Winter bird communities in the Aland archipelago: an island biogeographic point of view

The distribution of wintering land birds was examined in the archipelago of Aland (60°N, 20°E) along two ecological dimensions: (1) the island continuum from small skerries ((0.5 ha) to the mainland of Aland (97000 ha), and (2) the habitat range of the mainland of Aland. In the species-area relationship, both the exponential and power function models fitted well. The increase of the number of species with island size was interpreted as a consequence of increasing habitat diversity on larger islands. The smallest islands supported only few seed-eating species utilizing the islands in a fine-grained fashion; none of these birds belonged to the breeding fauna of the islands. With increasing island size and habitat diversity, insectivores and species tied to the human culture were added. On the mainland of Aland, more than half of the birds were observed near human habitations: bird communities wintering in pine forests were richer than in deciduous forests. Proportion of wintering species was greatest in the breeding communities of pine forests. The proportion of wintering species was presumably regulated by the degree of structural change in the habitat between summer and winter, this change being most drastic in deciduous habitats The species observed could be broadly classified into three feeding categories: opportunists., omnivores and scavengers, dependent on the diverse food items provided by man; food specialists, species eating seeds and berries, ranging widely in the islands and habitats; and insectivorous habitat specialists, inhabiting coniferous habitats, and islands with coniferous (pine) forests. Opportunists, in particular, gel increasing wintering opportunities by the impact of man.