Table 7.- Data from aerial surveys of deer on the Rock River range, principally in Ogle and 
Winnebago counties, Illinois, 1949 and 1950. 
Range Flown, 4 ae Estimated 
Date Square Miles Visibility Deer Seen Population 
February, 1949 63 Good 661 1,002 
February, 1950 83 Fair 559 847 
December, 1950 63 Good 656 994 
correction factor for the Rock River range, the 
calculated correction factor, 66 per cent, was used. 
It is interesting to note that the estimated 
numbers in the herds varied by only eight in the 
2 years when visibility was rated “good.” In 
February of 1950, when the visibility was consid- 
ered only “fair,” the estimated number was about 
150 short of the estimated number in the other 
years. The low count was thought to have reflect- 
ed a shortcoming of the aerial census, rather than 
an actual reduction in population, for the writer, 
while conducting field studies, did not observe a 
population decline between the fall of 1948 and 
the spring of 1951. Nor did landowners and con- 
setvation officers in this area who were inter- 
viewed believe that the deer population had 
changed materially during this period. 
Population on the Horseshoe Lake Game 
Refuge.—The earliest information available 
concerning the deer population on the island of 
the Horseshoe Lake Game Refuge, fig. 12, is 
that supplied by Leopold, Sowls, & Spencer (1947: 
166), who stated that the population had in- 
creased from an original planting of 1 buck and 3 
does in 1933 to a total of 250 deer by 1944. The 
herd was reported to have been reduced by trap- 
ping until about 150 were present at the outset of 
the winter of 1947-48 (Swears 1948:15). 
More information on the status of the herd on 
the island of the Horseshoe Lake Game Refuge 
was brought to light in 1950. A drive on April 22 
of that year revealed 168 deer. The next census, 
December 9, 1950, produced a count of 195 deer. 
Nineteen animals had been removed during the 
fall of 1950 by live trapping; thus, the total popu- 
lation in early fall, 1950, is estimated to have 
been 214. This represented a 27.4 per cent in- 
crease over the spring population. 
The area per deer on the island in the spring 
of 1950 was approximately 7.1 acres; by early fall 
the area per deer had been reduced to approx- 
imately 5.6 acres. These figures: represent popu- 
lation densities considered high for deer range of 
average quality. The gravity of the population 
ptoblem on the island became especially apparent 
14 
when it was realized that about 750 of the ap- 
proximately 1,200 acres there were being used to 
grow field crops, which in themselves were not 
desirable deer foods. 
The effects of a deer population on its range 
were discussed by O’Roke & Hamerstrom (1948) 
in their paper on the George Reserve deer herd. 
This southern Michigan herd had its beginning in 
March, 1928, when four does and two bucks were 
released in the enclosure on the reserve. Ap- 
proximately 1,200 acres in size, the reserve con- 
sisted of about 46 per cent grassland, 43 per cent 
woody vegetation, and 11 per cent marsh and bog. 
“In the fall of 1933 it became apparent that 
the deer had increased phenomenally and that 
vegetation was being drastically overused,” wrote 
O’Roke & Hamerstrom (1948:79). The first deer 
drive and count on the area indicated that a deer 
population of about 160 inhabited the area in the 
early part of the winter of 1933-34. The area per 
deer for this population was approximately 7.5 
acres. 
In the winters beginning with 1933-34 and 
ending with 1940-41, the early winter herds fluc- 
tuated between 112 and 210 deer. O’Roke & 
Hamerstrom (1948:85) related that “There has 
been a noticeable improvement in the under-story 
since the herd was cut back to an average of about 
55 [per section] in the winter of 1941-42.” The 
area per deer for this population was about 11.6 
acres, 
Writing in 1947, O’Roke & Hamerstrom (1948: 
86) stated that “there has been some recovery 
during the last four or five years, . . . . but the 
invasion of brush and trees into the old fields is 
still almost at a standstill.” The deer density 
had declined until, early in the winter of 1946-47, 
the area per deer was about 16.2 acres. 
Sex and Age Ratios 
Data on the sex ratios of deer on the Rock 
River range were obtained from three sources: 
(1) live trapping, (2) fatality counts, and (3) field 
observations, table 8. In 4 years of trapping, 78 
