There is a growing concern about recent population declines in forest breeding songbirds in North America. The absence of many of these species from small forest tracts has led to questions about the effects of forest fragmentation on songbird reproductive success. Increased nest predation and brood parasitism by brown-headed cowbirds appear to be associated with increased forest fragmentation. There is, however, a paucity of data on the impact of cowbird parasitism and nest predation on the reproductive success of individual species of forest-nesting birds. The purpose of this study is to examine the population dynamics of the wood thrush, a rapidly declining forest songbird. The factors determining reproductive success on three study sites in a moderately fragmented region, the Shawnee National Forest of southern Illinois where cowbird parasitism and nest predation rates are high will be studied. These data will be used to evaluate a model of population dynamics that proposes that some populations (sources) regularly produce surplus offspring which immigrate to other populations (sinks) which would otherwise become extinct as a result of low reproductive success. This model will be evaluated at regional, local and temporal scales.
