Opportunistic predation on bats by the tawny owl Strix aluco

The presence of bats in the diet of tawny owls inhabiting the entrance parts of a large bat hibernation site in central Poland was compared in two periods. In the years 1985-1988 the median total number of hibernating bats per survey (522) was nearly 3 times lower than in the years 2002-2008 (1455). Along this trend the share of bats in tawny owls' diet increased respectively from 21.9% of vertebrate prey items to 36.5%. In areas without large bat concentrations they usually comprise up to 1% of vertebrate prey. This finding could be an evidence of an opportunistic feature of bat predation by owls. (C) Koninklijke Brill NV, Leiden, 2009