Long-term datasets reveal declines in many populations of landbird migrants, with declines especially evident in shrub-nesting species. Moreover, transitional shrub-dominated habitats are becoming scarce, reducing breeding habitat for these species. Because migrants often occupy habitats en route similar to those used in the breeding season, the persistence of shrub-dominated habitats may be especially critical for these species during passage. Because little is known about migrant-habitat relationships, especially at inland, unforested stopover sites, we compared the importance of forested and upland shrub-dominated habitat to spring migrating landbirds by comparing: (1) temporal and habitat-related variation in resource abundance, and (2) migrant use of forested vs. shrub-dominated habitat during stopover. We collected data on bird-habitat relationships and invertebrate abundance from both habitats at two sites in or near Lackawanna State Park, Lackawanna County, Pennsylvania. We captured and counted more short-distance and long-distance migrants in shrub habitat (256 birds per 1000 mist-net hours and seven birds per 100 m of survey transect) than in forested habitat (67 birds per 1000 mist-net hours and three birds per 100 m of transect), and shrub habitat contained a somewhat greater diversity of migrants (H′ = 4.0 vs. 3.9). We also measured greater flying invertebrate biomass in shrub vs. forested habitat, although there was no difference in flightless invertebrate biomass between the two habitats. Our results support the hypothesis that migrant habitat use reflects spatial differences in resource abundance and indicate that shrub-dominated habitats may be important stopover habitat for landbird migrants.
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