STEVE INSKEEP, HOST:
Every year around this time, tens of thousands of people take part in a kind of birdwatching marathon. It's happening in Canada, Latin America and throughout the United States. Many participants get up in the middle of the night, brave frigid winter temperatures, and do whatever else it takes to count as many birds as they can in 24 hours.
And as NPR's Véronique LaCapra reports, these counts are telling scientists a lot about how birds across the Americas are doing.
BRUICE HILL: I have a flashlight here. Oh, you've got one there.
VERONIQUE LACAPRA, BYLINE: It's just after 6:30 on a cold winter morning in northern Virginia. It's still really dark, and a hazy, almost full moon hangs just above the horizon. Members of the Loudoun County bird count are standing in the middle of a gravel road. They're trying to trick an eastern screech owl into calling out.
(SOUNDBITE OF SCREECH OWL)
LACAPRA: That's a Smartphone app recording that Bruce Hill and his bird count team are using.
HILL: If they hear their own call, they will typically respond, either territorially or because they're interested in finding a mate, something like that.
LACAPRA: Hill says he can recognize about 300 different species just by hearing them. He's been doing this bird count for at least 20 years.
HILL: It is definitely addictive.
LACAPRA: It's known as the Christmas Bird Count, but teams have from mid-December to January 5 to get their counting done. Geoff LeBaron directs the program for the National Audubon Society, which consolidates the data.
GEOFF LEBARON: There are people that fly across the country or crisscross the country to do counts.
LACAPRA: LeBaron did his first holiday bird count in the late '70s and has hardly missed a year since. He says the count started back in 1900, as a reaction to a very different holiday tradition. The Side Hunt, as it was known, had teams competing not to count birds but to shoot and kill them. That first year, 27 people participated in the count. LeBaron says this winter they're expecting more than 60,000.
LEBARON: And it's pretty much the same people doing it the same way in the same areas at the same time of year every year, so you get really good trend data over time.
LACAPRA: David Bonter is a bird scientist with the Cornell Lab of Ornithology.
DAVID BONTER: We're seeing some really big changes in where birds are being seen across North America.
LACAPRA: One big change is that many birds are expanding their winter ranges further north.
BONTER: Very likely the warming climate has something to do with that. The birds are often limited at the northern edge of their range by severe winter weather, and with a series of mild winters, that allows birds to survive at the northern limits of their range where they may not have in the past.
LACAPRA: Scientists are also seeing changes due to spreading industrial-scale agriculture and urban development. Prairie birds are declining, but birds that go to feeders - like cardinals and jays - are doing great.
HILL: So we're going to walk down this way a little ways, and try again for a barred owl.
LACAPRA: Back at the Virginia bird count, it's almost sunrise. Bruce Hill and his team have been at it for about an hour. The group has found plenty of birds: nuthatches, chickadees, a flicker, even a bald eagle perched on a distant tree. But no owls.
HILL: A lot of times you'll just walk right by owls and not even know it, they usually just sit very still, and they're usually near a trunk somewhere, and unless you're really looking for them hard, you'll miss them.
LACAPRA: My hands and feet are starting to get a little numb. Then suddenly we see it.
HILL: There he is. There's a barred owl right there, flying.
LACAPRA: The large brown bird glides across the road in front of us, then disappears into a patch of cedar trees.
LACAPRA: By the end of the day, Hill and his team will have racked up a total of 67 species - almost 4,300 birds. Not a record, but still pretty good. Veronique LaCapra, NPR News. Transcript provided by NPR, Copyright National Public Radio.