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Dead Birds Don’t Fly

bald_eagle.jpg
Illustration by Adelaide Tyrol

I see him high above the Wilder Dam, which spans the Connecticut River between Lebanon, New Hampshire, and Hartford, Vermont. With wings flat and primary flight feathers extended, he circles the edge of a thermal, head and tail white, body and wings brown. Around and around he flies, with eyes keen enough to take in the whole Upper Valley.

I no longer think of the open water below Wilder Dam as just another riverine attraction for wintering ducks. The arrival of the bald eagle changed all that. I first saw the big bird in December of 1981. The eagle, an immature bird, stayed for a few days and then vanished. Each year since, one or two bald eagles have returned to the Upper Valley for the winter. But our neighborhood was once not so hospitable; bald eagles were missing from the Upper Valley for nearly four decades.

By the late 1940s, wintering bald eagles had mysteriously disappeared from this stretch of the Connecticut River and from most of the United States. Use of the synthetic compound DDT (dichloro-diphenyl-trichloroethane) had increased dramatically after World War II to combat everything from mosquitoes to head lice, and researchers in the upper Midwest were convinced that DDT was causing problems for the eagles. Data from their studies confirmed that the dead and dying birds they were studying - robins, marsh hawks, and eagles - had large concentrations of DDT in their fat and brains. The smaller birds died outright from the chemical while the larger birds laid eggs with thin shells that broke under the weight of the incubating parent.

But was there evidence from the Northeast? Beginning in 1960, Betty Sherrard, an energetic naturalist, had fretted over the robins that convulsed and died each spring on her lawn in Hanover, New Hampshire. In 1962, she read Rachel Carson’s Silent Spring and suspected that DDT was to blame; every April, Dartmouth College in Hanover sprayed its beloved elm trees with DDT to try to control the spread of Dutch elm disease. Sherrard asked two Dartmouth College biochemists, Drs. Charles and Doris Wurster, to investigate the situation.

They agreed to help. For several days in April 1963, approximately 1,300 pounds of DDT were sprayed on more than 1,000 elms on the Dartmouth campus. Just across the river, the town of Norwich, Vermont, was not spraying their elms. The Wursters thus had the perfect setup for a scientific experiment: a control population of birds free from DDT just across the river from the affected population in Hanover.

The Wursters and two friends surveyed a 15-acre section of Hanover and an ecologically similar section of Norwich. They counted robins on their way to work, at lunch, and on their daily rounds between April 8 and mid-May. After mid-May, the counts were spaced at intervals of two and three days until mid-June. The results were stunning.

Nothing appeared to happen the morning after spraying the elms; the DDT took weeks to travel from the elms into the soil and into the earthworms. By mid-May, however, dead robins began to appear in Hanover, and all the casualties were found to have high levels of DDT in their tissues. Although Norwich had a few dead robins here and there, none had high levels of DDT.

The Hanover study area lost more than 70 percent of its breeding robins in 1963. Norwich’s study area, relatively free from the effects of the chemical, actually gained a few birds that year.

Not everyone was convinced by the results of the study. One California scientist claimed that the robins had simply flown across the river, thereby explaining the population decline in Hanover and the slight rise in Norwich. But dead birds don’t fly. Fortunately, the ground crew at Dartmouth heeded the Wursters’ warning and switched to a less-toxic compound, methoxychlor, in 1964. Although a few robins died that spring, the result perhaps of residual DDT, the population rebounded, and there was no evidence of the chemical in other species of birds.

Across North America, populations of bald eagles (as well as osprey, peregrine falcons, and brown pelicans) have staged a remarkable comeback. Today, several thousand pairs of eagles nest in the lower 48 states, many at sites that had been abandoned for decades. In 1988 (and every year since), along the shore of Lake Umbagog in northern New Hampshire, a pair has built their nest in the same towering white pine that had stood empty of eagles since 1949.

There are three eagles at Wilder Dam this winter. For that, we all owe a big thanks to the local researchers and naturalists who came to the eagles’ aid forty years ago this spring.

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