In 1900, on Christmas Day, 27 birders in 25 locations across the United States donned winter gear and binoculars, then stepped outside to list all the birds they could see in 24 hours. They didn’t know it then, but they were launching what would become the world's longest-running community science project on birds. Much has changed since that first count: Global temperatures have risen by 1 degree Celsius, or 1.8 degrees Fahrenheit, the U.S. population has more than quadrupled, and 64 percent of the world's wetlands have been destroyed. But the birders keep counting. Last year, at the end of December 2020, the 吃瓜黑料’s Christmas Bird Count (CBC) entered its 121st year with 54,533 birders observing winter birds in 1,842 sites across the United States, even during a pandemic. The field notes created over the past century are now helping researchers answer a critical question: How have climate change and land-use change affected birds? The question has...