templeton | Nuthatches eavesdrop on variations in heterospecific Chickadee mobbing alarm calls[END_REF]. A next step is to study how passerine birds encode information about predator threat in their mobbing calls (see [START_REF] Carlson | A comparative study of how British tits encode predator threat in their mobbing calls[END_REF] for Paridae) and how this information is used by heterospecifics.

We have demonstrated that season influenced decision making in Tits. Overall, mobbing is a complex antipredator strategy, and many parameters, such as prevalence in flocks, similarity of mobbing calls or learning process among species sharing the same habitat, may influence the expression of this behaviour. Further research identifying mobbing and non-mobbing species would provide information on species interdependence and avian community organization.

Figure Captions

Fig. 1

 1 Fig. 1 Percentage of trials in which Great Tits and Blue Tits approached within 15 m of the

Table 1

 1 Details of the number of tests conducted for each combination in this study(n = 140)

			Playbacks
			Nuthatch		Wren
			Breeding season	Autumn	Breeding season	Autumn
		Blue Tit	15	21	15	15
	Tested
	species	Great Tit	15	21	18	20

Table 2

 2 Generalised linear mixed model type II Wald Chi-square results

	Explanatory terms in the fixed part of the models	χ2	p
	season	9.65 a	0.002
	receiver species	4.521 a	0.033
	mobbing stimuli	9.236 a	0.002
	season*receiver species	0.229 a	0.632
	season*mobbing stimuli	0.036 a	0.849
	receiver species*mobbing stimuli	0.011 a	0.915
	season*receiver species*mobbing stimuli	0.593 a	0.441

a df = 1

Acknowledgments

This study was supported by French Ministry of Research and Higher Education funding (to M.D. PhD grants 2015-2018). It was conducted with the approval of the Prefecture du Rhône, in accordance with the current laws in France. We thank David Wheatcroft for providing the wren mobbing call data.

this article. The datasets generated during and/or analysed during the current study are available from the corresponding author following a reasonable request.


Mylène Dutour
email: mylene.dutour@hotmail.com

Thierry Lengagne

Jean-Paul Léna

Marion Cordonnier

Michaël Girard

Syntax manipulation changes perception of mobbing call sequences across passerine species

Keywords: acoustic communication, call combination, interspecific communication, mobbing, syntax, tits

come

| Introduction

During the last decades, accumulating evidence has revealed that animal vocalizations share several features with human language [START_REF] Collier | Language evolution: syntax before phonology?[END_REF]. Duality of patterning, otherwise known as double articulation [START_REF] Martinet | La double articulation linguistique[END_REF], is a property of human language that makes possible a combinatorial structure on two levels: (i) phonological syntax, when meaningless sounds called phonemes (syllable or note) are combined to form meaningful acoustic structures called morphemes (motif) and words; and (ii) compositional syntax, which is the combination of meaningful motifs into a larger structure, whose meaning depends on the motifs involved and the syntactical rules used to put them together [START_REF] Berwick | Evolution, brain, and the nature of language[END_REF][START_REF] Marler | The Origin and Diversification of Language[END_REF][START_REF] Ten Cate | Revisiting the syntactic abilities of non-human animals: natural vocalizations and artificial grammar learning[END_REF]. The involvement of this property in the acoustic signal enables much more information to be conveyed using a finite set of vocal elements [START_REF] Berwick | Evolution, brain, and the nature of language[END_REF]. In animals, vocalizations involve a far less complex level of organization than human language. Although more and more evidence for syntax is becoming available for some birds and mammals (Coye et al., 2005;[START_REF] Ouattara | Campbell's monkeys use affixation to alter call meaning[END_REF], the ability of species to use compositional syntax (or compositionality; see [START_REF] Suzuki | Syntactic rules in avian vocal sequences as a window into the evolution of compositionality[END_REF] is still debated (Bolhuis et al., 2018;[START_REF] Petkov | Birds, primates, and spoken language origins: behavioral phenotypes and neurobiological substrates[END_REF][START_REF] Petkov | On the pursuit of the brain network for proto-syntactic learning in non-human primates: conceptual issues and neurobiological hypotheses[END_REF][START_REF] Suzuki | Call combinations in birds and the evolution of compositional syntax[END_REF]. More recently, three studies have examined this issue by studying mobbing calls produced by the Japanese tit, Parus minor, and the southern pied babbler, Turdoides bicolor [START_REF] Engesser | Meaningful call combinations and compositional processing in the southern pied babbler[END_REF][START_REF] Suzuki | Experimental evidence for compositional syntax in bird calls[END_REF][START_REF] Suzuki | Wild birds use an ordering rule to decode novel call sequences[END_REF]. Mobbing calls are particular forms of alarm signals. They are widespread, especially in passerine birds [START_REF] Klump | Acoustic behaviour of birds and mammals in the predator context. 1. Factors affecting the structure of alarm signals. 2. The functionalsignificance and evolution of alarm signals[END_REF]. They are emitted by animals trying to chase away a predator [START_REF] Pettifor | The effects of avian mobbing on a potential predator, the European kestrel, Falco tinnunculus[END_REF]. To be efficient, these calls often share particular features allowing listeners to join the mob, features that are thought to be involved in interspecific communication (Dutour et al., 2017a;[START_REF] Hurd | Interspecific attraction to the mobbing calls of black-capped chickadees (Parus atricapillus)[END_REF][START_REF] Marler | Characteristics of some animal calls[END_REF]. In this situation, where calling birds and receivers have a certain level of shared interest, and communication should be clear and as detailed as required to bring about an appropriate response that involves multiple joint behaviours [START_REF] Cunha | Predation risk drives the expression of mobbing across bird species[END_REF], [START_REF] Griesser | From bird calls to human language: exploring the evolutionary drivers of compositional syntax[END_REF] have recently suggested from findings from two avian species that compositional syntax may evolve.

Mobbing calls have been well described in the Paridae (Carlson et al., 2017a;[START_REF] Jung | Variation in chick-a-dee calls of bridled titmice (Baeolophus wollweberi): Frequent use of non-combinatorial calls in a combinatorial calling system[END_REF] and are usually composed of combinations of frequency modulated elements [START_REF] Hetrick | Antipredator calls of tufted titmice and interspecific transfer of encoded threat information[END_REF] referred to hereafter as FME, followed by a string of a repeated loud broadband elements [START_REF] Templeton | Allometry of alarm calls: black-capped chickadees encode information about predator size[END_REF], referred to hereafter as D notes. As suggested by several authors, such a pattern could well arise from a hierarchized organization following a 'syntactic-like' rule [START_REF] Hailman | The 'chick-a-dee'calls of Parus atricapillus: a recombinant system of animal communication compared with written English[END_REF][START_REF] Hailman | Combinatorial animal communication with computable syntax: Chick-a-dee calling qualifies as" language" by structural linguistics[END_REF]Lucas & Freeberg, 2007). In the Japanese tit, each motif is also used alone in context other than mobbing. The FME motif is used in vigilance situations and it serves as an alarm signal to which receivers respond by scanning for danger (alert calls), while the D notes are involved within foraging flocks and by nest mates to recruit social partners and elicit an approach of the receivers (recruitment calls) [START_REF] Suzuki | Experimental evidence for compositional syntax in bird calls[END_REF]2017). When the Japanese tits heard the FME motif combined with the D notes as the compound FME-D, which is also a natural vocalization, they showed both scanning and approach behaviours. However, there was little or no response when the call order was artificially reversed to D-FME (i.e. a sequence with unclear or ambiguous meaning; Bolhuis et al., 2018b;[START_REF] Suzuki | Call combinations in birds and the evolution of compositional syntax[END_REF]. From this, authors suggest that compositional syntax is a mechanism for information transmission, although some authors have argued that Suzuki and colleagues do not provide any evidence for genuine compositionality in these bird vocalizations (Bolhuis et al., 2018a;Bolhuis et al., 2018b). One may ask whether the use of a compositional syntax in mobbing calls is specific to Parus minor or extends to other Paridae species. Furthermore, using compositional syntax to encode information in mobbing calls with several motifs, including ones involved in social cohesion with conspecifics, could limit the ability of heterospecifics to correctly identify mobbing calls. This is particularly intriguing since mobbing calls can communicate the presence of a predator to heterospecifics as well as conspecifics [START_REF] Dutour | Mobbing behaviour varies according to predator dangerousness and occurrence[END_REF]Dutour et al., 2017a;[START_REF] Hurd | Interspecific attraction to the mobbing calls of black-capped chickadees (Parus atricapillus)[END_REF], prompting the question whether there is actually a similar underlying compositional structure across bird species, enabling them to decode information in heterospecific mobbing calls [START_REF] Griesser | From bird calls to human language: exploring the evolutionary drivers of compositional syntax[END_REF][START_REF] Russell | Communication: Animal Steps on the Road to Syntax?[END_REF]. It may also be relevant to ask whether species that do not produce combinatorial calls can nevertheless decode them.

The main objectives of the present study were first to test whether the great tit, Parus major, uses compositional syntax in mobbing calls. The great tit is the closest relative of the Japanese tit [START_REF] Johansson | A complete multilocus species phylogeny of the tits and chickadees (Aves: Paridae)[END_REF], and these species have similar calls and similar social structures, and also rely on both conspecifics and heterospecifics during mobbing [START_REF] Randler | Asymmetries in commitment in an avian communication network[END_REF][START_REF] Suzuki | Referential calls coordinate multi-species mobbing in a forest bird community[END_REF]. We investigated whether receivers extract different meanings from FME or D motif alone, and from combined motifs both natural or artificially-reversed order (i.e., FME-D or D-FME sequences). Secondly, we investigated whether heterospecifics are sensitive to syntax alteration of great tit mobbing calls. In this second experiment, we compared the responses of coal tits, Periparus ater, and common chaffinches, Fringilla coelebs, to playbacks of natural and artificially reversed great tit mobbing calls. We chose coal tits rather than other Paridae species since mobbing calls of this species are particularly complex and composed of multiple motifs (Carlson et al., 2017a;Dutour et al., 2017a), and also because our previous work showed that coal tits are especially prone to respond to heterospecific mobbing calls (Dutour et al., 2017a). We selected the chaffinch as an extragroup member (i.e., non-Paridae species) since it is often found in heterospecific mobs although it is less prone than tits to join heterospecific callers (Dutour et al., 2017a). Since the mobbing calls of the chaffinch were composed of a single "chink" note [START_REF] Randler | Heterospecifics do not respond to subtle differences in chaffinch mobbing calls: message is encoded in number of elements[END_REF], we predicted that coal tits would be more sensitive to syntax alteration than chaffinches.

| METHODS

| Experimental design

Data was collected during playback experiments conducted at the onset of the breeding season (experiment 1, February/March 2018) and at the end of the breeding season (experiment 2, July/August 2017) on wild passerines inhabiting mixed deciduous-coniferous forests located in south-east France (45°80'N, 4°52'E). In order to examine whether great tit mobbing calls involve compositional syntax (experiment 1), we used five playback types. First, we examined whether each motif alone (FME calls and D calls) induced a distinct behaviour, whether vigilance and recruitment. We then examined whether tits hearing the combined motifs in the natural order (i.e., FME-D calls, the natural mobbing call sequence) display a combination of the behaviours they exhibit when hearing each motif alone, and we tested whether this is also the case when the combined motifs are presented in the reverse order (i.e., D-FME calls, artificially reversed mobbing call sequence). Finally, we also performed control tests for which the playback contained only background noise (hereafter referred as BN tests).

We conducted these tests with 100 adult great tits (20 individuals for each test type); each bird received a single treatment. The goal of experiment 2 was to test whether heterospecific receivers (coal tits and chaffinches) are sensitive to syntax alteration in the same way as intraspecific receivers (great tits). To this end, we investigated how individuals of each species behave when hearing a playback of natural FME-D calls and artificially reversed D-FME calls of great tits. We also replicated these tests with great tits as receivers in order to permit comparisons across species. This second experiment involved 90 different individuals (15 individuals per test and for each species; each individual received a single treatment).

| Field test procedure

Field tests were done following a similar methodology used by [START_REF] Dutour | Mobbing calls: a signal transcending species boundaries[END_REF] to investigate the response of passerine birds to allopatric mobbing calls. After a focal bird was located, a loudspeaker was placed 30 meter away from the bird at the bottom of a tree. An experimented ornithologist and a field assistant were positioned opposite each other at vantage points at least 15 meters from the loudspeaker and the focal bird to avoid any disturbance during the test. Before the beginning of the experiment, the baseline behaviour of the focal bird was observed during a pre-trial period lasting at least 1 minute. If the bird was found to show alarm behaviour (i.e., emit mobbing calls, which happened in less than 5% of the cases) the test was abandoned. Otherwise, the playback was started when no other passerine was observed near the focal individual. Then, during 1 min of playbacks, two behavioural variables were recorded so as to infer vigilance effort and recruitment propensity, respectively: (1) the number of horizontal scans (we counted the number of obvious movements that birds made with their heads from left to right or right to left, approximately a 180 turn; [START_REF] Suzuki | Experimental evidence for compositional syntax in bird calls[END_REF] and (2) an approach within a radius of 15 m of the loudspeaker.

Horizontal scanning is a good indicator of perceived danger in birds [START_REF] Curio | The adaptive significance of avian mobbing: II. Cultural transmission of enemy recognition in blackbirds: effectiveness and some constraints[END_REF] and both behaviours, i.e. scanning and approaching, are common during mobbing events (Carlson et al., 2017b;Dutour et al., 2017a;[START_REF] Suzuki | Experimental evidence for compositional syntax in bird calls[END_REF][START_REF] Suzuki | Wild birds use an ordering rule to decode novel call sequences[END_REF]. Moreover, we set the approach distance to 15m since this approach distance was previously found to be a relevant measure of mobbing propensity (see Dutour et al., 2017a for more details). All observations were carried out with binoculars. The concordance of the number of scanning between observers was evaluated in a complementary study (see Supporting Information S1).

Since this study revealed a very high concordance despite a systemic bias between observers (see Table S1 Supporting Information S1), only the records done by the experienced ornithologist who participated to the whole study were used for data analyses. All trials were conducted between 6 a.m. and 1 p.m. during calm and dry weather. All focal animals were selected to be separated by more than 100 m to minimize pseudo-replication risk. In addition, we never went back twice to the same forest path, and during the breeding season the tits' territories are separated by 50 m in our study area. Once a test was done, we went on more than 100 m before trying to detect another bird which is calling or foraging. No bird was seen following us. Hence, although birds were not individually ringed, the probability of testing the same individual twice was low, and we are confident that our observations were performed on different individuals. Moreover, playback sequences were evenly distributed across the study period to avoid any temporal confounding effect.

| Playback stimuli and playback materials

We used mobbing calls produced by three great tits previously recorded in response to intraspecific mobbing calls (Dutour et al., 2017a). Calls were recorded with a Fostex FR2LE digital recorder connected to a Sennheiser ME67-K6 microphone (see Dutour et al., 2017a for more details). We also used mobbing calls obtained from the Xeno Canto online database (http://www.xeno-canto.org) recorded in different European countries (n = 6) located along the species' range in order to encompass the call variation range that a local bird community might experience and to generalize our conclusions. From these recording files, we built 20 unique soundtracks of natural mobbing calls (FME-D sequences) using Avisoft-SASLab software (i.e., 20 soundtracks with 1 individual per soundtrack). Recordings were in 16-bit WAV format (44.1 KHz sampling rate). These soundtracks were then used to construct three others, respectively FME calls, D calls, and D-FME calls as follows: FME and D calls were constructed by removing either D or FME calls from each FME-D calls and the D-FME calls were constructed by reversing the order of the motifs in the original FME-D calls of great tits (Figure 1). Within each sound track, calls were repeated at a rate of 26 calls per minute (this calling rate is within the range of the natural repetition rates, unpubl. data). We used a series of five to eight D motifs to construct playback (mean ± SE = 7.11 ± 0.06; Figure 1). Each track D notes were placed 82 ± 40 ms before FME (no difference from the time between FME and D notes in the natural sequence; t = 1.789, p > 0.05). The number of D notes could relate to the perceived level of threat [START_REF] Templeton | Allometry of alarm calls: black-capped chickadees encode information about predator size[END_REF] which could artificially bring about variation in the response of focal birds. With this mind, all playback sequences were arranged to adjust the ratio of the D notes over the FME notes in a mobbing call sequence, making this ratio slightly lower in our study (range: 5/8 -8/11) than in Suzuki et al., (2016, range: 7/10 -10/13, see Figure 1 in both manuscripts for comparison). We also constructed 20 control soundtracks using the parts where no birds were calling in the same recordings as natural mobbing calls (BN). In order to avoid pseudoreplication [START_REF] Kroodsma | Pseudoreplication in playback experiments, revisited a decade later[END_REF], we played back each soundtrack only once using a Shopinnov 20 W loudspeaker (the probability of testing an individual twice was low, see above). The average amplitude used for these playbacks was obtained with a sound level meter placed 1 m from the loudspeaker (~ 83 dB, Solo 01dB Metravib, Z weighting, re: 20 μ m Pa).

| Statistical analyses

In the analysis of experiment 1, we used generalized linear mixed models (GLMMs) treating the individual records from which were constructed all subsequent soundtracks (i.e. the test types FME calls, D calls, FME-D calls, D-FME calls and control) as a random effect. The test type was introduced as an explanatory term and the observation duration (i.e., the time during which we could observe the bird) as an adjustment covariate in the fixed part of the model.

All analyses performed on the number of scans were performed using a log link function and a generalized Poisson distribution (Proc GLIMMIX; SAS institute inc, 2012) to circumvent the overdispersion of count data relative to a Poisson distribution (see Joe & Zhu, 2005 for justification). For the approaching behaviour, the individual binary response (i.e., approaching versus not approaching) was introduced as the dependent variable using a logit link and a binomial distribution for the error term. For both response variables, the effect of explanatory terms was examined using a non-sequential F test, and subsequently removed from the model if non-significant. Although the random effect was never found significant (results not shown), it was always kept in the model if possible (see below). Multiple comparisons between test types were performed on least square mean estimates using a Dunnett-Hsu correction in the case of pairwise comparisons against the control situation (i.e. BN test) or the Sidak correction otherwise. Finally, for each test type, we used a Wilcoxon test to determine whether scanning behaviour differed between approaching individuals and those staying away, or conversely whether each approach was associated with scanning. Tests were not performed for the FME and the BN playbacks, given the lack of approaching birds in these sequences (see Results).

We proceeded in a similar way for the analyses of experiment 2. We first verified that the mobbing call sequence reversal resulted in the same behavioural pattern for conspecific receivers as observed in experiment 1. More specifically, we compared the number of scans and propensity to approach using generalized linear mixed models including the test type and the period (the onset of the breeding season i.e., experiment 1, or the end of the breeding season, i.e., experiment 2) as explanatory terms in the fixed part of the model and the individual from whom the recording came as a random effect.

We then compared the responses (scan and approach) of the three species to the great tits mobbing calls presented in the natural order (i.e., FME-D calls) and reversed sequence (i.e., FME-D calls). For this purpose, the receiver species (great tit, coal tit and chaffinch), the test type (FME-D and D-FME calls), and their interactive effect were introduced as explanatory terms in the fixed part of the model and the individual from which the recording came as a random effect. We then performed contrast analyses to test whether each heterospecific receiver specie (i.e. coal tit and chaffinch) responded differently than the conspecific one (i.e. great tit) to the mobbing calls sequence reversion. Multiple comparisons between test types were subsequently performed on least square mean estimates using a Sidak correction. Concerning the approach behaviour, none of the coal tits approached when exposed to one of the two test types (i.e., D-FME calls; n = 15) resulting in a sparse data structure. For this reason, likelihood optimization in the presence of a random effect is made impracticable and all tests based on the standard deviation associated to the estimates are made unreliable. We therefore used a GLM instead of a GLMM to analyse the approaching behaviour, and we used likelihood ratio tests instead of the F test to assess the significance of explanatory terms introduced in the model as well as the significance of contrast analyses.

Partial analyses for each heterospecific receiver species were then performed, and in the case of the coal tits, the difference between both test types was examined used a Fisher exact test.

We also completed these analyses by Wilcoxon tests to determine for each species whether scanning differed between approaching individuals and those staying away or conversely whether each approach was associated with scanning within each test type (i.e., FME-D and D-FME calls). These tests were not performed for coal tits in response to D-FME calls since no individual approached during the test, and nor for chaffinches since almost none of them approached during the test (see Results).

| Ethical note

This work was approved by the Prefecture de l'Ain (DDPP01-15-230) and by the ethical rules set by University Lyon 1 in accordance with the current laws in France.

| Results

| Experiment 1: compositional syntax in great tit mobbing calls

Overall, our analyses indicate that the scanning behaviour varied significantly according to the test type (respectively for the effects of the test type and the observation duration: 𝐹 4,23 = 14.9, 𝑝 < 0.0001; 𝐹 1,85 = 5.65, 𝑝 < 0.0197 ; see Figure 2a). Pairwise comparisons between each test type and the BN control reveal a significantly higher number of scans for the FME calls and FME-D calls tests (respectively for FME, D, FME-D calls and D-FME: t = 5.28, p < 0.0001; t = 0.38, p = 0.98; t = 3.96, p = 0.0021; t = -0.14, p = 0.99). There was also no significant difference between FME-D calls and FME calls (t =1.80, p = 0.59) and pairwise comparisons confirm that both FME and FME-D calls triggered significantly more scans than D calls and D-FME ones (all p < 0.01). The probability of approaching varied significantly between the test types (respectively for the effects of the test type and the observation duration: 𝐹 4,23 = 4.17, 𝑝 = 0.011 ; 𝐹 1,85 = 0.94, 𝑝 = 0.33 ; see Figure 2b). Pairwise comparisons between each test type and the BN control reveal a significantly higher approach propensity for the D and FME-D calls tests (respectively for FME, D, FME-D calls and D-FME: t = -0.01, p >0.99; t = 3.05, p = 0.0154; t = 2.54, p = 0.0464; t = 1.99, p = 0.136). Moreover, there was no significant difference between D calls and FME-D calls tests (t = 0.96; p = 0.88) and pairwise comparisons indicate that both D calls and FME-D calls significantly increased the approach propensity when compared to FME calls but not when compared to D-FME calls (D calls versus FME calls : t = 3.16, p = 0.022; FME-D calls versus FME calls : t = 2.64, p = 0.071; D calls versus D-FME calls : t = 1.87, p = 0.32; FME-D calls versus D-FME calls : t = 0.96, p = 0.88). Finally, regardless of the test type, scanning did not differ between approaching individuals and those staying away (Wilcoxon tests respectively for the D, FME-D, D-FME calls: W = 49.5, p = 0.78; W = 25.5, p = 0.07; W = 37, p = 0.52; test not performed for the FME and the BN playbacks given the lack of approaching birds in these sequences).

| Experiment 2: effect of syntax alteration on heterospecific perception

As experiments on heterospecific responses to the syntax alteration of great tit mobbing calls were done at the end of the breeding period while experiment 1 was performed at the onset of the breeding season, we first controlled that great tits response to the alteration of conspecific mobbing calls does not vary during the breeding season. Our results indicate that, whatever the study period (i.e. at the onset or at the end of the breeding season), great tits approached the loudspeaker with the same propensity in response to FME-D calls or D-FME calls (period effect: 𝐹 1,8 = 0.82; 𝑝 = 0.39; test type effect: 𝐹 1,8 = 1.44; 𝑝 = 0.26; interactive effect: 𝐹 1,8 = 0.01; 𝑝 = 0.94; see Figure 3a). However, if the study period also did not alter the effect of the mobbing call inversion on their scanning behaviour, great tits exhibited a higher vigilance effort at the end of the breeding season than at the onset (period effect: 𝐹 1,8 = 5.45; 𝑝 = 0.0478; test type effect: 𝐹 1,8 = 32.27; 𝑝 = 0.0005; interactive effect: 𝐹 1,8 = 0.01; 𝑝 = 0.93; see Figure 3b). Moreover, as in the case of experiment 1, scanning did not differ between approaching individuals and those staying away (Wilcoxon test respectively for the FME-D and the D-FME calls: n = 15, W = 29; p = 0.86; n = 15, W = 25; p = 0.77).

These results indicate that the intensity of the subjects' reaction to FME-D calls compared to D-FME calls remained unchanged before and after the breeding season for conspecifics, and offers stable ground for interspecific comparison.

Comparing the response to the inversion of great tit mobbing call sequence among receiver species, our results reveal a significant interactive effect of the receiver species and the test type on the number of scans displayed during the test (receiver species effect: 𝐹 2,16 = 2.32; 𝑝 = 0.131; test type effect: 𝐹 1,8 = 9.36; 𝑝 = 0.0156; interaction term: 𝐹 2,16 = 4.73; 𝑝 = 0.0243). As revealed by the contrast analyses, only the difference of the number of scans between the two test types exhibited by chaffinches significantly varied from the one exhibited by the great tits (respectively for chaffinches versus great tits and coal tits versus great tits : 𝐹 116 = 7.55; 𝑝 = 0.0143; 𝐹 1,16 = 0.01; 𝑝 = 0.93). The difference of approach propensity between the two test types also varied significantly according to the receiver species (receiver species effect: 𝜒 2 2 = 20; 𝑝 < 0.0001; test type effect: 𝜒 1 2 = 2.98; 𝑝 = 0.084; interaction term: 𝜒 2 2 = 6.31; 𝑝 = 0.042). However, only the difference of approach propensity between the two test types exhibited by the coal tits significantly varied from the one exhibited by the great tits as revealed by the contrast analyses (respectively for chaffinches versus great tits and coal tits versus great tits: 𝜒 1 2 = 0.82; 𝑝 = 0.36; 𝜒 1 2 = 4.16; 𝑝 = 0.0413). Partial analyses indicate that chaffinches did not scan the surroundings differently according to the order of the mobbing call sequence presented (𝐹 1,16 = 0.60; 𝑝 = 0.45). Chaffinches also rarely approached the loudspeaker, regardless of the sequence order (Figure 3c). In contrast, the coal tits displayed both increased vigilance and a higher propensity to approach the loudspeaker when hearing the FME-D calls than when hearing the D-FME calls (for the number of scans: 𝐹 1,16 = 9.08; 𝑝 = 0.0082; for the approach propensity : Fisher exact test p = 0.042 , see Figure 3b). Finally, unlike the great tits, coal tits approaching the loudspeaker also scanned significantly more intensely (14.2 ± 3.9) than those staying away (8.1 ± 5.2) when hearing the great tit mobbing calls in the natural order (Wilcoxon test: n = 15; W = 8; p = 0.042; test not performed for the D-FME calls given the lack of approaching birds in this sequence).

| DISCUSSION

We found that great tits behave distinctively when hearing respectively the FME motifs and the D ones. They scan the environment when hearing the former, and approach the sound source when hearing the latter. As previously seen in the Japanese tits [START_REF] Suzuki | Experimental evidence for compositional syntax in bird calls[END_REF][START_REF] Suzuki | Wild birds use an ordering rule to decode novel call sequences[END_REF], these results indicate that these two motifs also convey distinct meanings to the great tit. One corresponds to an alarm call (i.e., the FME motif) which elicits increased vigilance from the receiver, while the other corresponds to a recruitment call (i.e., the D motif) which elicits approach behaviour from the receiver. In response to the naturally ordered mobbing call sequence (i.e., FME-D calls), great tits exhibited both increased vigilance (with a high proportion of scanning) and a rapid approach (n =19; mean ± SE = 30.22 ± 3.38 seconds) toward the sound source, potentially to support the simulated caller opposing the putative threat, indicating that tits extracted both meanings from the naturallyordered combination of the two motifs. To confirm that the mobbing sequence constitutes a combination of two individual calls, a second step is to construct combinations by artificially merging both calls (with the calls originating from the same individual) and show that natural and artificial versions elicit the same response. Both the nature and the intensity of the responses we observed in the present study are similar to those obtained for the Japanese tit [START_REF] Suzuki | Experimental evidence for compositional syntax in bird calls[END_REF] and for the pied babbler [START_REF] Engesser | Meaningful call combinations and compositional processing in the southern pied babbler[END_REF]. However, while in the Japanese tits both the vigilance effort and the propensity to approach vanish when tits hear the mobbing call sequence in the reversed order [START_REF] Suzuki | Experimental evidence for compositional syntax in bird calls[END_REF][START_REF] Suzuki | Wild birds use an ordering rule to decode novel call sequences[END_REF], the results are different in the case of the great tit. The vigilance effort vanishes but the propensity to approach did not change when the combination order was reversed. Although the percentage of trials in which great tits approached the loudspeaker during the D-FME calls is reduced by 14% compared to the FME-D calls, this difference is low when compared to the one reported by [START_REF] Suzuki | Experimental evidence for compositional syntax in bird calls[END_REF] on the Japanese tits (i.e. 41.2% of reduction). Since we observed similar responses in both experiments, which were performed in two breeding seasons, this behavioural pattern is reproducible. Our results therefore suggest that great tits are sensitive to the ordered combination of the two motifs. However, as noted by several authors [START_REF] Berwick | Evolution, brain, and the nature of language[END_REF][START_REF] Ten Cate | Revisiting the syntactic abilities of non-human animals: natural vocalizations and artificial grammar learning[END_REF], this is not to say that tits actually exploit compositional syntax to convey more information. To our knowledge, great tits never use reversed mobbing calls sequences in the wild, and several hypotheses could explain a sequence order reversal effect without invoking compositional syntax usage. First, these results could be explained by the occurrence of a perception bias [START_REF] Grafe | The function of call alternation in the African reed frog (Hyperolius marmoratus): precise call timing prevents auditory masking[END_REF][START_REF] Klump | Mechanisms and function of call-timing in malemale interactions in frogs[END_REF] when the sequence order is reversed. Indeed, D notes, which are large frequency bandwidths enhancing location, may operate as auditory masking on the FME notes (frequency modulations resistant to degradation; [START_REF] Brown | Acoustic signal amplitude patterns: a computer simulation investigation of the acoustic adaptation hypothesis[END_REF][START_REF] Marler | Characteristics of some animal calls[END_REF] given the relative short delay between both sequences. Thus tits could no longer perceive the FME notes when they are artificially placed after the D notes. Such a phenomenon could well explain why great tits reduced vigilance behaviour when hearing the artificially reversed sequence of mobbing calls. This hypothesis could be tested directly by replacing D notes by a distinct high-bandwidth note (e.g., a burst of loud white noise). Auditory masking by D notes could also be the reason why great tits do not produce D-FME calls and, therefore, explain why the ordering rule is the way it is. Mobbing call responsiveness (and/or interpretation) may also depend on the social context, for instance according to the presence of the receiver's mate in the vicinity [START_REF] Suzuki | Experimental evidence for compositional syntax in bird calls[END_REF] or the seasonal activity [START_REF] Dutour | Mobbing behaviour in a passerine community increases with prevalence in predator diet[END_REF][START_REF] Dutour | Seasonal variation in mobbing behaviour of passerine birds[END_REF]Lucas et al., 2007). In the Japanese tit, the sequence reversal effect was examined in flock members during the non-breeding season [START_REF] Suzuki | Experimental evidence for compositional syntax in bird calls[END_REF] while in the present study tests were conducted on great tits at the onset and at the end of the breeding season.

Since the sensitivity to recruitment calls may vary between these two social contexts, further work is required to examine this point.

Chaffinches were barely sensitive to great tit mobbing calls although this species is regularly observed in heterospecific mobbing groups [START_REF] Dutour | Mobbing behaviour varies according to predator dangerousness and occurrence[END_REF]. Moreover, we recently found a very low responsiveness of chaffinches to heterospecific mobbing calls when compared to their own mobbing calls (Dutour et al., 2017a; see also [START_REF] Randler | Asymmetries in commitment in an avian communication network[END_REF]. We suggested that such a lack of response could have a causal explanation (the strong dissimilarity between mobbing calls of the chaffinch and other species), and a functional explanation (the exploitation of a distinct ecological niche). The second hypothesis cannot explain our results because chaffinches and great tits usually share the same ecological niche.

It is therefore likely that chaffinches are sensitive only to their own species, although the absence of a background noise test in experiment 2 cannot allow us to safely conclude on this point.

Conversely, coal tits were not significantly less responsive than conspecific subjects to the naturally-ordered great tit mobbing call sequence (i.e., FME-D calls) regardless of the behaviour being measured (i.e., vigilance or approach). However, their behaviours differ from that of conspecific great tit subjects since approaching coal tits were also more vigilant than those staying away, whereas approaching great tits were not significantly more vigilant than their counterparts staying away. Furthermore, and above all, when the mobbing call sequence order was reversed, coal tits not only reduced their vigilance effort but also no longer approached the loudspeaker. If these results indicate that coal tits are sensitive to great tit mobbing call sequence reversal, they also suggest that coal tits do not extract the same meaning from the mobbing calls sequence as great tits do. In particular, their responsiveness seems more binary or less versatile than that of great tits. This could indicate that some information (i.e., social information) is present in great tit mobbing calls, inducing conspecific responses, whereas coal tits do not pay attention to it. If auditory masking can explain great tit responses to D-FME calls, we hypothesized that wouldn't the same hold true for coal tits. Coal tits could be indeed more sensitive to FME notes when they are artificially placed after the D notes compared to great tits, because their mobbing calls are more complex (Carlson et al., 2017a), composed of combinations of many different calls; and they are more high pitched (unpublished data). One other hypothesis could be used to explain the behaviour of coal tits. It is possible that coal tits acquire the meaning of FME, D and FME-D calls via associative learning (i.e., coal tits learn to associate these calls emitted by great tits with a vigilance situation, a foraging flock or mobbing; e.g., [START_REF] Potvin | Birds learn socially to recognize heterospecific alarm calls by acoustic association[END_REF] because these signals can be heard repeatedly in the field. In contrast, coal tits cannot have acquired any meaning for D-FME sequences as they are never heard. This may explain their lack of reaction to these stimuli. This hypothesis would also be consistent with the 'correlation' between scanning and approach in coal tits (i.e., the binary response depending on whether the birds have been sufficiently exposed to great tits to associate these calls with a danger, or not). Although coal tits are sensitive to great tits mobbing call sequence reversal, our results did not provide evidence for compositional processing. Additional playbacks to verify coal tits' and chaffinches' reactions to FME and D notes, as well as to artificial stimuli starting with an FME-or a D-motif (but finishing with another motif that is normally not used in combination with these motifs) would be most valuable to disentangle this question.

Our results also suggest that the combination order of the repetitive loud and broadband notes (i.e., the D motif) preceded by the frequency modulation notes (i.e., the FME motif) within the mobbing call sequence is of prime importance to elicit a response in the receiver. In this respect, it is particularly striking that, in the few bird species for which the effect of compositional syntax has been investigated, all studies reported that natural mobbing call sequences follow this combination ordering [START_REF] Engesser | Meaningful call combinations and compositional processing in the southern pied babbler[END_REF][START_REF] Suzuki | Experimental evidence for compositional syntax in bird calls[END_REF][START_REF] Suzuki | Wild birds use an ordering rule to decode novel call sequences[END_REF][START_REF] Templeton | Allometry of alarm calls: black-capped chickadees encode information about predator size[END_REF]. Furthermore, as suggested by the present study and recent studies on the Japanese tit [START_REF] Suzuki | Wild birds use an ordering rule to decode novel call sequences[END_REF], it seems that this rule is more important than the acoustic similarity of each motif between species to enable heterospecific response. Nevertheless, whether this rule applies more broadly to bird communities remains to be established and further studies on a larger set of bird species will be necessary. To this aim, comparisons across allopatric species should be especially informative. Furthermore, as birds may not only eavesdrop on heterospecific communication to extract information about predator threats but also about food resources [START_REF] Magrath | Eavesdropping on heterospecific alarm calls: from mechanisms to consequences[END_REF], future work is needed to clarify how each motif alone (i.e., alert calls and recruitment calls) is understood by

FIGURE CAPTIONS FIGURE 1 FIGURE 2 FIGURE 3

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heterospecifics.

and Camille Locatelli for assistance in the field. This work was supported by French Ministry of Research and Higher Education funding (to M.D. PhD grants 2015-2018).