About ten years ago I was working in Cambridge as a research assistant with Dr Alex Thornton, studying a population of wild jackdaws. During the winter months we started observing the huge numbers of jackdaws roosting together, and got fascinated about the incredible timing of the birds all arriving and leaving the roosts at once. We started with some pilot studies of the roost at dawn and dusk, which showed the jackdaws were very vocal in the early morning and seemed to increasingly call more the closer to the moment they were to leave.
Many years later, Alex, who had now started a professorship at the University of Exeter, started looking at our questions again and decided to start a proper project to investigate the mechanisms underlying the massive departures of jackdaw roosts. Together with his master student Alex Dibnah, he was able to collect detailed observational data from a number of roosts across two winters. They also now ran a full-on playback experiment in the field to test our hypothesis that the jackdaws use vocalisations as a means to reach consensus decisions.
We observed the departures of roosts ranging from a couple hundred to a couple thousands of birds. While on some mornings jackdaws departed in a stream of small groups of individuals, on most mornings a majority or even the entire roost departed together and this happened almost at an instance, within a few seconds. By using audio recorders in the roost, we were able to record the calling of the birds and observed that the exact timing of departure of the roost was strongly linked to the calling of the jackdaws: the steeper the increase in calling intensity, the earlier the mass departure.
By a playback experiment in which we played calls of jackdaws in the roost, we were able to make the whole roost leave many minutes earlier than the predicted time of departure. Control playbacks of noise or wind did not induce any earlier departure. Hence, our playback experiments provides further evidence that it is the calling that mediates the timing of the jackdaws’ mass departures. Through their calls, jackdaws appear to effectively signal their willingness to leave, providing large groups with a means of achieving consensus to perform cohesive, collective departures from their roost.
Our study is published in Current Biology:
Dibnah, A. J., Herbert-read, J. E., Boogert, N. J., Mcivor, G. E., Jolles, J. W., & Thornton, A. (2022). Vocally mediated consensus decisions govern mass departures from jackdaw roosts. Current Biology, 32(10), R455–R456. https://doi.org/10.1016/j.cub.2022.04.032