In colonial tree-dwelling bats, is vital to prevent disintegration of the group during frequent roost-switching. Thus some mechanisms which maintain group cohesion are expected. Dawn swarming is a set of behavioural displays observed in many such bats before they enter the roost. It is suggested that this behaviour plays a role in transferring information about the roost position. However this phenomenon had not been explored in detail. Based on qualitative and quantitative description of behaviour we suggest its potential function. Using field-based video-recordings of swarming sessions made on maternity colonies of Leisler's bat (Nyctalus leisleri), we constructed ethograms which revealed remarkably similar behavioural sequences among individuals. For more than two hours prior to sunrise, individual flybys in front of the roost entrance predominated, followed by landings and leaps, which preceded the final entering of the roost. Interestingly, no obvious peak of behavioural activity was found at any particular time during swarming but a wave-like pattern was observed. We suggest that individuals are swarming in close proximity to the roosting tree with some purpose, most probably serving as a beacon for other group members and thus marking the current location of the roost.