CHANGES IN THE WINTER DISTRIBUTION OF THE ROUGH-LEGGED HAWK 
cover. Thiel (1985) showed a negative correlation between snow depth and 
Rough-legged Hawk numbers in Wisconsin. Watson (1986a) documented 
movements just after a major snowfall, and numerous authors observed 
changes in foraging behavior in apparent response to increased snow cover 
(Schnell 1968, Klein and Mason 1981, Temeles and Wellicome 1992, Wat- 
son 1986b, Lingle 1989). We found no evidence in our data for short-term 
(within a year) shifts in response to snow cover. For example, winters when 
the entire North Plains region experienced heavy snowfalls and many days 
with significant snow cover were not associated with any obvious decrease 
in Rough-legged Hawks. The shift was more gradual, apparently in response 
to the decrease in snow cover and improved opportunities for foraging over 
a longer term. Perhaps birds wintering in areas with reduced snow cover 
had higher survival rates or were more likely to return to a given area in 
subsequent years if the foraging conditions were good. This possibility is 
more consistent with the species’ winter site fidelity as described by Watson 
(1986a), Gatz and Hegdal (1986), and Garrison and Bloom (1993). 
Some regional trends suggest that factors other than climate may also 
be involved in this range shift. For example, snow cover decreased in the 
North Intermountain West (Figure 4) and, to a lesser degree, in the South 
Plains (Figure 5), but Rough-legged Hawk abundance in both these regions 
decreased. Other factors that might influence the winter distribution of the 
Rough-legged Hawk include loss of habitat, changes in the density or mix 
of prey, and competition with other open-country raptors. 
We had no access to data on prey density or mix and no practical means of 
examining specific land-cover changes over the 28 years of the study for all 
CBC circles. Because increases in human population may reflect conversion 
of open country to cities, suburbs, or more intense forms of agriculture, we 
used human population increase from 1980 to 2000 in the county associated 
with each CBC circle as a very crude proxy for local habitat loss. Logistic 
regression showed a negative effect (P = 0.04) of human population growth 
on Rough-legged Hawk abundance. 
To examine the potential influence of competition with other raptors, we 
analyzed trends in Red-tailed Hawk abundance in these same CBC circles. 
The Red-tailed Hawk is the most abundant raptor with which the Rough- 
legged Hawk shares most of its winter range. There is nearly complete 
overlap in the Red-tailed Hawk’s habitat use (Weller 1964, Fischer et al. 
1984, Bildstein 1987) and prey base (Craighead and Craighead 1956, 
Bildstein 1987) with those of the Rough-legged Hawk. Although Craighead 
and Craighead (1956) noted little direct competition between these species 
in Michigan, others have noted evidence of competition. Hogan (1983) 
reported a Rough-legged Hawk kleptoparasitizing a Red-tailed Hawk. 
Bildstein (1987) noted many agonistic encounters in Ohio, with Red-tailed 
Hawks pirating prey from Rough-legged Hawks more often than the con- 
trary. Schnell (1968) reported that these two species avoid competing for 
prey and perches when possible. The Red-tailed Hawk uses a wider variety 
of prey and habitats than the Rough-legged Hawk (Weller 1964, Bildstein 
1987), with the latter more restricted to open areas (Zarn 1975, Baker and 
Brooks 1981, Fischer et al. 1984, Lingle 1989). 
We found that numbers of the Red-tailed Hawk increased in all regions 
220 
