Vol. 6, No. 2 
Page 2 
3* Factors Inf 1uencing Pistribution and Abundance of Pheasants W. L. Anderson 
It has been postulated that high air temperatures during the egg laying period 
has an important influence in limiting the southern distribution of pheasants. Lab¬ 
oratory experiments by Ralph E. Yeatter, Illinois Natural History Survey, have demon¬ 
strated that the hatchability of eggs from game farm pheasants declined to near 50 
percent when exposed to preincubation temperatures between 78° and 83° for 9 hours a 
day during 7 consecutive days. If high temperatures inhibit hatching of pheasant 
eggs on the Neoga area, which is south of the contiguous range, the proportion of 
chicks that hatched from nests subjected to temperatures higher than 78° would be ex¬ 
pected to be less than the proportion of chicks from nests established during periods 
of lower temperatures. In 195l, 50 percent of 117 young pheasants captured on the 
Neoga area during late summer had hatched from eggs exposed to preincubation tempera¬ 
tures higher than 78°. Also, the number of chicks observed in 19 broods which hatched 
from clutches exposed to preincubation temperatures higher than 78° averaged 8.6, 
whereas 23 broods from clutches not exposed to such temperatures contained an average 
of 7.4 chicks. These findings do not preclude the possibility that high temperatures 
prevent entire clutches from hatching. However, these data cast some doubt on the 
role of high air temperatures during the nesting season in preventing the establish¬ 
ment of permanent pheasant populations at Neoga. 
4. Rabbit Management W. R, Edwards, D. A. Casteel 
The influence of hunting on cottontail abundance is being evaluated as part of 
the study on the dynamics of the rabbit population on the 4-H area of Allerton Park. 
A review of the estimated numbers of rabbits on the area for the years 1956-1962 
revealed a considerable annual fluctuation. Table 1 presents the data on the abun¬ 
dance of cottontails on the 4-H area. These data suggest' that hunting might be a 
factor controlling rabbit abundance. 
When the number of rabbits present in spring was plotted against proportionate 
mortality, it was found that rabbit abundance in spring was mortality dependent. 
There did not appear to be any strong relationship between total rabbits in the fall 
and the population the following spring. Apparently, mortality was not density pro¬ 
portionate nor was it dependent on numbers of rabbits taken by hunters. It was con¬ 
cluded that hunting was not the dominant factor controlling population size in spring. 
There was no strong relationship between the number of rabbits on the area in 
spring and the number present the following fall. However, the proportionate increase 
from spring to fall was inversely related to the density of rabbits present in spring; 
the more adults, the less the percentage of young in the fall population. The females' 
ability to reproduce, or the survival of young, are therefore density dependent. This 
phenomena, referred to as "inversity" by the late Dr. Errington, is typical of animal 
populations in general. It is responsible for the so-called "balance of nature," 
This principle of animal ecology is fundamental to the sustained harvest of game ani¬ 
mals. 
I 
The suspected primary cause of year-to-year changes in density of cottontails was 
annual fluctuations in weather. No consistent relationship was evident between winter 
weather and mortality of cottontails. Similarly, no relationship was evident between 
