Breeding Birds of Isolated Woodlots: Area and Habitat Relationships

We investigated breeding bird communities of isolated woodlots (1.8—600 ha) in east—central Illinois during three summers (1979—1981) to compare the influence of area and habitat on community structure. Woodlots supported from 9 to 43 species and composition was relatively constant among years. Ecological generalists dominated small woodlots, while more specialized species increased in importance with area. Area accounted for most variation (86—98%) in total species number in each year and the species—area relationship did not change significantly among years. The amount of variance accounted for by area was greater than in previous studies. Neither habitat nor woodlot isolation explained significant additional variation in total species richness after area. Area accounted for most variation in number of species in different migratory and breeding habitat categories, except for short—distance migrants, which correlated most strongly with habitat. Variation in habitat was not related to woodlot area and habitat accounted for additional variation in bird species numbers in most cases. Abundances of one—third to one—half of species examined correlated with woodlot area, but a greater proportion (66—72%) were influenced more strongly by habitat variables. Results from Illinois support previous conclusions that species that breed in forest interior habitat and winter in the tropics are most likely to be adversely affected by a reduction in forest habitat. Results also show that bird communities in isolated tracts of forest are not random assemblages, but rather that species found in smaller woodlots are subsets of species found in larger forests.