CBC Graph Legend
Annual Population Indices for North America: The Christmas Bird Count (CBC) Annual Index Graphs show the annual population indices of each species using CBC data from 1965-66 through 2003-04. The annual indices are determined by taking into account the number of birds seen in each circle, the detectability of the species, and the amount of effort that was spent looking for birds in each circle. The values can be thought of as the number of birds that could be expected to be seen on a CBC each year, assuming an average amount of effort in each circle, averaged across all CBC circles where the species occurs. These population indices are copyright National Audubon Society, Inc. 2006, all rights reserved.