The selection of roost cavities by Nyctalus noctula and N. leisleri, 2 widespread species of mainly Eurasian distribution, was examined in Białowieża Primeval Forest in eastern Poland from May to August in 1998–2002. Because N. noctula is one of the most common forest-dwelling bats in Europe, whereas the smaller N. leisleri is relatively rare across its range (except in Ireland) and more limited to ancient forests, we hypothesized that these 2 taxa may differ significantly in their tree-cavity selection. Twenty-five N. noctula and 26 N. leisleri were radiotracked to 52 and 50 roost trees, respectively. For each accessible cavity roost occupied by N. noctula (n = 28) and N. leisleri (n = 39), 16 features were measured and compared with potentially available cavities (n = 72). Both species were selective in roost choice, and preferred cavities located higher (averaging 19 m above the ground), in more open surroundings, with smaller entrances, and with greater safety distance (from martens) than available cavities. Nearly all roosts occupied by bats were dry inside. Both species slightly more frequently settled in cavities with entrances facing NE and SW, but the differences were not statistically significant. Compared to the pool of available cavities, N. noctula was statistically more frequently found in cavities with wider inside cross section and with 1 entrance, unlike N. leisleri, which often used cavities with more than 1 entrance (range 1–6 entrances). One of the most noticeable differences between the 2 species was roost origin. N. leisleri used natural cavities (90%) more often than woodpecker cavities (10%), whereas N. noctula showed the opposite tendency (woodpecker-made cavities accounted for just over half of roosts chosen by this species). The safety distance also was significantly larger in N. leisleri than in N. noctula. A logistic regression model for N. noctula incorporating 4 cavity variables (safety distance from martens, height above ground, cavity origin, and mean distance to nearest vegetation) classified roost and available cavities correctly 85% and 94% of the time, respectively. For N. leisleri, the use of 2 variables only (i.e., height above ground and marten distance) resulted in correct assignment of 85% of roosts and 88% of available cavities. These differences suggest that the 2 species use different antipredator strategies that may have important consequences for their different survival rates in younger forests. In general, roosts in Białowieża Primeval Forest are selected under pressures of predation and climate, and there bats tend to use safe and warm shelters.
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