North American Breeding Bird Survey annual summary 1990-1991

The North American Breeding Bird Survey (BBS) was used to estimate continental and regional changes in bird populations during 1989-1990 and 1990-1991, placing these short-term changes within the context of population trends since 1966. For the entire survey area, 51.7% of the species exhibited increases in population size during 1989-1990, reversing the decreasing tendency exhibited by the majority of species during 1988-1989. A similar percentage (51.4%) of species with increasing populations was noted during 1990-1991. These percentages were also analyzed for 12 guilds of North American birds and 3 broad geographical regions. Many regional patterns exist in these population changes, reflecting the variability inherent in short-term population fluctuations. Two-year (1989-1990 and 1990-1991) changes and long-term (1966-1990) continental trends are provided for 255 species.