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Shedding Light on Migration Connectivity

2009-10-08 18:14:45.0

Kent McFarland

Miniature, light-sensitive devices weighing less than a dime were strapped to the backs of Purple Martins and Wood Thrushes in their Pennsylvania breeding grounds and have yielded the first clear picture of songbird-migration routes and over-wintering areas.


The study, reported in the journal Science, marks the first successful tracking of songbird-migration routes and wintering areas and also shows that scientists have significantly underestimated their flying ability.


"Never before has anyone been able to track songbirds for their entire migratory trip," said study author Bridget Stutchbury, a professor of biology in York’s Faculty of Science & Engineering. "We’re excited to achieve this scientific first."  Songbirds, the most common type of bird in our skies, are too small for conventional satellite tracking.


Stutchbury and her team mounted miniaturized geolocators on 14 Wood Thrushes and 20 Purple Martins, breeding in Pennsylvania during 2007, tracking the birds’ fall takeoff, migration to South and Central America, and journey back to North America. In the summer of 2008, they retrieved the geolocators from five wood thrushes and two purple martins and reconstructed individual migration routes and wintering locations from the stored daylight timing data.


Data from the geolocators indicated that songbirds can fly in excess of 311 miles per day. Previous studies estimated their flight performance at roughly 90 miles per day. Spring migration rate was two to six times more rapid than in fall.For example, one martin took 43 days to reach Brazil during fall migration, but returned to its breeding colony in only 13 days.


"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 purple martins, which are members of the swallow family, had a stopover of three to four weeks in the Yucatan before continuing to Brazil. Four wood thrushes spent one to two weeks in the southeastern United States in late October, before crossing the Gulf of Mexico, and two other individuals stopped on the Yucatan Peninsula for two to four weeks before continuing migration.


The geolocators, weighing less than a dime and measuring only a half-inch long and a quarter-inch wide, detect light, allowing researchers to estimate the birds' latitude and longitude by recording sunrise and sunset times. The plastic-covered devices are mounted on the birds' backs by thin teflon straps looped around their legs, in a similar fashion to a day hiker’s backpack.


The geolocators can be small because they don't transmit a signal. They only collect and store data. The birds must be captured when they return to their breeding grounds to allow researchers to remove the devices and retrieve the precious data.

Stutchbury credits researchers with the British Antarctic Survey for miniaturizing the geolocators. "They hadn’t really been thinking of [attaching them to] songbirds, but when I saw the technology, I knew we could do this," she said.


The study also uncovered evidence that wood thrushes from a single breeding population did not scatter over their tropical wintering grounds. All five wood thrushes 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."


"Tracking birds to their winteringareas is also essential for predicting the impactof 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."


VCE Biologist Roz Renfrew knows this feeling all too well. "Of all the Bobolink flock sightings in South America over the years, they only account for about 5-10% of the entire breeding population. We don’t have a clue where up to 95% of the Bobolinks are wintering, " exclaimed Renfrew.


Renfrew aims to help change that using geolocators. She will be attaching them to Bobolinks across their breeding range, from Vermont to Oregon, to evaluate how migration routes and wintering grounds may differ among breeding populations. And by combining geolocator data with feather isotope levels (see the last issue of Field Notes), she will be able determine their diet among the different wintering locations.


During her recent expeditions to South America studying Bobolinks Renfrew found that some feed in native grasslands, but many dine in rice fields. "Geolcators will tell us whether birds from Oregon dine at the same restaurants as those from Vermont," says Renfrew. "If one population eats more rice than another, those birds might be in serious trouble."


VCE biologists have been lucky enough to capture two Bicknell’s Thrushes that they banded in Vermont on their wintering grounds in the Dominican Republic. But where are the other 50,000 or so thrushes wintering and how do they get there and back? Geolocators will help answer these mysteries.


VCE has teamed up with the Smithsonian Migratory Bird Center at the National Zoo, the Canadian Wildlife Service, Bird Studies Canada, and the University of New Brunswick, all members of the International Bicknell’s Thrush Conservation Group, to deploy geolocators on 120 male thrushes this summer across the breeding range from the Catskills to the Gaspe in Quebec across to Cape Bretton in Nova Scotia.


"The hardest part is going to be the wait," says VCE Director Chris Rimmer. "After we deploy them, we have to wait until the summer of 2010 to recapture them and download the light data. That will make for a long winter of patience."

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