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If you've ever looked at a flock of birds passing overhead, you've probably noticed that they tend to travel in the shape of a V. Well, scientists writing in today's issue of the journal Nature say they now know why. NPR's Geoff Brumfiel has this story of high-flying research.

GEOFF BRUMFIEL, BYLINE: You may have heard that birds fly in V's to save energy, that they're drafting like bicycle racers. And for a long time, scientists have thought the same thing. But they didn't have any proof.

STEVEN PORTUGAL: All it was was theory. No one ever actually was able to measure anything.

BRUMFIEL: That's Steve Portugal from the Royal Veterinary College in the U.K. Portugal is part of a lab that's built special devices that can fit on a bird's back and record the stuff you need to know if you're into bird physics. These gadgets have GPS and accelerometers. But they don't have transmitters. The only way to get hold of the measurements is to take the device off the bird's back.

And this meant Portugal needed just the right birds to study formation flying: birds that fly in a V, take off and land in a predictable place, who don't mind getting handled by people. And that's where these guys come in.

(SOUNDBITE OF NORTHERN BALD IBIS)

BRUMFIEL: Meet the Northern bald ibis. That's its mating call, and it looks about as ugly as it sounds.

PORTUGAL: Only a mother could love them, perhaps. But they are - they're very endearing. And they're just a bit different.

(LAUGHTER)

BRUMFIEL: Actually, the Northern bald ibis has a lot of fans in Europe. It disappeared from the wild there more than 300 years ago, and now, conservationists are trying to bring it back.

(SOUNDBITE OF BIRDS CHIRPING)

BRUMFIEL: These baby ibises are given human parents right after they hatch.

PORTUGAL: The human foster parents spend every day, every waking moment of every day with the ibis.

BRUMFIEL: Once the ibises are big enough to fly...

PORTUGAL: Then these foster parents hop in a Microlite, these little lightweight planes that you sort of see roaming around. And the birds then follow the Microlite because they're following their foster parent. And they teach them the migration routes that they would have done historically.

BRUMFIEL: OK. So these birds are the perfect test birds. And, as it turns out, they naturally organize themselves into a V formation.

PORTUGAL: When they first start flying around, they will naturally put themselves into a V formation, but it won't be great. But it seems like with practice, they gradually get better and better at flying in a good V.

BRUMFIEL: Portugal and his colleagues put their gadgets on the ibises. Then they compared the data to computer simulations. And the birds are drafting. They catch an uprush of air from the wing tip of the bird ahead. What's more, the birds were synchronizing their wing beats to maximize the effect. Portugal thinks there's a very good reason why the ibises do this. Flying is harder work than, say, running.

PORTUGAL: When we get exercising, our heart rate goes up to about 180 beats per minute on a good day. When birds are flying, it goes up to 400 beats per minute.

BRUMFIEL: Because they're working so hard, Portugal says he thinks ibises notice right away when they're catching a little lift from the bird ahead.

PORTUGAL: Suddenly they hit a spot where they can sense that they're not having to work as hard, and then they go, oh, this feels a bit easier. I'll stay here. And then if all the other individuals are doing the same, eventually they'll create a V formation.

BRUMFIEL: This isn't the end of the research, though. There are still more questions to be answered, like who's the bird that gets stuck at the front? Geoff Brumfiel, NPR News.

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