Acid soil blamed for thinning eggshells.

DDT became infamous in the 1960s and 1970s for causing birds to produce eggs with dangerously thin shells. The United States and many European countries have since banned the pesticide. Now, calcium deficiency due to acid soil appears to have similar effects on birds, according to a new study of a common European species of passerine known as the great tit.

An increasing number of great tits and other passerines living in forests with nutrient-poor, acidified soils in the Netherlands are producing eggs with thin and porous shells, researchers reported in 1989. A new team has unraveled the series of mishaps responsible for those defects, it reports in the March 31 NATURE.

After ruling out poisoning as the culprit, the scientists concluded that the birds suffer from lack of calcium in their diet because of the scarcity of snail shells on which they normally dine, writes Jaap Graveland of the Institute for Forestry and Nature Research in Wageningen and his colleagues.

"Snails have all but disappeared on acid, sandy soils," says coauthor Arie J. van Noordwijk of the Netherlands Institute of Ecology in Heteren.

The researchers blame the declining snail population on the decreased amount of calcium in the soil, which results from acid rain. The soil's calcium content is 0.3 gram per kilogram of dry matter; 5 to 10 grams is normal, Noordwijk says.

Adding lime to acidified forest soil boosts the snail population to levels comparable to those found in areas with rich soil, other studies find. And when Graveland's team provided the birds in nutrient-poor localities with shells from snails and chicken eggs, the proportion of nests with at least one defective shell dropped from about 65 percent to about 20 percent, the researchers report. Another source of calcium - spiders provides only 10 percent of a laying bird's daily requirement, even in areas with rich soil, they find.

"To our knowledge, this is the first experimental evidence for calcium limitation in wild birds," they contend. It also reveals a previously overlooked mechanism by which acidification harms species' nutritional status, they write.

Studies of North American birds that eat snail shells also suggest that fledglings may suffer from deficiencies, says David Winkler of Cornell University, who praises the Dutch study. The researchers from the Netherlands did not look at fledgling health, but they report that they found few shell fragments in the nests for the fledglings to eat.

The study "illustrates what to most people would be an unexpected link between acid rain and bird populations," says Kenneth V. Rosenberg, also of Curnell. "It's something that could go completely unnoticed." Europeans monitor their bird populations much more closely than Americans do, he added.

Though a poor substitute to any snail lover, chicken eggshells provide nourishment for birds from the impoverished forests. They raid local farms and picnic sites for these eggshells to try to make up for their lack of snails, the researchers report.

"They may fly up to a mile or more to get extra calcium," says Noordwijk.