Poor winter homes delay bird nesting.

A migratory bird that gets stuck with second-rate winter accomodations will achieve only lackluster breeding success the next summer, hundreds or even thousands of miles away, a new study suggests.

"Making that link before has been impossible to do," says Peter P. Marra of the Smithsonian Migratory Bird Center in Washington, D.C. He and his colleagues established the connection by using a technique that's novel among ornithologists: deducing where a bird has been by the carbon isotopes in its tissue. Isotopes are forms of atoms with different numbers of neutrons.

In the Dec. 4 Science, the researchers report that American redstarts that reach their New Hampshire summer breeding ground early--a big factor in nesting success--have isotope ratios typical of moist, food-rich winter habitats. Birds that arrive late, however, show ratios typical of poor, dry habitats.

"Things we're seeing in the breeding grounds are caused by events the previous winter," Marra explains.

Monitoring redstarts wintering in Jamaica, the researchers found that dominant males and the tougher females claim forest territories with abundant insects for good. An underclass of smaller, weaker redstarts gets pushed into scrub, which has fewer insects.

The researchers found that birds in prime winter territories increased or maintained their weight, but the birds in the slums lost up to 11 percent of their body mass. Come spring, redstarts in the poor habitat took longer to pack on fat for the flight north, leaving some 10 days later than birds in the forest.

The researchers could not track those particular birds north--a problem with many such studies. So Marra and his colleagues observed redstarts summering in central New Hampshire. Late-arriving males tended to be high in carbon-13, like the birds in the inferior winter habitats. There, birds ate insects that feed on dry-region plants, which typically build up extra C-13.

Trevor D. Price of the University of California, San Diego welcomes the report as the first evidence tying bad winters and bad summers in a way he and other researchers had suspected. "We've finally got some facts," he says.

Scott K. Robinson of the University of Illinois at Urbana-Champaign points out that the study "certainly indicates there isn't enough good habitat" for wintering redstarts. He predicts the results will bolster claims that problems in the tropics contribute strongly to declines of migratory birds. Some scientists still think breeding-ground problems are more important. However, Robinson says, "I think the jury is still out."