CHANGES IN THE WINTER DISTRIBUTION OF THE ROUGH-LEGGED HAWK 
increases in human population. In this region the Red-tailed Hawk’s average 
abundance (51 birds/100 party hours) and human population growth (15%) 
were both fairly low. Snow cover on the Atlantic Coast did increase slightly in 
the last two 7-year periods, but this slight change seems unlikely to have been 
a major factor. Other factors may be responsible for trends in this region, or 
the decline in Rough-legged Hawks may simply be the result of more birds 
remaining in the North Plains region and not migrating to the coast. 
One possible hypothesis to explain our results is the following. During the 
period we examined, increases in Red-tailed Hawks (possibly coupled with 
increased loss of open country) may have applied increasing competitive 
pressure on wintering Rough-legged Hawks. At the same time, decreasing 
snow cover in the North Plains made that area more productive for winter 
foraging, allowing Rough-legged Hawks to shift away from areas with high 
concentrations of Red-tailed Hawks. More broadly, this hypothesis suggests 
that while the northern limit of the winter range of the Rough-legged Hawk 
may be determined by climate factors (Bock and Lepthien 1976, Root 1988, 
Olson and Arsenault 2000), the southern, western, and eastern limits may 
be influenced by competition with the Red-tailed Hawk. More detailed data 
on actual historic land-use changes and interspecific interactions at the level 
of specific CBC circles could help test this hypothesis. 
ACKNOWLEDGMENTS 
We thank Ted Beedy, Zach Smith, and John Sterling for helpful suggestions on 
an early draft of the manuscript. Doug Faulkner and an anonymous reviewer made 
suggestions that greatly improved the final product. Special thanks to all the compilers 
and participants who record data on Christmas Bird Counts. 
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