The mass departures of vast numbers of roosting birds suddenly taking to the skies around sunrise are one of the most dramatic examples of collective behaviour in nature. How can such large numbers of individuals coordinate their movements so exquisitely? A new paper in Current Biology led by Master’s student Alex Dibnah, under the supervision of HFSP grant-holder Alex Thornton, uses observations and field experiments at jackdaw roosts to show that mass departures are underpinned by a vocalisation-based consensus decision-making mechanism.
Figure: Jackdaws in winter roosts - photo credit: Jolle Jolles.
Like many birds, jackdaws form large winter roosts containing hundreds or even thousands of individuals. On cold, dark winter mornings, the birds typically spend a long time calling before suddenly taking to the skies together. Studying multiple roosts containing hundreds to over a thousand wild jackdaws in Cornwall, UK, the study found that steeper rises in calling intensity led to earlier mass departures. Higher final calling intensity prior to departure produced a steep, non-linear increase in the proportion of roost members departing together. Playback experiments then confirmed that the calls had a causal effect on departures from the roost: playing back recordings of roosting calls resulted in earlier mass departures compared to playbacks of wind noise. This demonstrates that the birds were responding specifically to each other’s calls, and not to noise in general.
This research was borne from an HFSP grant to Alex Thornton (University of Exeter) and colleagues focusing on collective behaviour in heterogeneous societies, using jackdaw flocks as a model system. The bulk of the grant focused on flocks in flight, but as the grant progressed the researchers became interested in how groups reach the decision to take to the skies. Previous work on other species has investigated consensus decision-making in groups that are relatively small or made up of family members. In contrast, jackdaw roosts are vast, containing hundreds of individuals of different ages, sexes, family groups and colonies, spread across the treetops. All these different individuals are likely to have different preferences as to when they want to leave the roost (as a result of differences in size, hunger and so on), but sticking together can provide benefits in terms of reduced risks of predation and access to foraging information by following others. The work demonstrates that vocalisations could allow consensus decisions to be reached even in groups that are large, dispersed and heterogeneous.
In the Media
HFSP award information
Research Grant - Program (RGP0049/2017): Collective behaviour and information transmission in heterogeneous societies
Principal investigator: Alex Thornton, University of Exeter, Penryn, UK