Diet and reproduction in coastal and inland populations of the Tawny Owl Strix aluco in southern Finland

In generalist predators the energy needed for reproduction is derived from various prey categories, the profitability of which may vary according to the biomass or accessibility of the prey items. We examined the relationships between the diet and production of young in two Tawny Owl ( Strix aluco ) populations in southern Finland. We found a positive correlation between the amount of remnants of different prey in samples of litter collected from the nest bottoms after breeding and the production of young. Three important prey categories in the diet of owls during the nestling period, i.e. Water Voles, Field Voles and thrush-sized birds, seemed to be significantly related to the number of fledglings. However, only thrush-sized birds showed a significant relationship to the number of fledglings when all prey variables were analysed together in one model. The total prey mass was the best predictor of owl brood size. There were no significant local differences between the effects of different prey categories on the production of Tawny Owl young. Our results, combined with some earlier findings, outline the relationship between food supply and the production of Tawny Owl young in southern Finland. With regards to the Tawny Owl this outline can be generalized to other areas with other kinds of food supply within the distribution range of the species. Similar outlines are probably applicable also to many other generalist predators.