Responses of the long-eared owl Asio otus diet and the numbers of wintering individuals to changing abundance of the common vole Microtus arvalis

Opportunistic predator like the long-eared owl is able to respond to population fluctuations of its main prey. The composition of the winter diet of this owl species was investigated during the period of 13 winters (1992–2000, 2006–2011) in agricultural areas in Slovakia. In total, we found 23 mammal species and 33 bird species ( H ’ = 0.82) in pellets. The frequency of the dominant prey species, the common vole, varied from 57.7% to 92.4%. Our data show that the abundance of the common vole: (i) had biggest impact on the food niche breadth of the long-eared owl; (ii) when in decline, it was significantly compensated by the increase in the amount of 15 other accessory species (subject to the specific diet offered during the study winters); (iii) was positively correlated with the number of owls in the winter-roost, which varied during the 13 studied winters.