Lee re Qala TS ON aot Uplate. LN 13 
DARK INTERMEDIATE LIGHT 
Frequency Distribution 
of Rough-legged Hawks 
by Gary D. Schnell* 
During the winters of 1964-’65 and 1965-’66, I studied various 
aspects of the ecology of the rough-legged hawk (Buteo 
lagopus) in a 43.1 square mile study area near the center 
of DeKalb County, Illinois. 
Periodically during the two winters, I made car censuses 
along a 50-mile survey route in this area. The rough-legs 
that I observed varied greatly in plumage, from birds almost 
completely black in color to very light-colored individuals. 
Since there is a continuous range of color variations, I didn’t 
think it was appropriate to designate artificial plumage 
groups as color phases. Therefore, to record plumage types 
I separated them into three somewhat arbitrary color classes: 
dark, intermediate, and light. The birds placed in the inter- 
mediate class appeared somewhat mottled ventrally. I based 
the plumage classification on ventral and underwing plumage 
patterns, because in the field I could discern little difference 
in dorsal patterns. 
I was able to classify 441 of the 534 rough-legged hawks 
seen as to plumage, Table 1. Although the sightings do not 
necessarily represent different individuals, I did not know- 
ingly record any one bird twice in a single day. The dif- 
ference in frequency of birds in the three plumage classes 
for the two winters is statistically significant at the 95 percent 
probability level, but not at the 97.5 percent level. Con- 
sidering that the plumage classes do intergrade and that 
I had some difficulty in positively placing a few individuals, 
*The author is Graduate Assistant, Museum of Natural History, the Uni- 
versity of Kansas, Lawrence, Kansas 66045. 
