2.50 
NORTHERN 
ILLINOIS 
—=-—O——BIRDS PER OBSERVER-HOUR 
2.00 
—o—- BIRDS PER PARTY-HOUR 
—---O--BIRDS PER MILE 
1.50 
1.00 
NUMBER OF BIRDS 
.50 
/ / / ii / / / /9 /9 / 
Fig. 7.—Average numbers of raptors reported on Christmas counts in northern Illinois, 1905-1955, expressed as birds 
per observer-hour, birds per party-hour, and birds per mile. 
was necessary to use data from a number of localities 
in each section of the state. 
Analysis of Frequency of Occurrence 
Probably the simplest quantitative treatment of Christ- 
mas census data is the analysis of frequency of occur- 
rence of each species, the percentage of the total num- 
ber of counts in which the species was reported. The 
frequencies of occurrence for the species of raptors 
recorded in the period 1903-1955 in southern Illinois 
were based on 103 counts and in 1905-1955 in central 
and northern Illinois on 123 and301 counts respectively, 
figs. 5 and 6. 
If the Christmas census has any value in making 
determinations of relative abundance of species of rap- 
tors from early to later years, it should become apparent 
from an analysis of frequency of occurrence and density, 
the latter determined by number of birds per observer- 
hour, birds per party-hour, and birds per mile. 
Knowing that the southern region of the state is 
more heayily forested than the central and northern, we 
would expect higher frequencies there for such wood- 
land birds as the red-shouldered hawk and the barred 
owl and lower frequencies for such prairie-loving species 
as the short-eared owl and therough-legged hawk. These 
relative frequencies appeared as expected, figs. 5 and 6. 
In winter certain raptors tended to concentrate at 
latitudes farther south than in summer, regardless of 
habitat, as though responsive to the temperature cline. 
The Cooper’s hawk, marsh hawk, and sparrow hawk oc- 
curred with increasing frequency from north to south. 
Conversely, the snowy owl and the saw-whet owl oc- 
curred with decreasing frequency from north to south. 
Frequencies reported for the sparrow hawk and some 
other species tended to increase in later years, but 
these apparent increases do not necessarily indicate 
real increases in raptor populations. The increases re- 
ported in later years probably resulted largely from 
