MONTHLY WILDLIFE RESEARCH LETTER 
M- 3,f\o .lO 
J 
Department of Conservation and Natural History Survey, Cooperating 
T„ G. Scott, Editor 
Urbana, Illinois Nouemh&X, 1960 Vol. 3, No. 10 
W-30-P-14 R. F. Labisky, R. I. Smith 
The 1960 field investigations of the reproductive ecology of the pheasant 
on the intensively cultivated Sibley area were completed in October. The 100 
randomly- selected plots in this area have been searched for nests each year since 
1957. A total of 280 nests were found in 1960, which exceeds the previous maxi¬ 
mum for a single year (1958) by 14 nests or 3.7 per cent. In 1960, 30.36 per 
cent of the established nests were successful. Comparisons of nest establishment 
and nest success in each cover type for the past four years are presented in 
Table 1. 
W-42-R-10 R. D. Lord, Jr. 
A growth-rate curve of young cottontails has been calculated for use in 
estimating the date of birth of juveniles captured in the wild. The curve is 
based on 973 observations of 50 rabbits raised in captivity and 234 observations 
of 119 wild rabbits recaptured at intervals. The large number of observations 
makes it possible to calculate the 95 per cent confidence limits for the curve 
for application to an estimate. For example, a young rabbit weighing 200 grams 
(about 7 ounces) is estimated to be 33 days old and 95 out of 100 rabbits weigh¬ 
ing the same are between 25 and 44 days old. This curve has been developed to 
100 days of age. After 100 days the individual variation of gain in weight is 
so great that an estimate of age based on body weight is a poor one. 
W-55-R-4 F. C. Bellrose 
Four aerial censuses were made of waterfowl populations in the Illinois 
and Mississippi River valleys during October. Populations of ducks increased 
from 75,000 on October 5 to 421,000 on October 24. 
During the month of October, species of ducks most abundant were green¬ 
winged teal, widgeon, pintail, and mallard. Mallards made up 4.5 per cent of 
the duck population on October 5, 23 per cent on October 12, 56 per cent on Octo¬ 
ber 19, and 60 per cent on October 24. 
The numbers of diving ducks were much below normal for October, especi¬ 
ally in the Illinois River valley. 
