Effects of Forest Patch Size on Nesting Success of Wood Thrushes

Abstract

-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, we 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. We located 171 Wood Thrush nests. 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. We 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 (x = 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. Our 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. Received 6 July 1993, accepted 21 November 1993. MANY FOREST-DWELLING NEOTROPICAL migrant songbirds have undergone population declines in eastern deciduous forests of North America (Robbins et al. 1989a, Askins et al. 1990). Most declining forest species are found less commonly than expected in small areas of forest than in large (area sensitive), and some of these songbirds have decreased in abundance or disappeared completely from small forest patches (Ambuel and Temple 1983, Blake and Karr 1984, Askins et al. 1987). Because forest fragmentation results in smaller forest patches or fragments separated from each other by nonforest habitat, these declines have been attributed, in part, to fragmentation of forest habitat in North America (Robbins 1979, Askins et al. 1990). Numerous hypotheses have been proposed to explain why forest-dwelling Neotropical migrants decrease in abundance or disappear from small forest patches (Askins et al. 1990). As a forest becomes fragmented, the amount of edge habitat increases and the amount of interior de3Present address: Illinois Natural History Survey, 607 E. Peabody Drive, Champaign, Illinois, 61820, USA. creases. Mammalian nest predators such as eastern chipmunks (Tamias striatus) and raccoons (Procyon lotor; Bider 1968, Forsyth and Smith 1973) tend to be more abundant along the forest edge than in the forest interior. Whitcomb et al. (1981) suggested that avian nest predators, such as Blue Jays (Cyanocitta cristata) and American Crows (Corvus brachyrhynchos), show a similar pattern. The Brown-headed Cowbird (Molothus ater), a brood parasite, also is more abundant along edges (Brittingham and Temple 1983). In addition, rates of both nest depredation and brood parasitism are higher near forest edges than within the forest interior (Gates and Gysel 1978, Chasko and Gates 1982, Brittingham and Temple 1983, Temple and Cary 1988). Consequently, one hypothesis is that forest birds decline in number in small forest patches as a result of poor reproductive success due to high rates of predation on eggs and nestlings, and to brood parasitism (Brittingham and Temple 1983, Wilcove 1985, Askins et al. 1990). Experiments with artificial nests indicate that nest predation is more frequent in small forest patches than within extensive areas of forest, thus supporting this hypothesis (Wilcove 1985,