The CBC is an early-winter survey of birds. Although counts occur in Central and South America, most CBCs occur in North America. The sample area for a count is a circle that is 15 miles in diameter, and varying numbers of volunteers count all birds they see in the circle during a single day, which is within 2 weeks of 25 December. Although this analysis only considers the interval 1959 - 1988, the CBC was begun in 1900. The number of circles and participants has changed dramatically since the early years.
More research is needed into methods of adjusting counts.
Also, numerous articles have appeared in American Birds that summarize data for individual species and groups of species. Each of these categories has an icon that can be selected to display species information. You will then be presented with a list of species. All of the categories of information can also be accessed through the species accounts. Each species has an account, which can be accessed either from the icon on the main menu of the Home Page or from any of the categories of species information.