Collective decision-making in jackdaw winter roosts

Abstract

Animals living in groups must often make decisions collectively to maintain the benefits of group cohesion. Collective movement decisions require group members to reach a consensus before departure so that individuals can coordinate and synchronise their movement timing and direction. Therefore, behavioural mechanisms must be utilised that can efficiently transfer social information and integrate a variety of individual decision preferences. Some groups use quorum mechanisms to optimise both decision-making speed and accuracy, whereby the likelihood of the group performing an action increases sharply and non-linearly after a threshold number of individuals have indicated support. Quorum decisions have been well studied in eusocial insects, but empirical evidence is lacking that they play a role in large groups of vertebrates. Acoustic cues, such as vocalisations, may provide an efficient means for individuals to indicate support and reach a consensus when many individuals are dispersed or visually obscured. In this thesis, I investigated the potential for vocally-mediated quorum decisions to coordinate collective departures in jackdaw winter roosts. Here, many hundreds of birds often perform synchronised mass take-offs around sunrise that are preceded by high levels of vocalisations. Firstly, using audio and video recordings, I found that when calling intensity increased at faster rates, departures occurred earlier and greater proportions of the roost population departed together. Similarly, greater absolute calling intensities immediately prior to departure were associated with greater proportions departing cohesively, but calling intensity was unrelated to roost group size. Secondly, I used playback experiments to test whether artificially simulating an earlier onset of consensus through greater calling intensities just before departure would cause earlier departures. Experimental calling playbacks caused departures to occur on average 5-6 minutes earlier compared to control playbacks. In contrast, departure times under natural conditions (no treatment trials) showed no clear differences with those of control trials. This indicates that jackdaws were responding specifically to conspecific calls and that calling has a causal effect on departure timing. Overall, my thesis provides the first empirical and experimental evidence for collective movements being coordinated by vocally-mediated quorum decisions in large vertebrate groups.