Q. Why are some bird populations quoted as breeding pairs, others as breeding territories, and others as individual birds?
A. Counting birds is a difficult job at the best of times; and the best of times for counting different species occur at different points in the year. As a result, the way we count birds varies from species to species. Some are tallied by visually counting the birds as they fly over on migration, and for these we use a figure given as the number of individuals. Some data comes from backyard surveys, and for these too the results are best given as the number of birds. With others, such as ducks and seabirds, the easiest way to count them is to extrapolate the numbers based on the number of breeding pairs or nests. In some cases, a mix of the two methods is used; in the case of several species such as Canada and Barnacle Goose, the estimated number of birds counted in the winter is divided by three to give the number of breeding pairs. In short, whether the quoted figure appears as breeding pairs, individuals or territories relies largely on the method of data gathering.