Many investigators have used bird survey data to develop contour maps of bird abundance based on mean counts on survey routes. Root (1988) provided a grid of smoothed relative abundances for species observed on Christmas Bird Counts. We display relative abundance (birds/100 party hours) averaged over the interval 1966- 1989 for each circle.
The smoothing procedure is identical to that used in the Breeding Bird Survey Home Page maps, and we refer the users to the help files from the BBS Home Page for additional information. Because CBC circles tend to be quite near each other along the coasts and near cities, but far apart in remote areas (reflecting the number of people available to do the counts), CBC relative abundance maps must be quite highly smoothed to create continuous surfaces in the remote areas. Consequently, the maps often do not define sharp edges of bird distributions. Also, there are many occasional sightings of accidental species or late migrants on count circles. Over time, these observations accumulate and extend edges of ranges. Consequently, users are cautioned that regions covered by the lowest abundance category may not routinely contain the species.
Remember, the "counts" on the maps represent birds/100 party hours!
Root, T. 1988. Atlas of wintering North American birds. University of Chicago Press, Chicago, Il.