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This winter has brought an unexpected invasion from the North, the largest migration of snowy owls observed in decades. The Arctic birds usually winter in Southern Canada, but this year they've been spotted as far south as Florida. As NPR's Adam Cole reports, scientists are taking advantage of this unexpected visit by equipping a few of the owls with GPS tracking devices.

ADAM COLE, BYLINE: It's a chilly winter evening on the coast of Maryland, but naturalist Scott Weidensaul and two volunteers are taking a walk on the beach. One of the volunteers cradles a pigeon.

SCOTT WEIDENSAUL: Looks like the lunatics have escaped the asylum here. He's taking his pet pigeon for a walk and I'm walking around with a giant mouse trap over my arm.

COLE: That giant mousetrap actually looks more like a hockey goal, a light frame that supports a loose netting. It's designed to harmlessly capture large birds of prey and the pigeon is the bait. It will sit at the center of the trap wearing a sturdy leather jacket to protect it from piercing talons. A few hundred yards away there's a bright patch of white perched on the crest of a dune, a large female snowy owl.

Weidensaul hopes she's hungry.

WEIDENSAUL: That's kind of a typical, I'm-getting-ready-to-hunt perch.

COLE: He sets up the trap as stealthily as he can and then he waits for the owl to strike. Like most birders, Weidensaul was surprised when snowy owls suddenly started showing up in the United States, mostly in the North and the East, around Thanksgiving.

WEIDENSAUL: There was no preliminary. They're just suddenly, there were snowy owls everywhere. And actually I got a call from Dave.

COLE: That's Dave Brinker, a biologist with the Maryland Department of Natural Resources.

WEIDENSAUL: Who said this obviously more than just the typical every three to five year irruption.

COLE: That's irruption with an I. It means a sudden increase in the abundance of a species. This year's snowy owl irruption wasn't caused by the bouts of freezing weather we've been having or by climate change. It was caused by lemmings. Owls eat lemmings and Dave Brinker says when there are more lemmings, the owls have more babies.

DAVE BRINKER: When they get a big lemming high, they can lay up to 14 eggs and that's what happened this summer.

COLE: Huge numbers of owlets hatched and come winter they spread far to the south.

BRINKER: And that's when Scott and I got on the phone and said, we need to take advantage of this opportunity 'cause we won't see something like this for a long time, probably in the rest of our lifetime.

COLE: So Brinker and Weidensaul launched Project Snowstorm, their goal to tag as many owls as possible with GPS transmitters. By studying the owls' movements - for example, the fact that they hang out at airports, the scientists hope to better understand their behavior and protect them from hazards. In less than two weeks, Project Snowstorm managed to cobble together a national team of volunteers and $20,000 in donations.

Which brings us back to that Maryland beach where Weidensaul has been waiting for more than an hour.

WEIDENSAUL: Here she comes, here she comes, here she comes.

COLE: The big female owl launches herself at the trap, hovers above it for just a moment and then flies back to her perch. After another half hour she flies inland and out of sight.

WEIDENSAUL: After all that. And the next time somebody quotes Dave Brinker to me that anything worth doing is never easy, I'm going to hit them.

COLE: But further down the beach, another birder, Steve Huy, does manage to catch an owl.

STEVE HUY: This is very large. It's a female, pretty good size.

COLE: Like many members of Project Snowstorm, Huy isn't a professional ornithologist.

HUY: I'm an analyst for Marriott International.

COLE: But he is specially trained and permitted to catch owls. Huy and Brinker measure the big female's wingspan, tail length, fat stores and then they take a blood sample. The owl is surprisingly calm, but every so often she voices her disapproval.

HUY: She says, oh, you've got cold fingers.

COLE: And sometimes she shows her disapproval with her beak.

HUY: Ow, ow, ow, ow, ow. Man, that hurts.

COLE: Then comes the GPS transmitter. It's a small light box that fits easily in the palm of your hand and it's equipped with a solar panel and a chip that can hold five years worth of data. The owl wears it like a backpack with two thin Teflon straps wrapping around her wings. Brinker carefully sews the straps by hand so they fit the owl's frame and don't interfere with flight.

BRINKER: At first they're going to notice it, but they get used to it pretty quick.

COLE: As soon as the transmitter is snug, the owl is released on the beach where she was caught. Are there any solemn words spoken on a time like this?

BRINKER: This is a high tech owl so we'll use "Star Trek." Live long and prosper.

COLE: The owl takes off, gliding out into the dunes. Already her GPS unit is collecting data - longitude, latitude and altitude every half hour, and every three days her transmitter and the transmitters of the other 15 owls tagged so far will connect to a cell phone network and send all their data back to Project Snowstorm.

BRINKER: We're all sitting there, 7 o'clock every third day. It's like, you know, come on, who's checking in? And look where that bird went.

COLE: You can follow all the tagged owls on the Project Snowstorm website. The big female tagged on Maryland's coast has already started her journey back up to the Arctic Circle. At last check-in she was hunting in a Pennsylvania field. Adam Cole, NPR News.

GREENE: This is NPR News.

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