The Occupation of Small Islands by Passerine Birds

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Abstract
Breeding landbirds on 12 small (0.1-4.0 ha), forested islands in Muscongus Bay, Maine [USA], were censused from 1967-1975. All 17 spp. that bred also occupied spruce forests or adjacent disturbed habitats on the nearby mainland. Similar numbers of species came from both areas, but since most islands were spruce-covered, the contribution from the disturbed areas exceeded that predicted by chance. Size of forest accounted for the greatest amount of variation in numbers of species present, while the total area vegetated, amount of ground cover and forest height made lesser contributions. Total vegetated area accounted for the greatest amount of variation in numbers of species present from the disturbed areas, even though this study only treated the species that occupied the forested parts of the islands. When the smallest islands (< 0.2 ha) were removed from the calculation, z (slope of the log-log regression of species against area) fell near 0.20, which approaches a typical mainland figure. Including the smallest islands in the calculation, z was about 0.50, probably because these islands were too small to support most species. Some species occurred only on islands at least as large as their territory sizes on the mainland, while others decreased their territory size considerably. Those bird species not predicted on the basis of the size of their mainland territories often were low in the social hierarchy and demonstrated ecological release on the islands. Following 2 poor breeding seasons, spruce-forest species declined markedly, while those of disturbed areas increased; consequently, the islands remained in equilibrium when taken as a group. Turnover rates of species averaged 16.7%/yr. Numbers of vagrants actually observed were adequate to saturate the islands, at least in some years. Overall density, biomass and metabolic demand generally changed inversely with forest size and exceeded those on the adjacent mainland. [The following birds are discussed: red-breasted nuthatch, brown creeper, golden-crowned kinglet, ruby-crowned kinglet, red-eyed vireo, black-and-white warbler, northern parula, yellow warbler, magnolia warbler, yellow-rumped warbler, black-throated green warbler, blackburnian warbler, Canada warbler, American red start, purple finch, winter wren, catbird, robin, hermit thrush, Swainson''s thrush, yellow throat, dark-eyed junco, white-throated sparrow and song sparrow.].