Population declines of migratory songbirds have been attributed to either breeding-ground factors based on forest fragmentation leading to increased rates of nest predation and nest parasitism or wintering-ground factors based on tropical habitat loss and degradation. We analyzed patterns of habitat use by Nearctic-Neotropical migrants, temperate migrants, and permanent residents in isolated deciduous-forest patches (0.25–3400 ha) in Pennsylvania during 1994–95 to determine distribution patterns and to identify the primary cause of these declines. Nearctic-Neotropical migrants were found primarily in larger forest patches (≥ 40 ha) compared to small patches (≤ 2 ha). In contrast, with few exceptions, temperate migrants and permanent residents were found equally in small and larger forest patches. Based on predictions by Rappole and McDonald (1994) to determine the cause of population declines, our results indicate that wintering-ground factors most likely are responsible. However, we suggest that such results should be viewed with caution. Specifically, we recommend that patterns of habitat use during migration should be studied more thoroughly to elucidate the potential impacts of stopover-habitat modification on population declines of migratory birds.