World and Nation

Charting Bird Migrations by Using Tiny Backpacks

Birds are famous for airborne speed and endurance. Some have been clocked flying 60 mph or more. Others make annual migrations from Alaska to New Zealand, nonstop.

But for scientists, tracking birds as they perform those feats has been an intractable problem. Now researchers think they have cracked it with a novel device — a tiny bird backpack that contains sophisticated sensors and weighs less than a dime.

The new technology has opened up vast new possibilities for bird researchers. Already, it is yielding surprising findings — for example, that some birds fly even faster than previously thought. But its real importance, biologists say, is the opportunity to unlock mysteries of bird migration that could help preserve species threatened by habitat loss and climate change.

“We knew that purple martins went to Brazil and wood thrush went to Central America,” said Bridget J.M. Stutchbury, a biologist at York University in Toronto, who with colleagues fitted birds from the species with the sensors and mapped their migrations last year. “But the details of how an individual gets there, what routes they take, how fast they fly, how often they stop to rest — these are the kinds of details we have never been able to have.”

The research, reported Friday in the journal Science, involved 34 birds, but only seven were recovered with their sensors. Still, the work “is an important step,” said David W. Winkler, an ornithologist at the Cornell Laboratory of Ornithology, where he said researchers were developing similar techniques. “This represents a whole new level of accuracy,” Winkler said.

The tracking system relies on instruments called solar geo-locators that collect and store data on where the birds are in relation to the sun. Researchers remove the sensors, download the information and calculate where the birds were, and when they were there.

“If the bird were on a hillside you’d get a slightly wrong time,” Stutchbury said. “If it were a cloudy day you would get a slightly wrong time. But these devices are accurate enough, within 5 or 10 kilometers,” about 3 to 6 miles.

Bird migration is a subject of fascination for scientists and the public alike. Jacques Perrin’s 2003 film, “Winged Migration,” which used remote control gliders and ultralight aircraft to follow birds as they traveled the globe, attracted a large cult following. But while much is known about where birds nest and where they spend the winter, figuring out how they get from point A to point B has been a challenge that, overall, researchers have been unable to meet, especially for small species like songbirds.

Researchers have tried banding birds’ legs, tracking flocks with radar and even using satellites, all to little avail. The new system was developed by engineers at the British Antarctic Survey for use tracking wandering albatrosses, birds that inhabit the waters around Antarctica.

But the wandering albatross is about the size of a large dog, Stutchbury said. For her research, she needed instruments small enough and light enough for a tiny songbird. Then, at a 2006 conference, the British researchers said they had miniaturized their sensors to 1.5 grams. “That for me was a magic number,” she said.