Resource Use by Tawny Owls Strix aluco in Relation to Rodent Fluctuations in Bialowieza National Park, Poland

Abstract

Tawny Owl diet was studied in the primeval deciduous forests of Bialowieza National Park (eastern Poland), where the Tawny Owl was the most numerous predator and the forest rodents were non-cyclic with recurrent waves of outbreaks-crashes (triggered by heavy seed crops at 6-9-year intervals). The study (1985-1992) covered both noncyclic and outbreak-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. Between-year changes in the Tawny Owls' diet were mainly influenced by the abundance of yellow-necked mice, which thus determined the proportion of bank voles Clethrionomys glareolus taken and of alternative prey such as birds, amphibians, and shrews. Snow cover decreased the efficiency with which owls hunted for bank voles but not for mice and shrews. The alternative prey hypothesis, proposed to explain the linked cyclicity of microtines, shrews, and small game in northern Europe, may also be applied to animal population fluctuations in the temperate oak-dominated forests of Europe. During rodent crash years, predation by Tawny Owls may heavily affect the population dynamics of other small animals.