For Neotropical migrants, species richness is highest in eastern North America, especially from the northern Great Lakes into New England and south along the Appalachian Mountains. The fewest numbers of species are associated with habitats in the southwestern deserts, Great Basin region, and central Rocky Mountains.
Negative trends appear to predominate from the Great Plains eastward, but positive trends do appear in the Coastal Plain, the Allegheny Plateau, Northern New England, the Boreal Forest, and the Great Lakes Plain. Other regions with negative trends include Wyoming, the Pacific coast from California north into British Columbia, and the northern Rocky Mountains of British Columbia. Positive trends predominate in the Great Basin and southwestern deserts, where there are relatively few Neotropical migrants. Similar positive trends are also evident in the northern Great Plains of Montana, Alberta and adjacent states/provinces.
This group is also composed of species from most habitat types. As a whole, 45% of Neotropical migrants have positive trend estimates over the entire survey period. However, this number is not statistically different from 50%, indicating that over the 1966 - 1994 interval there is no evidence that Neotropical migrants have experienced consistent declines.
Robbins et al. (1989) documented quite consistent declines in Neotropical migrant birds in eastern North America for the interval 1978 - 1987. These declines followed a decade of generally increasing populations. However, Sauer and Droege (1992) summarized the geographic patterns in trends of Neotropical migrant birds for the longer interval 1966 - 1988. At that time, species with positive trends outnumbered those with negative trends in both eastern and western North America. Results presented here are consistent with the Sauer and Droege results, but the slightly more negative long-term perspective may reflect an increasing importance of the recent declines in Neotropical migrants in the eastern U.S.
Robbins, C.S., J.R. Sauer, R.S. Greenberg, and S. Droege. 1989.
Population declines in North American birds that migrate to
the neotropics. Proc. Natl. Acad. Sci. 86:7658-7662.
Sauer, J.R., and S. Droege. 1992. Geographic patterns in
population trends of Neotropical migrants in North America.
Pp. 26-42 in J.M Hagan, III and D.W. Johnston, eds. Ecology
and conservation of neotropical migrant landbirds.
Smithsonian Inst. Press, Washington, D.C.