100 
CENTRAL ILLINOIS 
® 90 SOUTHERN ILLINOIS 
= 
5 80 
[e) 
oO 
i770 
[e) 
=a GO 
2 
Ww 
© 50 
cr 40 
a 
+30 
oO 
4 
Ww 20 
=) 
sy 
=a) 
we 
fe) 
93 5 ‘940 
/9 Os /9 “0 /9 "55 /9 eS /9 ee 
Fig. 16.—Frequency of occurrence of the marsh hawk in central and southern []linois calculated for 5-year periods begin- 
ning in 1905 and ending in 1954. The year indicates the beginning of the 5-year period. 
.05 
a fi 
—=O- CENTRAL ILLINOIS 
—w~— SOUTHERN ILLINOIS 
wo 
a 
a 
@o 
uw 
ow 
x 
WwW 
oO 
= 
=) 
_ 
/9 /9 /9 /9 / /9 /9 /9 /9 /9 /9 
Os /0 15 20 925 3o 35 Fo %5 So Ss 
Fig. 17. — Average numbers of marsh hawks reported per observer-hour in central and southern Illinois, 1903-1955. 
15 
