Wood Thrush Declines Linked to Acid Rain
Citizen scientists collected key data


A Wood Thrush forages for pillbugs, earthworms, and snail shells. The birds need these calcium-rich foods to produce viable eggs. Because acid rain leaches calcium out of the soil, researchers suspect that Wood Thrush population declines are linked to the rarity of calcium-rich food in areas receiving acid rain.
John Schmitt

A recent study from the Cornell Lab of Ornithology has implicated acid rain in the troubling population declines of the Wood Thrush. Published in the Proceedings of the National Academy of Sciences, the study was the first large-scale analysis to show a clear link between acid rain and the widespread decline of a songbird.

The study's broad scope was made possible in part by citizen-science participants of the Lab's Birds in Forested Landscapes project, who collected data at more than 700 study sites across the Wood Thrush's breeding range.

The Wood Thrush's Paradox
The Wood Thrush is a migratory bird that breeds in eastern North America, from the Midwest and Plains states to the Atlantic, and from southern Ontario and Quebec to eastern Texas and northern Florida.

Between 1966 and 1999, Wood Thrush populations declined 1.7 percent per year across the species' breeding range. But the cause of the decline was a mystery.
Like many birds, the Wood Thrush breeds more successfully in large forested areas than in smaller patches. In small patches, Wood Thrushes lose more eggs and young to predation and are parasitized more often by the Brown-headed Cowbird, a species that lays its eggs in the nests of other birds. But the Lab's previous work on Wood Thrushes demonstrated no significant negative effect of habitat fragmentation.

Also, paradoxically, some of the steepest declines occurred where Wood Thrushes are most abundant, in areas along the slopes of the Appalachians, from the Adirondacks and White Mountains in the North to the Smoky Mountains in the South. As it turns out, those areas typically receive high levels of acid rain.

Acid rain and the calcium connection
Acid rain occurs when water in the atmosphere combines with pollution, mainly oxides of sulfur and nitrogen created by burning coal, oil, gasoline, and other hydrocarbons. Acid rain affects the landscape on which it falls in numerous ways, including leaching calcium out of the soil.

A female songbird requires 10 to 15 times more calcium to lay a clutch of eggs than a mammal of similar size requires to nurture its embryos. Birds that eat mainly insects or seeds usually depend on calcium-rich supplemental foods, such as snail shells, which are rare in acidified areas.

In regions with heavy acid rain and calcium-poor, well-drained soils, as in some parts of the Netherlands, the soil is so leached of calcium, and snails so rare, that some bird species suffered complete reproductive failure. Eggshell defects may be one of the causes of tremendous declines in avian biodiversity in heavily-polluted regions in the Netherlands.
Small-scale studies have shown negative impacts of acid rain on North American aquatic birds and terrestrial birds such as the Tree Swallow and the Eastern Kingbird. But no effects as severe as those documented in the Netherlands have been seen on this side of the Atlantic.

Occurence of acid rain, based on data from the National Atmospheric Deposition Program. Regions depicted in red received the highest levels of acid from precipitation; gray areas received lower levels. Note the high levels of acid rain in New England, the Adirondacks, the Allegheny Plateau, and the Smokies. Wood Thrush populations are declining in many parts of these regions.
Merging the data
The Lab's Birds in Forested Landscapes project (BFL) asks citizen scientists to collect data on breeding forest birds and habitat characteristics, to help assess the effects of forest fragmentation and other human influences on birds. Initially, acid rain was not a specific focus of the study. But at the suggestion of Diane E. Black, a BFL paticipant and retired scientist from the Environmental Protection Agency, Lab scientists began an investigation using the BFL data.

To address the effects of acid rain on the distribution of the Wood Thrush, data from several other sources were necessary. To estimate the amount of acid deposited by rainfall at each site, the research team used data collected by the National Atmospheric Deposition Program's National Trends Network. This network monitors a number of inorganic substances that fall from the sky while dissolved in rain, including the chemicals that may acidify raindrops.

Because the abundance of the Wood Thrush varies across its range, Lab scientists also needed a measure of how likely it was to find a Wood Thrush by chance, based on geographic location alone. These data were provided by the Breeding Bird Survey, a volunteer-based survey of more than 3,500 routes for breeding birds each spring, coordinated by the United States Geological Survey and the Canadian Wildlife Service.
Finally, the study required an estimate of the pH of the soil (a measure of its acidity and, hence, the availability of calcium). This was provided by a soils properties database called STATSGO, compiled by the Natural Resources Conservation Service.

The study's authors combined all of these data using GIS, a type of computer software used for mapping and studying data that vary at different locations. They then created a statistical model that predicted the probability that a Wood Thrush would attempt to breed at a given BFL study site based on the acidity of the rain there. The model also took into account soil pH, vegetation characteristics, regional abundance of Wood Thrushes, and the amount of habitat fragmentation in the surrounding landscape.

The study found that increases in the acidity of rain were accompanied by strong and statistically-significant decreases in the probability of breeding by the Wood Thrush at a given site. Also, the negative effects of acid rain were magnified in fragmented landscapes, and at higher elevations. Rain generally increases with elevation, so it makes sense that in areas with acid rain, more acid would be deposited at higher elevations.

But the combined negative effects of acid rain and forest fragmentation were somewhat surprising, because they were greater than what would have been expected if the magnitudes of the two impacts were simply added together.

Looking ahead
The Lab's research team is now conducting field studies to investigate how acid rain and habitat fragmentation factor into Wood Thrush declines. Lab scientists took to the woods at 40 sites across New York state this summer, collecting soil to test its nutrients and calcium availability. In an attempt to understand the connections between Wood Thrushes and calcium limitations, they surveyed for breeding Wood Thrushes and sampled for potential sources of calcium-snails and slugs. The fieldwork also served as a test of sampling techniques, laying the groundwork for further research by Lab staff or citizen scientists.

The research team hopes to formulate a protocol soon to enable citizen-scientists to help decipher how acid rain and calcium are intertwined with the fate of breeding birds in eastern North America. Participants of The Birdhouse Network (formerly The Nestbox Network) have already furthered our knowledge about birds' need for calcium, as part of the Lab's Birds and Calcium project, completed in 1998. Participants offered calcium supplements to birds and noted which species took the supplements, and when. Through the Lab's programs, we hope that volunteer citizen scientists will continue to play an important role in helping us to understand how human-caused environmental change affects breeding birds, and in working for their conservation.

Citation: Hames, R. S., K. V. Rosenberg, J. D. Lowe, S. E. Barker, and A. A. Dhondt, 2002. Adverse effects of acid rain on the distribution of the Wood Thrush Hylocichla mustelina in North America. Proceedings of the National Academy of Sciences 99:11235-11240

Suggested citation: Chu, Miyoko and Stefan Hames. Wood Thrush Declines Linked to Acid Rain. Birdscope, newsletter of the Cornell Lab of Ornithology, Autumn 2002. <>

For permission to reprint all or part of this article, please contact Miyoko Chu, Editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, New York. Phone (607) 254-2451. Email