Started in 1900, the Christmas Bird Count is North America's longest-running Citizen Science project. Counts happen in over 2000 localities throughout the Western Hemisphere.
The information collected by thousands of volunteer participants forms one of the world's largest sets of wildlife survey data. The results are used daily by conservation biologists and naturalists to assess the population trends and distribution of birds.
Each Christmas Bird Count is conducted on a single day between December 14 and January 5. Counts are carried out within a 24-km diameter circle that stays the same from year to year. They are organized, usually as group efforts, at the local level, often by a birding club or naturalist organization.