This Saturday, birders across the Western Hemisphere will wake up early and grab a pair of binoculars for the 120th annual Christmas Bird Count, the oldest and largest count of its kind -- and one of the longest-running community science programs in history.

For birders (and those who would like to be), the way it works is simple. Established counts exist all over the Western Hemisphere, although they’re mostly concentrated in the United States. Each one takes place over the course of one day; sometime between December 14th and January 5th; groups count as many birds as they can within a prearranged 15-mile diameter circle. There are options for everyone: newcomers to the count can join a group with at least one experienced birdwatcher, and people with limited mobility can opt for a stationary count or feeder watch.

Listen to reporter Erin Woo's story on WNYC:

Christmas Bird Counts become a tradition for their participants, says Geoff LeBaron. He’s been running the program for the Audubon Society for the past 32 years and participating since long before that.

“They have a real sense of place in the areas they’re counting,” LeBaron said. “They’re oftentimes in areas that they love. Plus, they’re getting out to bird, and they’re also getting out to bird with friends they may only see during the Christmas Bird Count period. A lot of people actually travel pretty long distances to do their traditional Christmas Bird Counts.”

But beyond the binoculars and the bonhomie, there’s a lot that goes on behind the scenes. The Christmas Bird Count draws tens of thousands of volunteer birders every year, and those volunteers generate hundreds of millions of data points. That wealth of information is invaluable to the scientific community.

Christmas Bird Count data informed the Cornell Lab of Ornithology’s landmark report from this past fall, which found that North America has lost 3 billion birds in the past 50 years. A project from the Audubon Society, Survival by Degrees, combines the data with climate models to let users see what birds are most at risk in their area.

The scientific community wasn’t always willing to trust the Christmas Bird Count data. They worried that variations between different groups of birders would make the data unreliable, and that unpredictable weather conditions at the tail end of the fall migration could skew results as well.

LeBaron disagreed.

“It doesn't matter if the people in Boston are doing it the same way as the people in Dallas or the people in Quito or the people in Anchorage,” he said, “because it's the same people in Boston doing it the same way every year.”

The Christmas Bird Count is used for trend data, revealing the way populations in one area change over time. For that kind of analysis, variations across groups aren’t as important. And because the data is often analyzed in decade-long chunks, weather variations tend to factor out, too.

Over time, scientists came to agree with LeBaron and trust the data. Now, his work is less about convincing the scientific community that the Christmas Bird Count is important and more about fielding requests from researchers who want to use its findings -- and about reminding his tens of thousands of volunteer birders why their work and the count is important, too.

“Birds provide a unique opportunity for people to get really hooked on caring about nature,” LeBaron said. “They do amazing things, they’re beautiful, they sing, they fly, they have these incredible migrations. And they’re there! They’re with you! With the possible exception of someone at South Pole Station, birds are probably the only life form that somebody anywhere on the surface of the Earth can see on a given day.”