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Masters of Flight: Songbirds' High-Speed Massive Migration

A new light-sensing backpack has mapped songbird migration routes for the first time



TIMOTHY J. MORTON

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In the midst of a brutal winter, it makes sense that anyone would want to head south ASAP, including, it seems, some common American songbirds, which new research shows, zip to winter hotspots far speedier than previously believed. Researchers report in Science today that, for the first time, they were able to map the entire flights – some of which are more than 4,600 miles (7,500 km) – of wood thrushes and purple martins from their summer North American breeding grounds to their winter homes in balmy South and Central America.

Perhaps even more astonishing, says lead study author Bridget Stutchbury, a biology professor at York University in Toronto, is that the return trip was even faster: One purple martin flew from Brazil to Pennsylvania in just 13 days, clocking more than 310 miles (570 km) a day.

Of the billions of charming chirpers in yards, forests and fields across the continent, very little is known about the great migrations many of them make semiannually. Martin Wikelski, director of the Max Planck Institute for Ornithology's Department of Migration and Immuno-ecology in Radolfzell, Germany, says he suspected the critters were fast, but says the study "marks a new era for migration research." For the first time, he notes, scientists were able to map the entire route of these songbirds, whose population have declined by as much as 30 percent over the past 40 years. He estimates that some five billion songbirds take the intercontinental trips each year.

To conduct the study, which was backed in part by the National Geographic Society, researchers captured 20 purple martins and 14 wood thrushes in their respective breeding grounds in northern Pennsylvania in 2007 and outfitted them with tiny geolocators (a small device that records and stores data on daylight), which were strapped on around their legs like teensy bird backpacks. Early trials found that the accessory, which weighs about 0.05 ounces (1.5 grams) or about 3 percent of the birds' body weight, didn't affect flying, nesting, mating or food-gathering behavior, Stutchbury said during a teleconference.

When the birds returned in spring 2008, the scientists were able to recapture two purple martins and five wood thrushes equipped with the mini data packs. Researchers were able to glean from the information in the geolocators – which provided clues on the time of day based on the levels of sunlight recorded – the birds' approximate location within about 180 miles (300 km) during their journeys.

"The conservation implications of this research are enormous," Stutchbury said, noting that it will provide insight into where the birds winter as well as the habitat threats they may face there. Conservationists have speculated that tropical deforestation, pesticides and climate change might be affecting the winter habitats of the birds, and forest fragmentation and loss of grasslands might be a problem in the breeding grounds. And a sub-par habitat, if it's not providing enough food, rest or shelter, can make for weaker birds that might not be able to make the grueling journey – especially those that the research showed take a shortcut over the Gulf of Mexico, which is a 12-to-14-hour non-stop flight.

A clearer understanding of bird migration routes can also help monitor the spread of bird-borne diseases as well as better prevent collisions between birds and aircraft, Wikelski says.

Until now, there has been no way to track these diminutive birds as they make their international treks because Global Positioning System (GPS) devices are too big and bulky, and small radio transmitters only work short-range. Currently, Wikelski notes, larger birds such as osprey and other raptors are some of the best-understood, because they're large enough to carry more sophisticated equipment – some of which can even monitor heart rate and environmental information.

Wikelski calls this study, "a major step forward" but says it's "just the beginning" of such work. He hopes that researchers will soon be able to create GPS tracking or other more specific devices puny enough to fit onto these birds and, eventually onto hummingbirds and even large insects.

In the meantime, Stutchbury and her team are eagerly awaiting the return of geolocators that were strapped on the backs of 35 wood thrushes and 20 purple martins last summer.

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