Just before 8 a.m. on Sunday, December 15, a crowd is gathering at the southeast rim of the reservoir in New York City’s Central Park. Most are bundled in puffer coats, huddled together in murmuring clusters, though a few eager individuals already have binoculars aimed at treetops or across the reservoir’s vast, glass-smooth surface. The morning is bright and gray, a crisp 30° Fahrenheit. One of the assembled, David Ringer, clutches a pair of hand warmer packets: “Christmas Bird Count survival trick number one.” The Christmas Bird Count, or CBC, has survived—and thrived—for 125 Decembers. The inaugural counters may have lacked portable chemical warming devices, but they turned out for the same reason as birders today: To identify and enumerate as many birds as possible in one winter outing. Central Park has been a CBC site since the very first count in 1900, with a checklist submitted by a single birder. More than a century later, there are enough participants...