ROBERT SIEGEL, Host:

We're going to the beach now and not just to any beach, this one is cold, but popular and crowded with a few thousand penguins.

(SOUNDBITE OF PENGUINS)

SIEGEL: It's a beach in Argentina. Every year around this time, penguins congregate in the far Southern Hemisphere to build nests and raise families. There are plenty of biologists on hand, too, wrapping tags on the penguins' flippers so they can follow the birds for life.

NPR: swimming.

CHRISTOPHER JOYCE: I was lucky enough to stand on a cliff overlooking one of these Argentinean beaches, where thousands of Magellanic penguins had gathered. Like awkward mannequins, they waddled down to the water. But when they dived in, they were transformed into black and white torpedoes.

Rory Wilson has watched penguins for 30 years.

RORY WILSON: Even people that work on penguins don't appreciate how stunning they are underwater, how maneuverable and how fast. You know, they're just - it's hard to describe it.

JOYCE: Wilson and other scientists have shown that penguins have a very, very low coefficient of drag. That's a measure of how well a body moves through air or water.

WILSON: If you were shaped like a penguin, you could kick off the side of a swimming pool and you would just go, you know, gliding for yards and yards and yards.

JOYCE: But Wilson and other biologists say some tags seemed to increase drag and slow down penguins. These tags can be plastic, aluminum or other metal. They're usually about an inch wide and a few inches long and wrap around the narrow base of a flipper. It's hard to imagine they'd slow down a penguin. Wilson, from Swansea University in Great Britain, says they do.

WILSON: Think of it like this: If you had a speedboat or something and then you took one of your bands and you stuck it to a propeller, would you expect the boat to perform in the same way?

JOYCE: And now a research team from the University of Strasbourg in France has evidence to back up what Wilson has seen. The French team put traditional metal bands on 50 King penguins that live near Antarctica. Fifty others had much smaller radio-frequency transponders. Ten years later, the survival rate for banded birds was 16 percent below the unbanded birds.

Yvon Le Maho was the chief biologist.

YVON LE MAHO: In other words, only the super athletes are surviving.

JOYCE: The numbers were worse for breeding. Banded birds produced 39 percent fewer chicks.

Yvon Le Maho found that banded birds took longer to forage for food in the ocean and they were slower to get to breeding sites in the spring. So adults had less time to raise their chicks before heading off for lengthy foraging trips in the winter.

LE MAHO: At some times, they will have to leave while their chick is still too young and too poor in body fuels of reserves to withstand the winter.

JOYCE: The study appears in the journal Nature, but the verdict on tags is by no means clear. Other scientists say they've got different results. Among the world's leading penguin experts is Dee Boersma, at the University of Washington.

DEE BOERSMA: This study shows that the bands that they used on King penguins harmed the King penguins. I have no doubt about that. But all bands are not created equal. It depends on what material that they're made of, it depends on how they're shaped, it depends on how they're fitted to the individual penguin. It depends on what penguin species it is.

JOYCE: Boersma has also studied bands on Magellanic penguins. Aluminum bands were harmful, but stainless steel ones were fine. She says eliminating all tags would be throwing the baby out with the bath water.

BOERSMA: In almost all cases, whenever we do science, we would like to do no harm. But in fact we do have to do some harm if we want to follow individuals.

JOYCE: And following individual penguins is increasingly important. They are sentinel species that are likely to show the first effects of climate change. And biologists agree they don't want to blame climate change for some effect that's really caused by a bad flipper band.

Christopher Joyce, NPR News.

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