Spatial consequences of relatedness and age in buzzards

Many studies have concluded that buzzards, Buteo buteo, are strongly territorial but a minority have suggested that some buzzards can share ranges. We radiotracked 146 buzzards from fledging for up to 4 years. In their first year, distances of buzzards from the nearest nest were least if it was their natal nest. To determine if this represented positive association with their own nest, or avoidance of strange nests by birds that had dispersed, we compared observed buzzard locations with random locations within a predefined distance from nests. There was strong association of buzzards with their own nests in their first autumn and winter; dispersed birds did not avoid other nests significantly but spaced themselves between them. In their first autumn, neighbouring sibling buzzards had closer activity centres and had a greater tendency to overlap their home ranges than nonsiblings, irrespective of distance from the natal nest. However, both effects disappeared with age: core ranges, which included the densest 90% of locations, become strongly exclusive in older buzzards. These results show that buzzards are able to share territories with their parents and siblings in their first year, but not when older. (C) 2001 The Association for the Study of Animal Behaviour.