REPRODUCTIVE SUCCESS OF MIGRATORY BIRDS IN HABITAT SOURCES AND SINKS

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 thre e forest-nesting migrants: Ovenbird (Seiurus aurocapillus), Red-eyed V ireo (Vireo olivaceus), and Wood Thrush (Hylocichla mustelina). We sur veyed birds and monitored reproductive success on 28 study plots in fr agmented and contiguous forests in true midwestern regions. Distributi on of individuals between fragmented and contiguous forests appeared t o vary among species and regions, but total nest failure was significa ntly higher in fragments than contiguous forests in both regions for a ll species (p = 0.053). We attributed greater nest failure to increase d nest predation (p = 0.093) and increased brood parasitism by the Bro wn-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 o f nest mortality appeared to be species specific, total nest failure a nd partial nest failure acted in concert to reduce the number of offsp ring per adult for all three species on fragments. We used simple popu lation growth models to assess the viability of the three species in f ragmented and contiguous habitats in both regions. In general populati ons on fragments appeared to be population sinks and populations on co ntinguous forests appeared to be population sources. Assuming constant mortality during winter, projected growth indicated that without immi gration Ovenbird and Red-eyed Vireo populations should become extinct on fragments in both regions and Wood Thrush populations should be mai ntained or slightly decline on fragments. Populations of all three spe cies should increase in contiguous habitats in both regions without em igration. We suggest that habitat fragmentation reduces local reproduc tion and may have ramifications for the entire population. A clear und erstanding of population demography depends on examination of demograp hic dynamics within and among sources and sinks. We emphasize that the long-term viability of these species depends on maintaining large tra cts of forest throughout the breeding range until the spatial scale at which source and sink populations interact can be determined.