At the close of the 115th National Audubon Christmas Bird Count, which took place between Dec. 14, 2014, and Jan. 5, 2015, Arey Professor of Biosciences Herb Wilson elaborated on the purpose of the tradition. Each bird count occurs on a single day within a circle with a diameter of 15 miles (an area of 78 square miles) in order to track changes in populations of migratory birds in North America and elsewhere. The data for all the counts are then archived online by the National Audubon Society and are available for download by researchers and birders—a “remarkable resource,” according to Wilson.