RESOURCE USE BY TAWNY OWLS STRIX-ALUCO IN RELATION TO RODENT FLUCTUATIONS IN BIALOWIEZA-NATIONAL-PARK, POLAND

Tawny Owl diet was studied in the primeval deciduous forests of Bialow ieza National Park (eastern Poland), where the Tawny Owl was the most numerous predator and the forest rodents were non-cyclic with recurren t waves of outbreaks-crashes (triggered by heavy seed crops at 6-9-yea r intervals). The study (1985-1992) covered both non-cyclic and outbre ak-crash years. Analysis of 1,116 pellets showed that Tawny Owls were specialised on forest rodents, mainly the yellow-necked mouse Apodemus flavicollis, but that they also exploited secondary resources: birds (especially Turdus, Parus and Carduelis), insectivores, and frogs. Bet ween-year changes in the Tawny Owls' diet were mainly influenced by th e abundance of yellow-necked mice, which thus determined the proportio n of bank voles Clethrionomys glareolus taken and of alternative prey such as birds, amphibians, and shrews. Snow cover decreased the effici ency with which owls hunted for bank voles but not for mice and shrews . The alternative prey hypothesis, proposed to explain the linked cycl icity of microtines, shrews, and small game in northern Europe, may al so be applied to animal population fluctuations in the temperate oak-d ominated forests of Europe. During rodent crash years, predation by Ta wny Owls may heavily affect the population dynamics of other small ani mals.