Residency, migration and a compromise: Adaptations to nest-site scarcity and food specialization in three fennoscandian owl species.

Fennoscandian owl species differ, among other things, with respect to nest-site, food, clutch size, mate and territory fidelity, sexual size dimorphism and longevity. All these life characteristics help shaping the pattern of mobility and wintering strategies found in owl species, but it seems justifiable to regard food abundance and nest-site availability as the most prominent ones.The Ural owl is a generalist feeder and nests in tree cavities which are scarce. The pair is faithful to their territory for life. The long-eared owl is a vole specialist, breeds in secondhand twig-nests, which are abundantly available, and is largely migratory. Tengmalm's owl is a vole specialist and breeds in tree holes. As an adaptation to the conflicting pressures of periodical food scarcity and of keen competition for the nest holes which therefore have better be guarded uninterruptedly this species has evolved a strategy of partial migration, adult males being resident and females and young being migratory.