Area‐Dependent Changes in the Bird Communities and Vegetation of Southern Wisconsin Forests

We studied avian biogeography and habitat selection in forests of southern Wisconsin ranging in area from 3 to > 500 ha. Bird diversity in these woodlots increases with area, due primarily to an increase in the number of forest—dwelling, long—distance migrants. We consider two possible explanations for this pattern: (1) area—dependent changes in forest vegetation, or (2) area—dependent change in interactions with competitors, predators, or brood parasites. We first describe vegetation structure and composition, then show that this description comprises important habitat features of forest birds. Bird habitat is characterized in three ways: (1) vegetational structure within bird territories is compared with that at random locations in the same woodlots, (2) structural characteristics of territories of different species are compared, and (3) factors related to species' abundance in different woodlots are analyzed. We found no area—dependent trends in vegetation structure or composition that seem likely to influence the bird community. However, forest—edge and farmland species increase in density as woodlot area decreases. We suggest that forest—edge and farmland species exclude certain forest—dwelling, long—distance migrants from small woodlots, and that this exclusion influences the bird community more than area—dependent changes in habitat or the degree of woodlot isolation.