Colonization of Islands by Land Birds: Prevalence Functions in a Finnish Archipelago

Land birds breeding on forty-four islands (size range 0.5-582 ha) in the Vargskar archipelago of the kland Islands (SW Finland) were censused in three (or two) breeding seasons in 1976-80 by one-visit methods. The islands were divided into five size classes and expected population sizes of land birds were estimated on the basis of (1) habitat composition of the islands and (2) population densities of each species in equivalent habitats in the source area, Main Aland. Prevalence is defined as the ratio between observed and expected population sizes. Observed densities of most of the fifty-six species studied did not deviate from expected values. Eleven species were more abundant on the islands than expected, including dominant generalists and species favoured by edges and bushy, heterogeneous habitat structure. Fourteen species had smaller populations on the islands than expected, but only seven of them consistently so in all census years and all island size classes. Habitat micro-structure seems important for three low-prevalence species, nest predation is invoked in two cases, in one case poor wintering capability seems a plausible factor, while one case remains obscure. Rarities were observed in the island censuses about as frequently as on Main Aland. Interspecific competition seems a plausible explanation for the absence of one species from the archipelago. One species, Turdus pilaris, changed its prevalence pattern dramatically in the study area by disappearing from the archipelago after the exceptionally cold winter of 1978/ 79; recolonization has taken place very slowly. The only species possibly showing density compensation is the dominant generalist Fringilla coelebs. Its populations on small islands are about doubled compared with the expecta- tions. Total densities are also higher than expected on small islands, but in most cases F. coelebs contributes less than half to the increase. In addition, there are no puzzling absences from small islands, but the poverty of their bird com- munities seems to result from reduced habitat diversity. Therefore autecological population responses rather than density compensation seem a plausible explana- tion of excessive population sizes of single species such as F. coelebs. We con- clude that, in these northern conditions, it seems appropriate to think of island communities as assemblages of species colonizing the islands according to their autecological (especially habitat) requirements. Ubiquitous habitat require- ments and flexibility in habitat utilization seem important correlates of coloniza-