Temporal changes in migratory bird use of edges during spring and fall seasons in Pennsylvania

Abstract

Edge created through forest fragmentation can have significant impacts on the avian community, increasing predation and nest-parasitism rates and changing species richness and abundance patterns near edges. Although considerable research has demonstrated edge effects during the breeding season, few studies have considered how proximity to an edge affects migrant communities in stopover habitat. We studied bird communities in southern Pennsylvania (Adams, Chester, and Montgomery counties) to determine if richness and abundance of migrants were impacted by edges during spring and fall 1999-2001. The three categories of study sites were north-facing forest-herbaceous edges, south-facing forest-herbaceous edges, and interior forest (>300 m from an edge); sites were located in Gettysburg National Military Park-Eisenhower National Historic Site (GETT-EISE) and Valley Forge National Historical Park. During fall migration, Nearctic-Neotropic migrant species richness was significantly (P = 0.03) higher in interior forest compared to edges, whereas species richness of other guilds was not significantly different among edge types. During spring migration, richness of both permanent residents (P < 0.001) and temperate migrants (P < 0.001) were higher at both edges compared to interior sites. Only two species, Dendroica caerulescens (Black-throated Blue Warbler) (P = 0.03 during fall, P = 0.002 during spring) and Vireo olivaceus (Red-eyed Vireo) (P = 0.03 during fall, P = 0.05 during spring), showed differences among edge types during both spring and fall migration. Although the parks differed in amount of forest and in landscape composition, differences in richness and abundance patterns of birds between the parks also may be based on active management of Odocoileus virginianus Zimmerman (White-tailed Deer) at GETT-EISE that has resulted in vegetation differences. Overall, we found differential impacts of fragmentation (edge) on guilds and during different migratory periods, illustrating the importance of considering habitat use during each season individually for conservation of migratory songbirds.