Think big to save birds on edge.

Bird lovers often take comfort in the small plots of forest left in their neighborhoods, thinking that these trees provide a haven for forest-loving birds.

But a new, large-scale study shows that small nature preserves fail to protect birds' nests from predators, such as cowbirds, which lurk along forest edges, report Scott K. Robinson of the state-run Illinois Natural History Survey in Champaign, Ill., and his colleagues.

Indeed, in some small woodland plots certain species now lose so many of their eggs and nestlings that their overall deathrates exceed their birthrates. Birds moving in from heavily forested areas help sustain the populations in the smaller forests, the scientists assert in the March 31 Science.

To do their study, Robinson and his many coworkers staked out more than 5,000 nests in five midwestern states between 1989 and 1993. They monitored eight species of neotropical migrant birds, which breed in North America and winter in the tropics, and observed nests of the northern cardinal. The species in the study have become scarcer in recent years, although none is yet in danger of extinction.

The nests occupied nine plots from 20 to 200 hectares in size on land ranging from more than 90 percent agricultural to more than 90 percent forested. Nests in areas with less forestation suffered higher rates of destruction and cowbird invasion, Robinson and his colleagues report.

Cowbirds lay their eggs in other birds' nests; the surrogate mothers raise the baby cowbirds and may neglect their own offspring. Nest predators include raccoons, snakes, and other creatures.

Previous studies have suggested that cowbirds and many nest predators avoid large, dense woodlands and instead frequent the smaller forests near farms and suburbia.

Most wood thrush nests in areas with less than 55 percent forest had several cowbird eggs in them, Robinson and his colleagues report. "In some landscapes, there were more cowbird eggs than wood thrush eggs per nest," they note. Every day, predators threatened the homes of warblers that nested near or on the ground in the most fragmented areas, which have only small patches of forest.

"Even the indigo bunting, which prefers forest edges, nests more successfully in less fragmented landscapes," the scientists report.

"Our results suggest that a good regional conservation strategy for migrant songbirds . . . is to identify, maintain, and restore the large tracts [of forest] that are most likely to be population sources," they conclude.

Other studies have also suggested that birds have better reproductive success in larger forests. Researchers see a "consistent, catastrophic loss of forest migrants" in smaller forests but not in woodlands larger than 100 hectares, Robert A. Askins of Connecticut College in New London notes in an accompanying article. Scientists have also found that old-growth forests attract a wider variety of species than do younger stands (SN: 6/18/94, p.399).

However, Askins adds, "although previous studies have suggested a [negative] relation between nesting success and the amount of nonforested habitat . . . Robinson and colleagues now report the first conclusive test of this hypothesis."

No one had previously monitored such a wide swath of the United States, says Ted R. Simons of North Carolina State University in Raleigh. The study by Robinson's team shows that the problem "is not just happening at a few isolated locations," he asserts.