Tiny birds go the distance
Tiny songbirds such as martins and thrushes can travel as far as 311 miles a day in their annual migrations between the Americas -- three times as far as researchers had previously believed -- biologists found in the first study to track the birds to their wintering grounds and back.
The birds fly two to six times as fast heading north in the spring as they do heading south in the fall, perhaps in a competition to reach the best breeding sites and attract the fittest mates, ornithologist Bridget Stutchbury of York University in Toronto reported today in the journal Science, which released the study online Thursday.
One industrious female martin flew the 4,660 miles from the Amazon basin to Pennsylvania in only 13 days -- with four of them spent on stopovers.
The new data were obtained using miniature geolocators, about the size and weight of a dime, attached to the birds’ backs much like a schoolchild’s backpack. The same technology was used in 2006 by Scott A. Shaffer of UC Santa Cruz to demonstrate that shearwater gulls fly a huge figure-eight over the Pacific Ocean during their migration, traveling as much as 46,000 miles in a year.
But the gulls are much bigger. Stutchbury’s paper marks the first time a newly miniaturized version of the geolocator has been used on birds so small.
“The data are exciting. It makes me drool,” said ornithologist Peter Marra of the Smithsonian Migratory Bird Center at the National Zoo in Washington, who was not involved in the research. “But this is really a proof-of-concept paper for the geolocators on a smaller-bodied bird.”
“The floodgates are opening” for a much larger number of studies, Marra said.
Stutchbury, Marra and other researchers have already placed the geolocators on other small species, such as bobolinks, seaside sparrows and other types of thrushes, that they plan to recapture this spring.
Small birds have been extremely difficult to track. Radar can monitor large flocks for short distances, but not individual birds, and the birds are too small to track by satellite. Some researchers have attached radio transmitters and attempted to follow birds in airplanes. One German researcher was able to follow individual birds for as long as four days.
The new geolocators were developed by James W. Fox and his colleagues at the British Antarctic Survey. Each self-contained unit weighs about 1.3 to 1.5 grams and carries a computer chip and a light sensor mounted on a small stalk that sticks up above the feathers. (The birds themselves weigh about 50 grams.) The device measures and records the times of sunrise and sunset.
“For any given place on the planet, we know the times of sunrise and sunset, so we can match up the locations,” Stutchbury said. “It’s a very simple concept.”
On their return to their breeding grounds, the birds are captured and the data downloaded.
Preliminary studies showed that the devices do not interfere with the birds’ mobility or their ability to catch prey, mate and feed offspring.
Stutchbury and her colleagues attached the sensors to 14 wood thrushes and 20 purple martins at their breeding grounds in northwestern Pennsylvania. The next spring, the researchers were able to recapture five thrushes and two martins. Two more thrushes came back but nested too high for the team to recapture.
As for the rest, “we have no idea what happened to them,” Stutchbury said. “We don’t know whether it was just bad luck or the birds were slowed down by the weight of the devices.”
The martins flew about 1,500 miles south to the Yucatan Peninsula in five days, then stopped over for three or four weeks before continuing on to the Amazon basin.
The thrushes spent one to two weeks in the Southeastern United States before continuing on to their wintering grounds in a narrow strip along the east coast of Honduras or Nicaragua.
“We were amazed to see that these birds did not scatter on the wintering grounds. We had always imagined they would,” Stutchbury said.
On the return flight, most of the birds stopped on the Yucatan Peninsula before making the 13- to 14-hour nonstop flight across the Gulf of Mexico to the U.S. Gulf Coast.
One thrush, however, “took the long way around, flying over land all the way and arriving in the breeding grounds very late,” Stutchbury said. She speculated that the bird might not have been able to accumulate sufficient stores of body fat for the flight because of environmental problems at its wintering site.
The research was sponsored by the National Geographic Society.
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