SCOTT SIMON, host: The Swainson thrush is a hardy songbird. It can fly strenuously for eight hours without anything to eat or drink, without becoming dehydrated. Scientists know how the birds get their energy for these long migratory flights, but were they get their water has been a bit of a mystery until now.

NPR's Joe Palca reports.

JOE PALCA: Swainson's thrushes spend their winters in South and Central America. But as spring approaches, they fly north to Canada.

ALEXANDER GERSON: The majority of them are nocturnal migrants, so they'll fly through the night for up to eight hours.

PALCA: Alexander Gerson is an avian researcher at the University of Western Ontario.

GERSON: As spring migration approaches, these birds start to eat a lot, and they put on a lot of fat. So they can almost double their body mass in fat.

PALCA: And that provides the fuel they need for the migratory flights.

GERSON: Fat is very energy dense, so you can carry a light load of fat and it'll provide a lot of energy.

PALCA: But burning fat doesn't get you much water. And any long-distance athlete will tell you that dehydration is the real enemy, not hunger. Unlike humans, birds don't sweat. But they do lose water when they breathe. Their flights are strenuous and long - 200 miles or so at a time - so the birds breathe heavily and water loss adds up. How do the birds replenish their water while they're flying?

Gerson says reports from the field provide a clue. Biologists were finding birds that actually lost muscle mass between migratory stops and, in some cases, their internal organs had shrunk.

GERSON: The hypothesis was that these birds might be using their lean mass, their protein, which is their muscles and organs, as a source of water.

PALCA: This would kind of make sense, biochemically speaking. Burning muscle tissue gives a bird about five times as much water as burning fat. Gerson had a way to test this hypothesis. There's a large wind tunnel at the University of Western Ontario. It's the avian equivalent of a treadmill. Put birds in the tunnel, turn on the wind, and the birds can fly for hours without going anywhere.

As Gerson and his colleagues report in the journal, Science, the birds that flew in conditions where they needed more water also lost more muscle mass.

Now if you think about it, it may seem like a mistake to burn away the muscles that you need to fly. Wouldn't it be easier to just take a pit stop? Gerson says that for the price of a bit of muscle, birds get the freedom to pick the best migratory route.

GERSON: So these birds can take advantage of tailwinds and not have to worry about becoming dehydrated.

PALCA: Scientists have suspected that burning muscle might be the secret for birds staying hydrated. But avian researcher Scott McWilliams, at the University of Rhode Island, says until now they didn't have proof.

SCOTT MCWILLIAMS: So it was surprising in the sense that they designed a nifty experiment to demonstrate that that could be the case.

PALCA: It's likely that other small birds, like the Swainson's thrush, burn protein to make water during flight. What's not yet clear is whether bigger, heavier birds would do the same thing.

MCWILLIAMS: You have something like a goose that would be one of the larger species and, yeah, they could very well do very different kinds of things, depending on the migration distance.

PALCA: In other words, what's good for the goose may not be the same as what's good for the thrush.

Joe Palca, NPR News, Washington.

SIMON: You're listening to WEEKEND EDITION from NPR News.

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