Twice a year, bar-tailed godwits migrate more than 7,000 miles so they can spend their summers in Alaska and their winters in New Zealand. Bar-headed geese fly about 2,000 miles between Mongolia and India, traveling at altitudes high enough to clear the top of Mount Everest.
Such flights are physically draining, requiring birds to expend enormous amounts of energy without stopping for food or water. For years, scientists have wondered how they do it.
Now researchers think they've figured out how birds stay hydrated on their marathon journeys.
Observing tiny songbirds as they flew for hours in a state-of-the-art wind tunnel, biologists at the University of Western Ontario in Canada discovered that the animals conserve water by burning muscle and organs instead of fat.
The protein in muscle doesn't provide as much energy as fat, but it can release five times as much water -- enough to keep birds going during their night-long flights, according to a study published Friday in the journal Science.
The idea that birds get water from their own muscles and organs isn't entirely new, but until recently scientists haven't had a way to test the theory, said Marcel Klaassen, an integrative ecologist at Deakin University in Australia who was among the early proponents of the idea.
That changed in 2009, with the completion of the world's most advanced avian wind tunnel at the University of Western Ontario. It resembles a "giant steel doughnut" 150 feet long and two stories high, said study leader Alexander Gerson, a graduate student in biology. Researchers can control flight conditions inside the wind tunnel, including humidity, temperature and simulated daylight and altitude.
In addition, scientists can now measure the amount of fat, protein and water in birds' bodies using a noninvasive technique called quantitative magnetic resonance analysis. QMR uses a technology similar to magnetic resonance imaging, or MRI.
"The reason this was possible is because we have a lot of fancy equipment," Gerson said.
The songbirds used in the study were Swainson's thrushes -- tiny creatures that weigh just over 1 ounce and stand only 4 or 5 inches tall. They breed in northern Canada and spend their winters in South America, migrating twice a year on an intercontinental path that takes them over the arid American Southwest.
For their experiment, biologists captured thrushes on the north shore of Lake Erie, Gerson said. They analyzed the body composition of each bird in the QMR device before putting the animals into the wind tunnel, letting them fly for an average of about 2 1/2 hours, and then measuring body composition again at the end of the flight.
Birds flew one at a time and completed flights at 80 percent relative humidity and at 10 percent relative humidity.
It wasn't easy to get birds to cooperate. Scientists tried to test 27 thrushes but were able to collect sufficient data on only five. Most went on strike, hunkering down on the bottom of the massive machine, Gerson said.
"They either want to fly, or they don't," he mused.
But the results gathered from the compliant birds demonstrated uniformly that the thrushes were burning muscle and organs to get water, Gerson said. No birds became dehydrated, and all burned significantly more protein tissue under dry conditions than under wet conditions. Blood tests confirmed the measurements.
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"It was surprising to see such a huge difference," Gerson said.
Researchers who weren't involved in the experiment praised its design. "They're showing that environmental variables directly affect the strategy of fuel use of birds during flight," said University of Rhode Island ecologist Scott McWilliams.
Klaassen said that understanding how birds use protein during migration could help scientists better understand the environmental challenges facing animals that migrate.
He added that it was "nice to be right" about the birds' water-conservation strategy.
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