In Brief:
The annual Audubon Society's Christmas Bird Count (CBC) is now over 107 years strong, making it the oldest and biggest " The CBC is designed with a series of "circular count areas" and has many counting birds each year within those circles. Some birds are counted in winter in CBC that are not counted in the BBS, so CBC gives an "alternate view of population distribution and change for many species that are well-surveyed by the BBS". These are grouped into "convenient categories of relative abundance" and "predict the average number of birds of the species that could be seen in about 100 party-hours of birdwatching in a CBC circle". There is a comparison of the standardized data on the CBC and the BBS data, for things included in both surveys.
