Separate vacations: birds winter apart but return in sync.

A mated pair of black-tailed godwits may fly off to separate wintering grounds a thousand kilometers apart, but they can return to their breeding grounds almost simultaneously, a migration survey has revealed.

The synchrony isn't surprising by itself, says Jennifer A. Gill of the University of East Anglia in England. Other shorebirds, such as pairs of black turnstones, do that. The surprise with godwits showed up because Gill and her colleagues had fitted the birds with color-coded leg bands and persuaded bird-watchers across Europe to report on the winter whereabouts of the birds. This massive amount of data revealed that the returning pairs weren't coordinating their returns the easy way--by spending the winter together. Instead, they ,typically spent the off-season in different countries, the team reports in the Oct. 7 Nature.

"It's very unusual to have this type of information," says Gill. And it reinforces her conviction that people seeking to protect migratory birds need to make sure they protect an ample network of habitats.

Almost all black-tailed godwits (Limosa limosa islandica) fly into the lowlands of Iceland to nest, starting about mid-April. The birds live up to 25 years, and about 90 percent of pairs mating one year reunite the next. The warm season doesn't last long, and in July, the birds start to leave for winter havens. Godwits spread along the coasts of Britain and Ireland, and some of them cross to mainland Europe.

Gill, Tomas Gunnarsson, also of East Anglia, and their colleagues invited dozens of bird-watchers to e-mail winter sightings of godwits and to note the colors of bands they wore. At the Icelandic breeding sites, birds arrived over a period of about a month. However, the researchers found that mates in 7 of 10 pairs of birds with known wintering locations arrived within 3 days of each other, even though these mates typically wintered 1,000 km from each other.

Two couples that had been mates the previous year arrived more than 8 days apart, and both pairs took new partners in Iceland. Breaking a long-term pair bond typically lowers the chances of nesting success, says Gill, and she proposes that the costs of divorce could have driven the evolution of synchronized arrival times. How the birds manage this timing isn't clear, says Gill.

The number of synchronized pairs in this study is small, cautions Allan J. Baker of the Royal Ontario Museum in Toronto. However, he considers the findings "extremely interesting" since other studies have demonstrated that some shorebirds have built-in clocks controlling their physiological preparations for migration. He speculates that mated pairs might have matched clocks.