Influence of temperature on maternity roost selection by noctule bats (Nyctalus noctula) and Leisler’s bats (N. leisleri) in Białowieża Primeval Forest, Poland

This study tests whether the temperature of tree cavities determines their selection by bats in Białowieża Primeval Forest (BPF), eastern Poland. Using a data logger, I simultaneously measured the temperature in cavities selected by bats as roosts and the temperature in available but unselected cavities. The maternity roosts chosen by noctule bats, Nyctalus noctula (Schreber, 1774), and Leisler’s bats, N. leisleri (Kuhl, 1817), during late pregnancy and lactation were warmer than unoccupied cavities, but temperature ranges in the two types of cavity did not differ. A logistic regression model showed that the mean cavity temperature during the night (2200–0400) and the minimum cavity temperature over a 24 h period were crucial for roost selection. This suggests that female noctule and Leisler’s bats selected roosts that promoted juvenile growth and used tree cavities that could save them energy while they were active because the cavity temperatures were close to the lower critical temperature of their thermoneutral zone. I also suggest that selection of warmer cavities minimized the bats' energy expenditure prior to emergence from the roost and that passive rewarming inside the cavity was an important factor in minimizing energetic costs of roosting by bats in BPF. Mean and maximum temperatures recorded near tree trunks increased with the height at which the temperature was measured (ground level, 10, 20, and 30 m), suggesting that bats can gain thermal benefits from insolation of the trunk by selecting highly placed cavities, as was observed in BPF.