MONTHLY WILDLIFE RESEARCH LETTER 
Vol. 3, No. 1 
Page 2 
W-42-R-9 
R. Lord 
Analysis of litter size in Illinois cottontails has shown a difference 
between those in the southern and those in the central parts of the state. Litters 
were smaller in the southern part and the difference was statistically significant. 
This finding led to an analysis of litter size in relation to latitude in rabbits 
in the United States. It was found that there was a statistically significant 
positive correlation between litter size and latitude. The most likely explanation 
of this phenomenon is contained in Errington's principle of inversity. When 
mortality is high, as in the north, there must be a compensating increase in 
reproduction to maintain the species. Thus, it appears that there is a higher 
mortality rate among rabbits in northern Illinois than those in southern Illinois. 
Analysis of mortality data shows that the months of May, June, July and 
August produce 73 per cent of the rabbits that are taken by hunters in fall. This 
indicates that an annual census performed in September will most nearly determine 
the size of the rabbit population. Most of the year's reproduction will have taken 
place and the young will have attained an age and size that will be present on 
roads for counting. Analysis of rabbit activity along roads indicates that rabbits 
in September were most active from 7:C0 p.m. to 1:00 a.m., which is therefore the 
best time to make a roadside census in that month. 
Eugene F. Dudley of the University of Michigan visited the Survey to confer 
with the project leader on the problems of cottontail research. 
W-55-R-4 
F. Bellrose 
The annual winter inventory of waterfowl in Illinois was conducted on 
January 4, I960, at which time 529,890 mallards and 13,770 black ducks were 
counted. The annual winter inventory on January 9 and 10 of 1959 resulted in 
counts of 461,000 mallards and 6,000 black ducks. 
V- 56 -R -4 
G. C„ Sanderson 
Through January, tags were obtained from 26 of the 71 raccoons live-trapped 
on the Piatt County study area during 1959. Thus, hunters killed at least 3 7 
per cent of the marked raccoons on or near the study area compared to 14.5 per 
cent during the previous season. 
Because the disease among the captive raccoons described in the November 
Newsletter was not active during the last half of December and the first half of 
January, we hoped that the susceptible animals had died and the disease had 
disappeared from the captive raccoons. This did not prove to be the case; in the 
latter half of January captive raccoons began to die again of the same disease. 
In this new outbreak, adult animals were dying which apparently were immune 
earlier. Some died even though they had not died in October or November when one 
or more raccoons died in the same cage with them. Thus far, no positive cases of 
this disease has been identified in wild raccoons. 
