Populations of rough-legged hawks (Buteo lagopus) have been declining throughout many portions of their winter range, including the Central Valley of California.  Suggested reasons for declines of this Holarctic breeder have included habitat loss in open country habitats, increasing human population pressure, competition from other raptors, and global climate change.  Therefore, our objective was to evaluate the effects of weather variables, human population density, and competition from red-tailed hawks (Buteo jamaicensis) on the winter abundance and distribution of rough-legged hawks throughout the U.S. using Christmas Bird Count (CBC) data.  We used mixed regression  to model the effects of average December temperatures, the number of December days with snow depths > 5 cm, and the relative abundance of red-tailed hawks on the relative abundance of rough-legged hawks within each count circle over four periods (1979 – 1985, 1986 – 1992, 1993 – 1999, 2000 – 2006).            

Results/Conclusions

Our results indicate that 91% (101/112) of the count circles on the western and eastern coasts and three southern regions showed decreases in the relative abundance of rough-legged hawks between the first (1979 – 1992) and second (1993 – 2006) 14-year periods.  Rough-legged hawks also increased significantly in the North Plains, which is a core part of their winter range.  There was a significant interaction of region and period and a negative effect of red-tailed hawks on the relative abundance of rough-legged hawks. Weather effects were not significant in the mixed regression, but isolation of count circles with increases in rough-legged hawk numbers and a minimum of 5 days of snow accumulation > 5 cm indicate December temperatures and the number of December days with snow accumulation >5 cm were negatively related.  Collectively, our results indicate warming temperatures and reduced lengths of snow accumulation likely associated with global climate change are resulting in a northward shift of the rough-legged hawk wintering distribution.  Furthermore, our results suggest a potential density-dependent effect of competition whereby rough-legged hawks begin to exhibit declines and a northward range shift when red-tailed hawks exceed 0.5 birds/party hour.