Every year at around this time, tens of thousands of people take part in a kind of bird-watching marathon. From Canada to Latin America and throughout the United States, participants will get up in the middle of the night. Some brave frigid winter temperatures, and many do whatever else it takes to count as many birds as they can in 24 hours.
This is the National Audubon Society's Christmas Bird Count, which lasts from Dec. 14 to Jan. 5. More than just an opportunity for bird-watching with friends, the annual counts are telling scientists a lot about how birds across the Americas are doing.
The group gathers on a cold, winter morning in Northern Virginia. It's still dark, and a hazy, almost-full moon hangs just above the horizon at 6:30 a.m.
Bruce Hill and his team are standing in the middle of a gravel road, using a smartphone app to play a recording of an Eastern Screech Owl.
They're trying to trick a real owl into calling back.
"If they hear their own call," Hill explains, "they will typically respond, either territorially or because they're interested in finding a mate, something like that."
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. "It is definitely addictive," Hill says.
"There are people that fly across the country, or crisscross the country, to do counts," says Geoff LeBaron, who directs the program for the National Audubon Society.
He's speaking from experience. LeBaron did his first holiday bird count in the late 1970s and has hardly missed a year since.
The count started back in 1900, as a reaction to a very different holiday tradition, LeBaron says. 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 bird count. LeBaron says this winter, they're expecting more than 60,000.
"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," LeBaron says, "so you get really good trend data over time."
Scientists anywhere can access those data through an online database. "We're seeing some really big changes in where birds are being seen, across North America," says David Bonter, a bird scientist with the Cornell Lab of Ornithology.
One big change is that many birds are expanding their winter ranges farther north. "Very likely the warming climate has something to do with that," Bonter says. "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."
Scientists are also seeing changes caused by the spread of industrial-scale agriculture and an increase in urban development: Prairie birds are declining, but birds that go to feeders like cardinals and jays are doing great.
By sunrise Bruce Hill and his team of Northern Virginia bird hunters 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.
"A lot of times you'll just walk right by owls and not even know it," says Hill. "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."
Then, suddenly, we see it.
"There he is!" Hill says. "There's a barred owl right there, flying."
The large brown bird glides across the road in front of us, then disappears into a patch of cedar trees.
"Excellent," Hill says.
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.