Seasonal and habitat variation in the diet of the tawny owl (Strix aluco) in central Poland during unusually warm years

In the temperate climate zone in Europe the composition of the diet of predatory vertebrates shows evident variability between the warm and cold season. However, the recently observed climate warming can mitigate the effect of snow cover and low temperatures on the winter foraging ecology of raptors, thus affecting trophic webs in ecosystems. We analysed diet variability in the tawny owl Strix aluco, between the warm and cold seasons of four unusually warm years (as compared to reference years of 1950-2000) in two habitats (forest vs. farmland) in Central Poland. The most important prey group in the tawny owl's diet were mammals, constituting over 80% of prey items. There were distinct diet differences between the two seasons: insectivorous mammals, birds and amphibians were caught more often during the warm season, and Muridae and Arvicolidae during the cold season. The proportion of insectivorous mammals, voles and amphibians was significantly higher in forest than in farmland. Diet diversity, analysed with rarefaction methods and expressed as the expected cumulative mammal species number for a given number of randomly sampled preyed mammals, was independent of season and higher in forest than in the agricultural habitat. We conclude that even during unusually warm years tawny owls change significantly their feeding habits between the warm and cold season. The effect of season, habitat and weather factors on diet variability in raptors are discussed.