Social preference persists at roosting aggregations in a cooperatively breeding bird

Animal sociality ranges from predominantly solitary life to complex multilevel societies in which stable core units merge into increasingly inclusive entities.

To understand the evolutionary pathways and mechanisms that promote and maintain animal sociality in different taxa, it is crucial to uncover whether conspecific aggregations result from true social attraction or nonsocial forcing factors (e.g. localized resource) and whether social preference is maintained in different contexts. Here, we show that, in cooperatively breeding carrion crows, Corvus corone, core social units persist at higher levels of aggregations (roosts). In our study population, individuals live in cohesive territorial kin groups year-round in an all-purpose territory. At night, crows from different territories may sleep communally in one of the four roosts available in the area. By radiotracking 73 individuals, we found that roosts were used mainly in winter and on cold nights in autumn and spring, suggesting a thermoregulatory function of nocturnal aggregations. Interestingly, we also discovered that members of the same social group preferentially used the same roost, showing a social cohesion that was not achieved by mere attraction to the same roosting site but was based on an active choice of joining groupmates. Therefore, our results indicate that social preference persists in different aggregation contexts in cooperative crows, revealing a further complexity of their society and suggesting a social function of roosts. However, different social groups did not merge in a stable and predictable way at the roost level, providing weak support for a multilevel society in this population.