2.00 
1.50 
=O 
° 
° 
NUMBER OF BIRDS 
.50 
Aleve 
9/5 
fae ’930 
CENTRAL ILLINOIS 
—O—=—BIRDS PER OBSERVER-—HOUR 
—e—— BIRDS PER PARTY-—HOUR 
ae aeeso Sa IaH} Ilalky YUE 2 
/ 4 / / / 
[ee tg ts 8p 85 
Fig. 8.—Average numbers of raptors reported on Christmas counts in central Illinois, 1905-1955, expressed as birds per 
observer-hour, birds per party-hour, and birds per mile. 
three changes in the Christmas census methods: (1) 
attempts by observers to “‘better’’ the records of pre- 
vious years; (2) increases in the number of observers; 
(3) wider coverage of the census locality areas through 
the use of automobiles. 
CHANGES IN RAPTOR POPULATIONS 
Although there is considerable fluctuation from year 
to year, a general decline in winter populations of rap- 
tors is apparent from early to later years. Peaks repre- 
senting years of high population are lower in later years. 
There are considerable gaps in the record for southern 
Illinois, particularly in the early years, so that no def- 
inite trend showed there, but the population trend of rap- 
tors in northern and central [llinois shows the same gen- 
eral decline when expressed by three different units of 
measure—birds per observer-hour, birds per party-hour, 
and birds per party-mile traveled, figs. 7 and 8. 
Stewart (1954:187) stated that, as a unit of measure 
for the Christmas census data, number of birds found 
per mile of travel was more meaningful than number 
found per hour of observation. As mileage was not given 
on all of the early counts this record is incomplete. 
Eifrig (1939:5) and Nice (1945:9-10) both pointed 
out a decline in hawk populations in Illinois. If there 
has been a decline in hawks, has it been general for all 
raptors and all species? If all raptor populations have 
declined, what has caused the decrease? 
To answer the first question, we must look at the 
data on those individual species for which the record 
is most nearly complete. 
For the sparrow hawk, frequency and density have 
been highest in southern Illinois, figs. 10 and 11. Al- 
though the record was incomplete for early years, the 
highest density peaks came in those years, with the 
peak in the late 1920’s lower, and peaks in the 1940’s 
and 1950’s lower still. No definite trend is evident for 
