The Fragmented Forest

Brood Parasitism and Nest Predation

Among the primary causes of lowered reproductive success of forest-interior birds in fragmented habitats are increased rates of both brood parasitism and nest predation. Brood parasites, notably Brown-headed Cowbirds (Molothrus ater), are birds that do not build nests or raise young on their own; instead, they lay eggs in a host bird's nest (Figure 4). The cowbird nestling generally hatches first, outcompetes the host's nestlings for food, and receives the majority of the parental care. As cowbirds expanded their historic range from midwestern prairies into newly created agricultural land and fragmented forests in the East, they encountered many new potential host species (Robinson et al. 1995). Forest birds that have had no long-term exposure to cowbirds often do not have mechanisms for rejecting cowbird eggs and are particularly vulnerable to parasitism. Researchers believe that parasitism by Brown-headed Cowbirds is a significant factor contributing to declining numbers of many songbirds in North America (Robinson et al. 1995).

Figure 4. Brown-headed Cowbird nestlings grow rapidly, frequently outcompeting the host's nestlings for food and parental care. This adult Common Yellow-throat is feeding a cowbird fledgling that's more than twice its size. Photo by John Gavin.

In addition to parasitic cowbirds, nest predators such as jays, crows, raccoons, and domestic and feral cats that are not usually found in extensive forests gain access to the interior of forest patches via roads, power-line cuts, and other openings. Studies consistently show that nest predators have a greater affect on the reproductive success of forest birds in forest fragments than in contiguous forest (Wilcove 1985, Small and Hunter 1988).