Scientists equipped song birds, like this purple martin, with small geolocators on their backs so they could study their migrations. [Credit: Timothy J. Morton]
Songbirds are familiar creatures, markers of the seasons. Their voices ring in the spring, and we note their absence in winter. But no one has known exactly where they go during the cold months or how fast they travel — until now.
In February, a team of researchers at York University in Toronto, Canada, conducted a study that used new technology to follow two species of songbirds — purple martins and wood thrushes — on their long migrations to and from South America. “We were the first people to be looking at a map of songbird migration,” said Bridget Stutchbury, a conservation biologist who led the study.
“My primary interest is conservation,” she said. She believes that this kind of study will help researchers identify key stopover points along the migration path. Conservationists can then target these areas.
Besides the migration route, Stutchbury’s study revealed that these birds fly back to the United States up to three times faster than anyone thought they could. One purple martin covered the 4,660 miles in only 13 days, traveling over 300 miles per day. The data shattered previous estimates that the birds would top out at 100 miles per day.
The study began during the summer of 2007 in northern Pennsylvania, where both species of birds breed. The researchers used tiny harnesses to strap light-sensing devices called geolocators on the backs of 20 purple martins and 14 wood thrushes. The geolocators look like small green computer chips, and they weigh less than a dime — under five percent of the birds’ body weight.
Unlike global positioning systems, these geolocators don’t continuously beam data back to a satellite, so researchers must recover the devices to get the data. In the summer of 2008, five purple martins and two wood thrushes returned to the Pennsylvania breeding grounds with their geolocators intact.
During migration, the geolocators measured light levels and recorded the time of each measurement. Stutchbury and her team then reconstructed the birds’ migration paths by matching the cycles of sunrises and sunsets with the time stamps taken by the geolocators.
“We got a nice tight map and really clean data, so we were thrilled,” Stutchbury said.
“It’s incredible, it’s what so many people have been waiting for, for a very long time,” said Abraham Miller-Rushing, a biologist at the University of Maryland who studies how animal populations respond to climate change. He was not involved in Stutchbury’s project.
The life cycles of seeds, fruits and insects that songbirds eat have been changing with the warming climate, Miller-Rushing said. Bird species that have adjusted by starting their annual migration earlier have been more successful than those that have stuck to their old migration habits. Birds that miss peak food production times can starve.
Researchers know that bird migration paths are linked to climate, said Miller-Rushing, but it’s difficult to figure out how. Up to this point, scientists that observed a changed route had to guess whether the birds were leaving at an unusual time or traveling at an unusual speed. “This tool will be able to answer those questions,” he said.
The first avian geolocators were used to track albatross — large birds that fly over vast expanses of sea. Stutchbury collaborated with the engineers at the British Antarctic Survey, a government-funded research group, to miniaturize the device they created so that a songbird could carry it.
“There is a limit to what songbirds can carry before they’re at a disadvantage in the food chain,” explained Steven Bremner, the head of engineering at the Survey. “The smaller we can make it, the more birds we can tag.”
The technology is timely, Stutchbury believes, because so many bird species are imperiled by habitat destruction, climate change and toxic pollution. “It’s across the board — forest birds, thrushes, migratory birds, they’re crashing,” she said.
The National Audubon Society reports that populations of the top 20 on its list of common bird species in decline in the United States have dropped by an average of 70 percent since 1967. The report blames human encroachment, pesticides and deforestation, but nobody understands how species are changing their behavior to cope with these problems. “That’s where this geolocator comes in,” said Stutchbury.
Backyard bird watchers are also thrilled about the new data, for reasons besides the new information about global warming. “Most of us feel that these birds have an inherent value simply because they are part of the ecosystem,” said John Tautin, the director of the Purple Martin Conservation Association, a non-profit organization that has collaborated with researchers since its formation about 20 years ago.
Many members of the association are “purple martin landlords.” They build and maintain birdhouses that serve as the only habitat for this species east of the Rocky Mountains. Purple martins began switching to man-made homes in the 1600s when northeastern Native American tribes hung hollow gourds where the birds could nest. Now, birds in this region no longer build homes in the wild.
“The purple martin landlords love their birds. They look forward to seeing them come back every year,” said Tautin. “But there’s always been this mystery about when they arrive in the spring.”
Stutchbury plans to use the geolocators to solve other puzzles too. Next, she wants to compare the songbird migration route against weather data to see how individual birds change course in the face of storms and prevailing winds.
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