Patterns of declining US breeding bird abundance since 1980

Background/Question/Methods

Effective conservation of avian diversity requires an understanding of the spatial and temporal dynamics of avian abundance.  Although researchers and conservation planners have identified individual species declines, spatially-explicit broad-scale assessments of trends in the abundance of breeding bird assemblages are lacking. Our objectives were to characterize spatial and temporal trends in the abundance of breeding birds overall, and in twelve functional and behavioral guilds in the US. Annual abundance counts were tabulated from North American Breeding Bird Survey (BBS) data collected along 3694 40-km transects across the conterminous US for the period 1980-2007. With year as the predictor variable, we fit linear models for all BBS routes having at least 10 years of data and including observations within five years of both the start and finish of our time period.

Results/Conclusions

Out of 1369 BBS routes meeting these criteria, 466 had significant negative trends in total avian abundance, while 280 had positive trends. Many of the declines were severe, with 127 routes seeing greater than 50% reductions in fitted abundance over the time period. Trends were highly variable in space and and among functional and behavioral guilds. Surprisingly, Neotropical migrants were the least likely migratory guild to experience declines (374 declining vs. 346 increasing routes) while short distance migrants were the most likely (422 declining vs. 277 increasing routes). Birds in the cavity nesting (mean abundance = 95) and forest habitat (mean abundance = 98) guilds demonstrated the most positive trends, with mean increases of 12 and 14 individuals, respectively, over the entire period. These increases were especially concentrated in the eastern hardwood forest region. Grassland birds (mean abundance = 82) were among the most severely declining guilds examined, having a mean per route decline of 30 individuals—a decline that swelled to 197 when considering only grassland bird-dominated routes. Areas of limited grassland habitat, such as the Southeast, saw major proportional decreases, with 170 routes collapsing to less than 5% of their 1980 grassland bird abundance by 2007. Taken together, these results confirm sizable and widespread declines in US breeding bird abundance. More importantly, they call attention to great spatial and guild-wise variability in abundance trends, which will be crucial for identifying causal factors and prioritizing conservation efforts.