Evening emergence of two nursery colonies of Leisler's bat (Nyctalus leisleri) in Ireland

At one nursery roost of Leisler's bat Nyctalus leisleri, the first bat often, and over half of the colony exceptionally, left before sunset. Departure was earlier during the period of lactation, when energy demands were highest. Consequently emergence data had to be standardized to allow for this before investigating possible associations with light intensity and meteorological factors. The importance of such standardization in general is emphasized. Emergence was earlier on overcast nights and was delayed with increased light intensity. The bats in a neighbouring roost, from which fewer data were obtained, also left earlier on overcast evenings, but departure was generally later than in the first roost, and there was no evidence of its being earlier during lactation. These differences between the roosts may be at least partially explained by the difference in size of the exit holes. At both roosts departure was mainly as a series of outbursts, and this was more marked during the first half of emergence. Rate of emergence increased with colony size. It was unusual for the first bat to leave more than 30 min, or for any bat to leave over 60 min, after sunset. Heavy rain inhibited or disrupted emergence.