In groundbreaking research, Canadian scientists have for the first time tracked the flight paths of migrating songbirds by fitting them with tiny geolocators carried in miniature backpacks.
"Never before has anyone been able to track songbirds for their entire migratory trip," said lead investigator Bridget Stutchbury, a professor of biology at Toronto's York University.
"One aspect of the research is looking at really this miracle of migration and how these little birds are able to make these long trips," Stutchbury said. "And these geolocators allow us for the first time to do a start-to-finish map."
Songbirds fly thousands of kilometres between their breeding and wintering grounds during the annual migration. But existing technologies like radio-frequency or satellite-tracking weren't suitable for tracking birds small enough to fit in the palm of a hand.
Stutchbury's team worked with the British Antarctic Survey to design a miniaturized geolocator — weighing less than a dime — that could be easily carried on the backs of songbirds in order to chart their migration routes and wintering destinations.
The research began in summer 2007, when the team captured 14 wood thrushes and 20 purple martins at their breeding grounds in Pennsylvania, then strapped on the geolocators before the birds took wing for Central and South America.
The devices are mounted on the birds' backs by looping thin straps around their legs. The geolocator rests at the base of the bird's spine to avoid interfering with its balance.
Backpacks don't slow birds down
"We know that birds wearing geolocators have successfully mated and laid eggs ... and were able to find food and feed their young," Stutchbury said in a teleconference arranged by the National Geographic Society. "Our observations showed that these birds were not slowed down or showed any signs of even knowing they were carrying a backpack."
Unlike global positioning systems that use satellites to pinpoint the location and movement of a vehicle, for instance, geolocators have an onboard sensor that records light levels at sunrise and sunset.
And since the times of sun-up and sunset are known for any given day anywhere on the planet, Stutchbury said, "we can match up the sunrise and sunset times to the location where that bird must have been on that day."
When bird flocks returned last spring, the scientists were able to retrieve geolocators from five wood thrushes and two purple martins and analyze the data to map individual migration routes and wintering locations.
What they found was a surprise, say the researchers, whose report is published in this week's issue of the journal Science.
More than 500 kilometres per day
The data showed that songbirds can fly more than 500 kilometres in a single day, far more than the 150 kilometres per day that previous studies had estimated.
What's more, the researchers discovered that songbirds' migration rate was two to six times faster when they headed north in the spring — to breed — compared with their southerly trip in fall. For example, one purple martin took 43 days to reach Brazil during fall migration, but in spring returned to its breeding colony in just 13 days.
"So this bird was able to fly over 7,000 kilometres in under two weeks," she said. "It's really stunning."
Researchers also found that prolonged stopovers were common during fall migration. The purple martins, members of the swallow family that catch insects on the wing, lingered for three to four weeks in the Yucatan Peninsula before continuing on to Brazil.
Four of the wood thrushes, which are forest dwellers, spent one to two weeks in the southeastern United States in late October, before crossing the Gulf of Mexico; two others stopped in the Yucatan for two to four weeks before taking to the skies again.
The team has already begun the second round of their research. Last summer, the scientists outfitted 35 wood thrushes and 20 purple martins with geolocators. Now they wait to see what spring will bring.
But the research isn't only about charting migration routes — it's also about species conservation, Stutchbury said.
Wood thrush numbers have declined by about 30 per cent since the mid-1960s, just one example of how songbird populations worldwide have plummeted over the last 30 to 40 years.
"They're in such a tailspin that there's a real sense of urgency to use geolocators to try to identify what's causing the problems," said Stutchbury, adding that scientists need to understand whether changes in habitat in either the breeding or wintering grounds are "driving the numbers down."
Ryan Norris, a biologist at Ontario's University of Guelph who studies migratory animals, said the successful use of geolocators should have "a significant impact" on future songbird research.
"Up until now, the ability to track animals over large distances has really relied on rough estimations," said Norris, who was not involved in the study. "Geolocators look to be more accurate."
While the research shows that individual songbirds can be tracked using the devices, he said the challenge is to map migratory pathways across the entire range of a single species.
"So the next question is where do the other breeding populations winter? Do they mix with the Pennsylvania population or do they winter in another separate area?" he said. "And that's important for conservation decisions and how to allocate resources for conservation."