Wood Thrushes in Decline

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."

Wood Thrush and Mercury

A study of netted songbirds in upstate New York has revealed alarmingly high rates of mercury poisoning. Mercury was found in the blood of 178 bird species, with the highest being in wood thrushes. Until the recent studies on landbirds, most of the concern about mercury pollution was focused on fish and their predators. Now we need to worry about land-based sources of the toxin also.

Dr. Evers’s work suggests that when mercury falls on land, it is absorbed by soil and by fallen leaves that are consumed by worms and insects. Songbirds then feed on the bugs, absorbing the mercury.

While all the birds he tested last year had mercury in their blood, wood thrushes had the most, Dr. Evers said, an average of 0.1 parts per million. That is below the federal safe standard for fish (0.3 p.p.m.) but high enough to affect the birds’ reproductive cycle.

With fewer songbirds to eat potentially harmful insects, the state’s forests would be at greater risk for damage by gypsy moths and other pests, Dr. Evers said.

As the article notes, wood thrushes have been in steep decline, up to 45 percent since the 1960s. Habitat fragmentation has been blamed in the past as the primary culprit. Results from this study suggest that mercury pollution is part of the problem as well.

I am not quite sure how this differs from a story produced last year on the same research. It seems that the National Wildlife Federation may be raising the story's profile again. Another study published last year found high levels of mercury and methymercury in Bicknell's Thrushes and Yellow-rumped Warblers.

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• Wood Thrush
• Mercury
• Pollution

Mercury and the Wood Thrush

Biologists in New York state are netting wood thrushes in the Catskill Mountains and testing them for mercury levels. The study is meant to assess the future health of the New York City water supply. Mercury is already present to some degree within the reservoirs due to pollution drifting from coal-fired power plants in the midwest. (Thus the state warns against fish consumption from lakes and streams even in the Catskills.) There is danger that levels of mercury and other contaminants could increase if birds like the wood thrush decrease due to mercury poison. Thrushes are one of many species that keep the insect population under control; barring that control, the ability of forests to filter pollutants out of the watershed could be greatly reduced.