Is Acid Rain Killing Off Wood Thrushes?

Robert Winkler
for National Geographic News
August 13, 2002
Acid rain may be forgotten, but it is not gone, and now researchers at
the Cornell Laboratory of Ornithology have linked it to the decline of
the wood thrush, a forest bird known for its beautiful song.

The wood thrush breeds in the eastern United States and southeastern Canada, and winters from central Mexico to Panama. Like many neotropical migrants, its population has been declining—nearly 2 percent a year between 1966 and 2000, according to Breeding Bird Survey (BBS) data compiled by U.S. and Canadian wildlife agencies.

Using data from the BBS, from government studies of acid rain and soil acidity, and from a Cornell Lab study of forest fragmentation, the researchers did a statistical analysis and found that increased amounts of acid rain make wood thrushes less likely to breed.

An article on the finding appears in the August 12 issue of Proceedings of the National Academy of Sciences.

"I was surprised that we uncovered a negative effect this big," said Stefan Hames, an ecologist and ornithologist at Cornell, and one of the article's five coauthors. "Like many people, I thought acid rain was a non-issue."

Robert Angus Smith, a British chemist, coined the term "acid rain" in 1852, but it didn't enter the public consciousness until the 1980s, when its destructive effects on trees and lakes caught the attention of the media. Acid rain results when sulfur and nitrogen compounds—products of fossil fuel combustion—rise into the atmosphere and combine with water. Prevailing winds may carry acid rain far; much of the acid rain that has fallen in the Northeast, for example, originated in power plants of the Midwest.

In the United States, the Clean Air Act has brought about a reduction in the emissions that cause acid rain, but as Hames and his colleagues point out, "many eastern regions of North America continue to experience heavy, wet acidic deposition, and many bird species breeding in these areas show unexplained population declines. Further, long-term acid deposition has depleted the available calcium in acid-sensitive soils, and current emission standards may be insufficient to ensure the recovery of these soils."

Despite rising awareness of acid rain's more insidious effects, such as calcium depletion in soil, research on declining populations of North American birds has continued to focus on the dangers of habitat loss and fragmentation. Indeed, the Cornell study claims to be the first in North America to present large-scale evidence linking the population decline of a land bird to acid rain.

The situation is reversed in Europe, where, perhaps owing to centuries of habitat fragmentation, researchers have been looking at more immediate threats: industrialized farming practices and pollution, including acid rain. Their studies of calcium depletion raised a number of red flags that the Cornell team suspects may apply to the wood thrush.

Acid rain, for example, can cause calcium to leach from the soil. The loss of this nutrient jeopardizes the breeding success of birds—to produce a clutch of eggs, a female bird may require up to 15 times more calcium than a pregnant mammal of equivalent size.

In areas where acid rain is most severe, the supplementary calcium-rich foods that female songbirds depend on—snail shells, isopods such as pill bugs, millipedes, and earthworms—may be in short supply. Lacking adequate calcium in their diet, females are more likely to lay eggs that are thin, brittle, and porous. If the weakened eggs can withstand the rigors of incubation, the parent birds will be hard-pressed to meet the very high calcium requirements of their growing nestlings.

Aside from depleting calcium, acid rain in soil can promote increased levels of potentially toxic aluminum, cadmium, and lead. Polluted soil, moreover, may slow the decomposition of leaf litter, which reduces the diversity and abundance of prey.

The Cornell study adds that "even more subtle effects caused by decreased winter survival of fledglings or lower return rates of adults in acidified regions would be sufficient to generate the patterns seen in the wood thrush."

Hames emphasizes, however, that patterns are all we have thus far.

"What's crucial for us as scientists is to understand the process that leads to these patterns," he said. "Our findings regarding the wood thrush are correlational. We need more focused studies to get down to exactly what is going on."

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