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 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. 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. 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.