Behaviour and social organization of little brown bats during dawn swarming

Abstract

Collective behaviour in animals, being the coordinated actions and movement of groups of individuals, has important implications for social structure and organization. Females of many temperate bat species roost together in maternity societies, exhibiting roost switching and fission-fusion dynamics throughout the summer season. In the pre-dawn hours, bats gather near roosts and participate in a variety of conspicuous behaviours as a collective, referred to as “dawn swarming”. Using thermal video recordings and roost co-occurrence data from a RFID-monitored maternity society of little brown bats (Myotis lucifugus), we quantitatively characterized behaviours of bats in the area around roosts sites during the dawn swarming period, and investigated the extent to which dawn swarming activity patterns reflects an underlying social structure in maternity societies. During dawn swarming, bats displayed several visual behaviours, including circling the roost and swooping up to its entrance, in dynamic patterns each morning. Dawn swarming activity peaked one hour before sunrise and occurred consistently throughout the maternity season. While some bat activity was observed in the area of unoccupied roosts, disproportionately more activity occurred around roosts that ended up occupied as day-roosts following sunrise. Although this species is known to form preferential roosting associations, the associations observed during dawn swarming were not consistent social subgroups and there was no compelling evidence that individuals preferentially associated with certain conspecifics during dawn swarming. Together, these results suggest that the dawn swarming period functions as a society-level collective behaviour used by bats daily before settling into a roost, facilitating the transition between active (night) and resting (day) states for these animals.