Birds' eggs started to thin long before DDT.

By combing museum collections for old birds' eggs, a researcher has found that thrush eggshells in Great Britain were thinning by the turn of the century, 47 years before DDT hit the market.

The pesticide, now banned in most countries, caused such dramatic shell thinning that populations of peregrines, ospreys, and other top predators began to decline.

Long before DDT was a glimmer in a farmer's eye, some other menace, as yet unknown, was sapping the strength of eggshells, claims Rhys E. Green of the Edinburgh office of the Royal Society for the Protection of Birds. In the April 22 Proceedings of the Royal Society for the London, B, he describes long, slow shell declines.

Eggshell thinning may have been an early consequence of industrialization, Green speculates. Acids formed when pollutants belch out of coal furnaces and smokestacks may have changed soil and water chemistry enough to reduce the availability of calcium, which is critical for eggshells. "Calcium can be a particularly bad pinch point," Green says.

Other research has linked acidified soil to frail eggs, he notes. J. Graveland of the Netherlands Institute of Ecology at Heteren reported in 1994, for example, that great tits lay weaker eggs as calcium-rich snails dwindle on acidified soils.

In an unusually broad survey, Green measured three museums' collections of well-labeled eggshells gathered since 1850 from four thrush species in Great Britain. Eliminating bad eggs still left Green several thousand to measure. "You get quite quick," he says.

From the weight and dimensions of the shells, he calculated an index of thickness similar to those developed for monitoring DDT effects. For two species, he also measured shell thickness by fitting a thin probe through the hole made to remove the material inside the egg in preparation for museum storage.

Both the index and the direct measurements show steady declines. Blackbirds (not the U.S. blackbird but Turdus merula, a cousin of the U.S. robin) have lost 7 to 10 percent on the shell index since 1850. Eggs from song thrushes thinned 6 percent, mistle thrushes 4 percent, and ring ouzels 2 percent. Only the ring ouzel leaves Great Britain for long migrations.

The research did not look for effects of weakened shells on bird populations, but Green points out that other species have withstood eggshell declines of up to 15 percent.

"I think he probably has something," says wildlife toxicologist D. Michael Fry, director of the Center for Avian Biology at the University of California, Davis. "There is a lot of scatter, but the trend is significant."

Acidification as a threat to eggshells sounds plausible to Lloyd Kiff, now science director at the Peregrine Fund in Boise, Idaho. More than 2 dozen recent studies show acid effects on songbirds abroad, but no research has been done in the United States, he says. "I don't know why someone hasn't picked it up and run with it," he adds. "We have similar conditions here."