Fragmentation of breeding habitat in North America has been implicated in the decline of forest-nesting, Neotropical migrant birds. We used a comparative approach to examine the effects of fragmentation on three forest-nesting migrants: Ovenbird (Seiurus aurocapillus), Red-eyed Vireo (Vireo olivaceus), and Wood Thrush (Hylocichla mustelina). We surveyed birds and monitored reproductive success on 28 study plots in fragmented and contiguous forests in true midwestern regions. Distribution of individuals between fragmented and contiguous forests appeared to vary among species and regions, but total nest failure was significantly higher in fragments than contiguous forests in both regions for all species (p = 0.053). We attributed greater nest failure to increased nest predation (p = 0.093) and increased brood parasitism by the Brown-headed Cowbird (Molothrus ater, p = 0.009). In addition to greater total nest failure partial nest failure due to cowbird parasitism led to a reduction in the number of host fledglings. Although the causes of nest mortality appeared to be species specific, total nest failure and partial nest failure acted in concert to reduce the number of offspring per adult for all three species on fragments. We used simple population growth models to assess the viability of the three species in fragmented and contiguous habitats in both regions. In general populations on fragments appeared to be population sinks and populations on continguous forests appeared to be population sources. Assuming constant mortality during winter, projected growth indicated that without immigration Ovenbird and Red-eyed Vireo populations should become extinct on fragments in both regions and Wood Thrush populations should be maintained or slightly decline on fragments. Populations of all three species should increase in contiguous habitats in both regions without emigration. We suggest that habitat fragmentation reduces local reproduction and may have ramifications for the entire population. A clear understanding of population demography depends on examination of demographic dynamics within and among sources and sinks. We emphasize that the long-term viability of these species depends on maintaining large tracts of forest throughout the breeding range until the spatial scale at which source and sink populations interact can be determined.
