A female Purple Martin wears a geolocator on her back. The device weighs less than a dime. Photo courtesy of Tim Morton.
Migrating Purple Martins can fly up to 358 miles (577 km) per day, and Wood Thrushes can cover 168 miles (271 km) per day, according to groundbreaking new research published today in the journal Science. Previous studies estimated songbirds could fly at roughly 93 miles (150 km) per day.
Bridget Stutchbury, a professor of biology at York University in Toronto, and her colleagues from the Purple Martin Conservation Association and the British Antarctic Survey mounted miniaturized geolocators on 14 Wood Thrushes and 20 Purple Martins in Pennsylvania in 2007.
The devices, which weigh about 1.5 grams, measure light continuously, allowing researchers to estimate birds' latitude and longitude by recording sunrise and sunset times. The units tracked the birds' fall migrations to Central America and South America and the journey back to Pennsylvania. In summer 2008, the scientists recaptured five thrushes and two martins and retrieved their geolocators. Then they reconstructed the individual migration routes and wintering locations.
“This is the first time anyone in the world has been able to map songbird migration routes to the tropics and back,” Stutchbury said.
These maps, from Stutchbury's paper, show the geolocation tracks of individual Purple Martins (A and B) and Wood Thrushes (C and D) that bred in northern Pennsylvania in 2007. BLUE: fall migration. YELLOW: winter range movements. RED: spring migration. Dotted lines link locations when latitude could not be determined. Inset shows winter territory locations of tracked Wood Thrushes (dots) and the winter range for the species (shaded); the standard deviation for one individual is shown. [Image © Science/AAAS]
Songbirds are too small for satellite tracking devices, which are used to study movements of falcons and other large birds. Thrushes have also been studied using radio transmitters, but to be effective, researchers who use them have to stay in radio contact with the birds by car or air.
The new study, funded by the National Geographic Society and from proceeds of Stutchbury’s 2007 book Silence of the Songbirds, found that songbirds' overall migration rate was two to six times more rapid in spring than in fall. For example, one martin took 43 days to reach Brazil during fall migration, but in spring it returned to its breeding colony in only 13 days. Rapid long-distance movement occurred in both species, said Stutchbury.
"We were flabbergasted by the birds' spring return times. To have a bird leave Brazil on April 12 and be home by the end of the month was just astounding. We always assumed they left sometime in March," she said.
Researchers also found that prolonged stopovers were common during fall migration. The martins, which are members of the swallow family, had a stopover of three to four weeks in the Yucatan before continuing to Brazil. One martin stayed in the Amazon Basin all winter. The other martin stopped at the basin and then flew about 600 miles south toward Bolivia. Stutchbury said its movement suggests that martins use multiple roosts in winter.
A male Wood Thrush wearing a geolocator attends young at his nest. Stutchbury said the devices did not prevent the birds from mating, building nests, or caring for chicks. Photo courtesy Elizabeth Gow.
Four thrushes spent one to two weeks in the southeastern United States in late October before crossing the Gulf of Mexico, and two individuals stopped on the Yucatan Peninsula for two to four weeks before continuing migration.
The study also uncovered evidence that Wood Thrushes from a single breeding population did not scatter over their tropical wintering grounds. All five birds wintered in a narrow band in eastern Honduras or Nicaragua.
"This region is clearly important for the overall conservation of Wood Thrushes, a species that has declined by 30 percent since 1966," said Stutchbury. "Songbird populations have been declining around the world for 30 or 40 years, so there is a lot of concern about them."
The researchers hope to learn more about the martins and thrushes this spring. They attached geolocators to 20 martins and 35 thrushes in 2008, and they’re hoping that at least half of the birds come back to Pennsylvania.
Other researchers are also using the devices, Stutchbury said. “The floodgates are open. I know a group that’s putting them on Bobolinks. I know a group that’s planning on putting them on Bicknell’s Thrushes. Most of the motivation for using these geolocators at the moment is the conservation value.”
Since the 2007 study began, the developers of the devices at the British Antarctic Survey have shaved a few tenths of a gram off the weight, which will enable them to be used on smaller birds. Stutchbury mentioned Eastern Kingbird, Olive-sided Flycatcher, and Veery as likely candidates for future studies.
She emphasized the importance of the research not only to protect at-risk species of songbirds but also to gauge environmental concerns.
"Tracking birds to their wintering areas is also essential for predicting the impact of tropical habitat loss and climate change," she said. "Until now, our hands have been tied in many ways, because we didn't know where the birds were going. They would just disappear and then come back in the spring. It's wonderful to now have a window into their journey." — M.M.