| This doctoral thesis focused on the contact call,a common vocalization type repeatedly produced by group members of many social animal species,and especially prevalent in birds and mammals.However,the precise functions of the contact calls,and particularly the information encoded in their rate of production,is still poorly understood.The objective for this thesis was hence to determine what information could potentially be transmitted through varying the contact call rate,and also which external factors,such as environmental or social variables,can influence this rate.For this purpose,I conducted both observational and experimental studies on the Swinhoe’s White-eye(Zosterops simplex),a groupliving passerine bird.I investigated several alternative hypotheses about the functions of contact calling and how that would influence contact call rate.First,contact calls could be used to keep regular spacing between foraging individuals.If so,one would predict an increase in the call rate in denser vegetation with more visual obstruction.Second,contact calls could provide information related to the predation risk,with the production of contact calls that suddenly stop when there is a predation threat,a strategy known as “adaptive silence”.In that case,I predicted birds would produce more calls in more dangerous open habitats,so that silence would be an obvious signal of danger.Third,rather than being associated with environmental conditions,contact calls could be related to the social context.When observing wild birds,this hypothesis suggests that contact call rate would be affected by the movements of group members.I made more than 800 audiorecordings on wild flocks across a gradient of vegetation density,recording also the movements of flying individuals.A total of 97 741 contact calls were counted.I found that the vegetation at the location of the flock had little influence on their call rate.Rather,I discovered that flying individuals produced a distinct flight call consisting of repeated notes of contact calls.I also found that the group-level contact call rate increased before individual movements,especially before peripheral birds flew into the group centre.These findings suggest that contact calling does not encode much information about the vegetation at the flock’s location,but may rather depend on the social context.When recording birds in the captive environment of an aviary,I was able to describe three different types of contact calls: a “flight call”,as found in the field,a soft “short-distance” call and a louder “long-distance” call.A Linear Discriminant Analysis was run using the acoustic characteristics of the different call types,and it correctly classified 97.8% of observations.To verify the absence of a vegetation effect on the production of these contact calls,I audio-recorded the captive birds with different density levels of artificial vegetation,and I found that the vegetation did not affect the group-level call rate,confirming the previous results.I also looked for a social effect on the rate of contact calls by manipulating the group size and group composition.A first objective here was to investigate whether individuals would modify their vocal production depending on the presence of particular individuals in the subgroup.By characterizing the social relationships between subjects,I predicted an increase of call rate with the presence of socially connected individuals.A second objective was to investigate whether individuals might regulate their call production to maintain a specific group-level call rate,as expected if birds would use the cessation of contact calls as a signal for danger.I found that individual-level rates were not consistent with the hypothesis that birds were maintaining a constant group-level call rate.However,a strong group size effect was found,with solitary birds making more long-distance and less short-distance calls than birds in groups.Further,I found that call rates decreased when birds were housed with individuals of differing dominance rank,but increased when housed with affiliated individuals.Together,these results confirm that contact calling is related to social context rather than to vegetation structure.Through an experimental approach,I also investigated whether the production of contact calls is related to predation risk,according to the adaptive silence hypothesis.The Swinhoe’s White-eye possesses an alarm call in its repertoire,yet alarm calls can be unreliable(produced by mistake or deceptively).Silence following an alarm call could then emphasize a real threat,and allow individuals to distinguish between false and true alarms.This hypothesis suggests that the production of contact calls would function as an “all clear” signal for safety,which can be found in some other social species.To test this,a playback experiment was conducted using calls of eight captive flocks.I found that birds did not respond to the sudden contact call cessation,nor did they have a stronger response to alarm calls followed by silence than to alarm calls followed by contact calls.These findings suggest that contact call rate does not encode information about predation risk.Together these results showed that,in the Swinhoe’s White-eye,contact calls are not strongly associated with the environmental conditions of vegetation and predation risk,but instead are modulated by the social dynamics of the group,suggesting that the rate of contact calls can be a source of information about social relationships,and may potentially have a recruitment function.This thesis work showed that the functions of the contact calls,which are among the simpler types of animal vocalizations,are still not fully known,and that they might vary between species.To highlight the functions of the contact calls in communication between conspecifics,studies investigating contact call rate in other group-living passerine species with different social systems are particularly encouraged.Ultimately,this would help to better understand the evolution of contact calling,a behaviour widely found across animal societies. |
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