The wood thrush has lost 43 percent of its population over the past 40 years, making it a species of conservation concern. As with other species in decline, the main culprit seems to be the loss and fragmentation of woodland habitats, largely due to development. Ongoing research at the University of Delaware compares the breeding success of wood thrushes at a university woodland preserve and in a nearby neighborhood.
Roth discovered that loss of habitat hasn't deterred the remaining wood thrushes from summering in Delaware. However, many must try to breed in less than desirable conditions; in the 1980s, Roth studied a sizable population of wood thrushes in the wooded Newark neighborhood of Arbour Park. These birds had the usual two to three nests with three to four eggs per year, but raised significantly fewer young than wood thrushes breeding in the UD Woods, located just two miles away. And, unlike the thrushes at the UD Woods, few of the banded adults in Arbour Park returned there the next year, which was more likely an indication of bird dissatisfaction than mortality.
"UD Woods' leaf litter fosters excellent food resources, such as larval and adult insects, earthworms and millipedes. And on the edge of the woods, the birds can forage for berries from spicebush, elderberry and other shrubs," notes Roth. "A suburban neighborhood often doesn't provide such an abundant food supply, much less native trees and shrubs for nesting and shade."
While many nests were lost to predation at Arbour Park, the nests there also were more frequently parasitized by the brown-headed cowbird. It lays its eggs in the nests of wood thrushes and some other song birds, often removing a host's egg for each one laid. The unwitting host incubates the cowbird eggs and feeds the cowbird chicks along with her own. The negative effect on wood thrushes is the loss of eggs.
"Cowbirds are more apt to parasitize nests in small forest fragments, wooded neighborhoods and other edge habitats created by humans than in deep deciduous forests, where the wood thrush thrives," says Roth. "Don't blame the cowbirds; we created the favorable conditions for them."