Bird species richness is greatest in the northern forest communities immediately south of the boreal forest, from the Maritime Provinces and New England across the northern Great Lakes and into central portions of the Prairie Provinces of Canada. Similar levels of species richness also occur in southern British Columbia and the adjacent U.S. The fewest numbers of species are associated with the southwestern deserts and the short-grass prairie communities of the western Great Plains.
This map is a mosaic of increases and declines with few apparent geographic patterns. Positive trends appear to prevail along the Atlantic coast from New York and Pennsylvania south to Georgia and the Carolinas, in the Great Lakes region, and along the Rocky Mountains. Negative trends may predominate from eastern Texas and Arkansas across the southeastern states and along the Appalachian Mountains to West Virginia, in Florida, and from northern California and Nevada northward to Washington and Idaho.
For all species of birds, the BBS shows equal percentages (50%) of increasing and decreasing species across North America. These equal percentages may be reflected in the mosaic of increases and declines shown in the trend map, with few evident geographic patterns. The geographic pattern to species richness is very similar to that shown by Robbins et al. (1986), based on BBS data obtained between 1966-1979.
Robbins, C.S., D. Bystrak, and P.H. Geissler. 1986. The Breeding
Bird Survey: its first fifteen years 1965-1979. U.S. Fish and
Wildl. Serv. Res. Publ. No. 157. 196 pp.