Findings not only pinpoint where the songbirds spend their time
but also reveal that they can travel faster than expected.
By Jackie Grom (excerpt from article in ScienceNOW Daily News)
12 February 2009
With the help of tiny position-sensing backpacks, for the first time
researchers have tracked songbirds throughout their annual migration
information may provide clues to what is causing the decline of some
Weighing in at between 40 and 50 grams, songbirds are too small for satellite tracking often used to monitor larger animals such as wolves, elephants, and whales, and too weak to carry long-term tracking devices. So researchers investigating migration have been limited to techniques such as tracking flocks of songbirds with radar over short distances and studying them in their stopover locations. Based on these studies, researchers were able to figure out when and where songbirds bred, rested, and spent their winters, but they could not nail down where individual birds went or determine their long-term rate of travel.
That's now changed, thanks to a serendipitous find. When Bridget Stutchbury, a biologist at York University in Toronto, Canada, attended an ornithology conference in 2006, she noticed that researchers were tagging a geolocator device to the legs of albatrosses to study their movement. The researchers had a 1.5-gram device that was light enough to strap to songbirds, says Stutchbury, but, because of the size of the birds, it would need to be tied to their backs, not their feet.
Over the winter, Stutchbury worked with the British Antarctic Survey, which had developed the geolocators, to optimize them for songbird use. In August of 2007, she and her team strapped geolocator backpacks to 14 wood thrushes and 20 purple martins. When they recaptured five of the wood thrushes and two of the purple martins the following spring, they were surprised to see that one of the birds made it from the Amazon to Pennsylvania in just 13 days, flying at an average of 577 km per day--about three times faster than estimates for other songbird species. And, as the researchers report today in Science, the return flights are anywhere between two and six times faster than in the fall, allowing the birds to beat their comrades to mating grounds.
The purple martins stopped over in the Yucatan Peninsula for 3 to 4 weeks before heading down to Brazil, where they, unlike the wood thrushes, continued to move around. Purple martin populations have not fallen as much as those of wood thrushes have, and this increased mobility, which gives them the ability to choose from different habitats, might explain why, Stutchbury says.
"It's never been quite clear whether [the declines are] happening in
one season or the other," but the new tracking technique "will help to
give us a better understanding of what's ultimately leading to those
population declines," says behavioral ecologist Dustin Rubenstein of
the University of California, Berkeley. This work helps to "catapult us
out of ignorance," adds physiological ecologist Martin Wikelski of the
Max Planck Institute for Ornithology in Radolfzell, Germany. However,
he cautions that the technology provides information on only the
songbirds that make it back to their breeding grounds. To find out why
some songbird species are declining, we need to find out what is
happening to the unsuccessful birds, he says.