EFFECTS OF FOREST PATCH SIZE ON NESTING SUCCESS OF WOOD THRUSHES

Declines of many forest-dwelling Neotropical migrants have been attrib uted, in part, to fragmentation of forest habitat on the breeding grou nds in North America. During 1990-1991, we determined reproductive suc cess of Wood Thrushes (Hylocichla mustelina) nesting within contiguous forest habitat (>10,000 ha) and in nine forest fragments ranging in s ize from 9.2 to 126.5 ha in Berks County, Pennsylvania. We located 171 Wood Thrush nests. Nesting success differed significantly among fores t size categories, with 86% of the nests successful in contiguous fore st, 72% successful in large fragments (>100 ha), and 43% successful in small fragments (<80 ha). The variable that best predicted nest survi val was forest area (R(2) = 0.86). Rates of predation differed signifi cantly among forest size categories, and predation was the primary cau se of nesting failure. We found 56% of the nests in small fragments we re lost to predators as compared to 22% in large fragments and 10% wit hin 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 pred ators was significantly higher in small forest fragments than in the l arge forest sites ((x) over bar = 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 s ize categories and had little influence on nesting success. Our result s suggest that reproductive success of Wood Thrushes nesting within co ntiguous forest is high and that severe reproductive dysfunction as a result of high rates of nest predation is an important consequence of forest fragmentation.