Effects of forest patch size on nesting success of wood thrushes

Declines of many forest-dwelling Neotropical migrants have been attributed, in part, to fragmentation of forest habitat on the breeding grounds in North America. During 1990-1991, [the authors] determined reproductive success of wood thrushes (Hylocichla mustelina) nesting within contiguous forest habitat (>10,000 ha) and in nine forest fragments ranging in size from 9.2 to 126.5 ha in Berks County, Pennsylvania. [They] located 171 wood thrush nets. Nesting success differed significantly among forest size categories, with 86% of the nests successful in contiguous forest, 72% successful in large fragments (>100 ha), and 43% successful in small fragments (<80 ha). The variable that best predicted nest survival was forest area (R² = 0.86). Rates of predation differed significantly among forest size categories, and predation was the primary cause of nesting failure. [The authors] found 56% of the nests in small fragments were lost to predators as compared to 22% in large fragments and 10% within the contiguous forest. Visitation by mammalian predators to scent posts was significantly different between small and large forest sites (41 vs. 14%, respectively), and relative abundance of avian nest predators was significantly higher in small forest fragments than in the large forest sites (x̄ = 1.04 vs. 0.41 birds per census point). Brown-headed cowbirds (Molothrus ater) parasitized 9% of the nests. Rates of brood parasitism did not differ significantly among forest size categories and had little influence on nesting success. [The authors'] results suggest that reproductive success of wood thrushes nesting within contiguous forest is high and that severe reproductive dysfunction as a result of high rates of nest predation is an important consequence of forest fragmentation.