Up through the 19th century, many North Americans participated in the tradition of Christmas “side hunts”, in which they competed at how many birds they could kill, regardless of whether they had any use for the carcasses and of whether the birds were beneficial, beautiful, or rare. In December 1900, the U.S. ornithologist Frank Chapman, founder of Bird-Lore (which became Audubon magazine), proposed counting birds on Christmas instead of killing them.
That year, 27 observers took part in the first count in 25 places in the United States and Canada, 15 of them in the northeastern U.S. from Massachusetts to Philadelphia. Since then the counts have been held every winter, usually with increasing numbers of observers. For instance, the 101st count, in the winter of 2000–2001, involved 52,471 people in 1,823 places in 17 countries (but mostly in the U.S. and Canada). During the 113th count (winter 2012–2013), 71,531 people participated in 2,369 locations. The National Audubon Society now partners with Bird Studies Canada, the Gulf Coast Bird Observatory of Texas (responsible for CBCs in Mexico), and the Red Nacional de Observadores de Aves (RNOA, National Network of Bird Observers) and the Instituto Alexander von Humboldt of Colombia.
More information on Audubon’s website.