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RENEE MONTAGNE, host:

We're going to hear, now, about hundreds of artificial bird's nests built by scientists in the Canadian arctic. They want to know why arctic shorebirds fly thousands of miles to lay their eggs. Here's NPR's Nell Greenfieldboyce.

NELL GREENFIELDBOYCE: Consider a bird like the White-Rumped Sandpiper.

(Soundbite of bird chirping)

GREENFIELDBOYCE: It weighs less than two ounces, but each year this little white and brown bird leaves the southern tip of South America and flies more than 9,000 miles to its breeding grounds in the arctic.

Ms. LAURA MCKINNON (University of Quebec at Rimouski): This is one of the reasons why I was interested in these birds, is because of their amazing migration strategies.

GREENFIELDBOYCE: Laura McKinnon is a researcher with the University of Quebec at Rimouski.

Ms. MCKINNON: I've always been interested in, you know, the cost and benefits of flying further north.

GREENFIELDBOYCE: Migrating to the arctic is exhausting and dangerous. But the benefits for shorebirds include lots of daylight, plenty of food for their young and fewer parasites.

McKinnon and her colleagues thought there might be one other advantage too.

Ms. MCKINNON: We thought that another one of the benefits of breeding further north could be reduced predation risk, and that's why we looked into this question.

GREENFIELDBOYCE: After all, a nest of eggs is a tasty snack, and the arctic has relatively few predators.

This is where the artificial nests come in. McKinnon and her colleagues wanted to watch nests at different breeding sites to see how long a nest could last without being plundered by an arctic fox or predatory bird. But they knew that some shorebirds are better than others at hiding their nests or fending off attackers.

To makes sure everything was equal at all of their study sites, McKinnon and her team decided to create fake nests full of tempting eggs.

Ms. MCKINNON: Luckily, shorebird nests are very, very simple and they are essentially just a small depression in the ground.

GREENFIELDBOYCE: So, the nest part was easy - even though they had to build more than 1,500 of them. Filling all those nests was a lot harder. The researchers used real quail eggs, which are similar in size and color to many arctic shorebirds' eggs.

Ms. MCKINNON: We have to take those in our hand luggage on the plane going up to the arctic. It's a little bit of a challenge to get them there in one piece.

GREENFIELDBOYCE: They took these eggs to seven remote breeding sites. The sites spanned 2,000 miles from the sub-arctic to the high-arctic. What the scientists learned is published in the journal, Science. McKinnon says the further north a shorebird's nest is, the less likely its eggs are to becomes someone's dinner.

Ms. MCKINNON: So, a bird flying to Akimiski, which is the most southern part, if it decides to fly another 3,350 kilometers to Alert, Ellesmere Island, it could reduce its predation risk by 66 percent.

GREENFIELDBOYCE: This means the scientist's initial hunch about the role of predators appears to be right. David Lank studies shorebirds at Simon Frazier University, near Vancouver.

Mr. DAVID LANK (Simon Frazier University): You know, they've taken us a step closer towards understanding why birds might go so far.

GREENFIELDBOYCE: But he says the study also raises some questions.

Mr. LANK: If it were so great, to always go as far as possible so that you'd have the highest probability of your nest hatching, why doesn't everyone do it?

GREENFIELDBOYCE: Some birds stop at the lower arctic, so maybe they prefer features like its longer nesting season. Still, he says all of this just goes to show a long trip that seems crazy to us can make sense for a bird that just wants to raise a family.

MONTAGNE: NPR's Nell Greenfieldboyce.

It's NPR News.

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