Living at the limit: Ecology and behaviour of Tawny Owls Strix aluco in a northern edge population in central Norway

The Tawny Owl Strix aluco was studied at the northernmost limit of its geog raphical range (63 degrees 20'N) in order to investigate how this extremely residential species has adapted to the marginal conditions of its northern outpost. The presence of nemoral forest vegetation was crucial for the occ urrence of the species. The mean annual home range size of radio tagged fem ales was much larger than reported in any other study. The winter home rang es were, on average, 54% larger than those in summer. In the summer, range size was negatively correlated with the proportion of mixed deciduous/conif erous forest within 1 km of the nest. This could be explained by the abunda nce of bird prey, which comprised 61% of the summer diet. In the winter, no significant correlation's with home range sizes were found. During this se ason, mammals comprised 95% of the diet. In extreme cases, females left the nesting area for prolonged periods during the non-breeding season. We conc lude that even a species known to be notoriously residential can express a high degree of plasticity in its ranging behaviour when population densitie s are low, at least among the females. However, the environmental and socia l factors leading to some females leaving their nest areas in the non-breed ing season are more complex than the result of mere lack of food.