Influence of seasonality, temperature and rainfall on the winter diet of the long-eared owl, Asio otus

In this study we analysed whether the diet composition of a wintering population (40-70 individuals) of long-eared owls (Asio otus) in northern Italy showed within-season variation, and whether it was influenced by ambient temperature and rainfall. Diet composition was determined by pellet content, and over 5500 prey items were analysed; pellets were collected at 2-wk intervals over two consecutive winters (October to April), 1996-1998. Three out of five main prey categories showed a marked within-season variability in relative frequency in diet, both considering the number of prey items and prey biomass, whereas between-year variability was shown only by a single prey category (Savi's pine vole). Although rainfall had no influence on diet composition, temperature affected negatively the prevalence of harvest mouse, a relatively unimportant prey category. Thus, the considered weather variables seem to have little influence on the winter diet composition (at the level of individual prey categories) of these owls. However, diet breadth (estimated by the Levins' index of niche breadth) increased with increasing rainfall and decreasing temperature, when calculated on the proportion of prey items: hence it seems that the owls become more generalists under unfavourable weather conditions.