Swarming behaviour associated with group cohesion in tree-dwelling bats

•A mechanism of how bats select new roost without group disintegration was studied.•Swarming activity of bats was performed during the whole night at roosting sites.•However, bats swarmed also at unoccupied sites.•Swarming has probably a signalling role in group cohesion during roost switching. Group living of social species increases the individual survival rate and enhances optimal exploitation of resources within their environment. Groups of cryptic tree-dwelling bats perform frequent roost switching on relatively long distances, whereas groups are temporarily disintegrated during foraging that precedes moving into new roost. In this study, we attempted to elucidate a mechanism in which group members can select new roost from a large set of potential roosting sites without disintegration of the whole group. Exploring the activity at the front of the tree hollows, recorded by automatic infrared monitors, we found that the swarming activity of Leisler’s bats was performed during the whole night and was concentrated before the civil twilight at occupied roosts. A generalized linear model revealed significant positive association between the group size and swarming activity while season had negative but smaller effect. As bats swarmed the whole night also at sites that were less occupied or even unoccupied, we suggest that members of a bat colony selected new roost from a potentially larger set of available roosting sites. Thus, we propose a possible signalling role of swarming as a threshold behaviour that ensures group cohesion during roost switching.