NESTED SUBSETS AND THE DISTRIBUTION OF BIRDS ON ISOLATED WOODLOTS

Distribution of bird species among isolated habitat patches (e.g., woodlots in an agricultural landscape) often appears to be nonrandom; species present in small, species-poor patches also are found in larger patches that support more species. Bird communities form "nested subsets" (after Patterson & Atmar 1986) if all species found in small faunas also are found in more species-rich assemblages. Occurrence of a nested subset pattern implies an underlying, nonrandom pattern of species distributions. I used computer simulations to analyze the degree of nestedness exhibited by bird communities in east-central Illinois. Results demonstrated that the distribution of bird species breeding in isolated woodlots (1.8 to 600 ha) differed significantly from that predicted by a random distribution model; species assemblages were more "nested" than expected by chance. Most species present in small, species-poor woodlots also were found in larger, species-rich woodlots. As groups, species requiring forest interior habitat for breeding and species wintering in the tropics showed highly nested distributions. In contrast, short-distance migrants and species breeding in forest edge habitat showed more variable distribution patterns; species recorded on smaller woodlots were not always recorded on larger, more species-rich woodlots. Apparent absences from larger woodlots may have reflected real distribution patterns or insufficient sampling of edge habitats. These results support previous conclusions that small habitat patches are insufficient for preservation of many species.