Birds are rapidly losing habitat, and their flight patterns can reveal large-scale changes in our planet’s health. But there are only two ongoing studies of the migration of American birds, one from the U.S. Geological Survey and another done by Audubon Society volunteers.

“The birds are like a barometer,” Brooklyn Bird Club member Paul Keim told VICE News correspondent Mary H.K. Choi in Brooklyn’s Prospect Park. “Much like the honeybees, much like frogs, the birds are telling us that environmentally the earth is changing.”

Keim is part of a nationwide effort coordinated by the Audubon Society. From Dec. 14 to Jan. 5, tens of thousands of volunteers participate in the National Audubon Society’s Christmas Bird Count. Last year, they clocked 60 million. The data set they create is vital for American ornithologists, supporting more than 200 peer-reviewed articles.