Factors Affecting Timing of Breeding in the Tawny Owl Strix aluco

Relevant variables characterising factors suspected to affect timing of breeding in birds may be difficult to measure. Then, easily available proxies, i.e. variables that are expected to vary consistently with some affecting factors, may be used instead. In the present study on tawny owls, I used besides individual characteristics of parent birds both de- tailed measurements of the prey base of territories, local and general indices of vole abundance as well as local and gen- eral weather conditions as explanatory variables. Owls tended to breed the earlier the older and the heavier they were. Breeding was the earlier the higher the abundance of water voles and the proportion of field voles in the territorial prey samples. Owls bred earlier when the local abundance of small voles in the preceding autumn was high. Regional vole in- dices showed no associations with timing of owls' breeding. Single winter weather variables did not show any significant associations neither. In combination with the advancing effect of water vole abundance, however, the delaying effect of the depth of the snow cover in March was significant. Models based on different data sets showed different kinds of asso- ciations between the abundance of small voles and timing of breeding in owls. The best models in which both intrinsic and extrinsic explanatory variables (characteristics of parent birds and environmental factors, respectively) were included did not differ considerably from each other. The occurrence of water voles in prey samples governed the best models. The results suggest, that without detailed knowledge on the prey base of territory, misleading results may emerge and the im- portance of small voles in governing breeding of owls may be overemphasised.