REGIONAL FOREST FRAGMENTATION AND THE NESTING SUCCESS OF MIGRATORY BIRDS

Forest fragmentation, the disruption in the continuity of forest habit at, is hypothesized to be a major cause of population decline for some species of forest birds because fragmentation reduces nesting (reprod uctive) success. Nest predation and parasitism by cowbirds increased w ith forest fragmentation in nine midwestern (United States) landscapes that varied from 6 to 95 percent forest cover within a 10-kilometer r adius of the study areas. Observed reproductive rates were low enough for some species in the most fragmented landscapes to suggest that the ir populations are sinks that depend for perpetuation on immigration f rom reproductive source populations in landscapes with more extensive forest cover. Conservation strategies should consider preservation and restoration of large, unfragmented ''core'' areas in each region.