Bird 'backpacks' help scientists discover the longest oversea migration
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Version 0 of 1. A tiny North American songbird migrates 1,500 miles non-stop over the Atlantic, scientists have discovered, in the longest known oversea journey for any land bird. The blackpoll warbler weighs only as much as a £2 coin and normally lives in forest environments, but once a year it embarks on a perilous three-day journey, scientists have found. By fitting the birds with tiny geolocator backpacks, they were able to track their route from North America and Canada to Puerto Rico and Cuba and then onwards to South America. Chris Rimmer, an ornithologist at the Vermont Center for Ecostudies and a co-author of the study, said: “There is no longer any doubt that the blackpoll undertakes one of the most audacious migrations of any bird on earth.” Although other migratory birds such as the albatross and arctic tern are known for travelling thousands of miles across oceans, the blackpoll warbler has the added challenge of making the journey without stopping, as a water landing would be fatal. “It’s a fly-or-die journey,” said Ryan Norris, the Canadian team leader at the University of Guelph, said. Other North American songbirds that travel south in winter take a less risky, continental route south through Mexico and Central America. However, ornithologists suspected the blackpoll warbler was taking a water route, based on observations of birds landing on boats during stormy Atlantic weather. It had also been noted that the birds put on significant amounts of weight in the autumn – sometimes almost doubling in size – hinting that they were preparing for a long journey ahead. “They get basically so fat that they don’t move,” said Norris. Until now, there was no conclusive proof that they were migrating across open ocean, however. “Some people said there’s no way they could do this,” Norris added. “They don’t seem particularly athletic birds.” In the latest study, published in Biology Letters, scientists equipped 20 birds in Vermont and 20 more in Nova Scotia with tiny backpacks fitted with a clock and a sensor to measure ambient light levels. The number of hours of daylight gives the bird’s latitude and the time of sunrise gives longitude. But the packs, weighing just 0.5g, were too small to include transmitters, meaning that the researchers had to search for the birds back at their nesting ground in the North American forest the following year. “These birds come back every spring very close to the same place they used in the previous breeding season, so with any luck you can catch them again,” said Norris. “Of course there is high mortality among migrating songbirds on such a long journey; we believe only about half return.” The team worried that the extra 0.5g – added to the bird’s roughly 12g body weight – could make an already challenging journey insurmountable. In the end, they were able to recapture three birds from the Vermont group and two from the Nova Scotia group for analyses. “It was pretty thrilling to get the return birds back, because their migratory feat in itself is on the brink of impossibility,” said Bill DeLuca, of the University of Massachusetts Amherst and the paper’s first author. The locators showed that the blackpolls took a route directly over the Atlantic, with distances ranging from 1,410 to 1,721 miles (2,270 to 2,770 km). Grahame Madge, of the RSPB, said the findings help explain why the songbirds have occasionally been seen in Britain: records show 45 sightings, mostly in the Scilly Isles, since the 1960s. “It’s quite easy to imagine how a storm system could sweep them up,” he said. “They set out thinking they’re heading for the Caribbean and end up in Europe.” |