CHANGES IN THE WINTER DISTRIBUTION OF THE ROUGH-LEGGED HAWK 
except the North and South Pacific coasts. This widespread increase in 
wintering Red-tailed Hawks has been noted by others (Preston and Beane 
1993, White 1994, McCay et al. 2001). Logistic regression of Red-tailed 
Hawk abundance versus changes in Rough-legged Hawk numbers showed a 
significant negative effect of the Red-tailed on the Rough-legged (P < 0.001), 
suggesting that competition between these species may be a factor in the 
range shift we observed. Given the finite amount of suitable habitat, it is 
reasonable to infer that large increases in the Red-tailed Hawk must, at some 
point, apply significant competitive pressures on the Rough-legged Hawk. In 
addition, the Rough-legged Hawk is more sensitive to urbanization than the 
Red-tailed Hawk (Bosakowski and Smith 1997, Berry et al. 1998, Schmidt 
and Bock 2005). Therefore, the increasing numbers of Red-tailed Hawks 
and increased conversion of habitat to urban uses may have a combined 
negative effect on Rough-legged Hawk abundance. 
Regional Trends 
Examination of trends for the ten regions suggests that changes in Rough- 
legged Hawk abundance might be influenced by some combination of the 
following: (1) Rough-legged Hawks staying in the North Plains region because 
of reduced snow cover in that area, (2) Rough-legged Hawks avoiding areas 
where Red-tailed Hawk density is high, and/or (3) loss of habitat associated 
with increases in human population. Our observation that numbers of the 
Rough-legged Hawk in the North Intermountain West and South Plains 
regions decreased in spite of reduced snow cover might be explained by 
these regions’ high average abundance of the Red-tailed Hawk (81 and 74 
birds/100 party hours, respectively). Also, both regions experienced high 
rates of human population growth (30% and 43%, respectively). Decreases in 
the Rough-legged in the South Intermountain West (Figure 4) were associated 
with high abundance of the Red-tailed (72 birds/100 party hours) and very 
high rates of human population growth (62%). The North and South Pacific 
Coast regions (Figure 3) had high abundance of Red-tailed Hawks (62 and 
145 birds/100 party hours, respectively), high rates of human population 
growth (26% and 59%, respectively), and large decreases in Rough-legged 
Hawks. The North Plains region showed almost parallel increases in Rough- 
legged and Red-tailed Hawks (Figure 5). However, the abundance of the 
Red-tailed was low throughout the period (18-45 birds/100 party hours), 
and rates of human population growth were also low (9%). Both Great Lakes 
regions showed no significant change in Rough-legged Hawk abundance 
even though Red-tailed Hawk abundance increased (Figure 6). Average Red- 
tailed Hawk abundance was generally low for both Great Lakes regions (41 
birds/100 party hours for the West Great Lakes; 48 birds/100 party hours 
for the East Great Lakes), however, and the human population grew slowly 
in the East Great Lakes (10%) and decreased in the West Great Lakes (-1%). 
The East region had low abundance of the Red-tailed (Figure 7) in the first 
7 years (37 birds/100 party hours) but high abundance during the last three 
7-year periods (66-84 birds/100 party hours) and a significant decline in 
the Rough-legged. Human population growth was low (8%). The Atlantic 
Coast was the one region where a decrease in Rough-legged Hawks was 
not associated with high Red-tailed Hawk abundance (Figure 7) and/or large 
221 
