MONTHLY WILDLIFE RESEARCH LETTER 
Department of Conservation and Natural History Survey, Cooperating 
Glen C. Sanderson and Helen C. Schultz, Editors 
Urbana, Illinois 
March, 1975 
Vol. ig, No. 3 
Ecology and Management of Sgu? rrels C. M. Nixon, 
S. P. Havera 
How serious is the loss of nestling squirrels because hunters shoot nursing 
females during the early weeks of Illinois' squirrel season? 
Using 1973 as a base year, we have estimated the loss of nestlings in both 
zones per 100 acres of forest land. We have based our estimate on reports from 
our hunter cooperators, on breeding rates of female squirrels examined during 
the summer and fall in Illinois, and on breeding rates reported for both gray 
and fox squirrels throughout their occupied range. 
We found that the loss averaged about eight nestlings per 100 acres of 
forest in both zones. 
To place this loss in perspective, we need to consider the density of the 
fall squirrel population that experiences such losses. Average fall densities 
of squirrels equal or exceed 100 squirrels per 100 acres in most of the Illinois 
squirrel range. Livetrapping and tagging studies have produced estimated densi¬ 
ties of 1.7 (fox squirrels, Piatt County), 1.7-2.2 (mixed population of fox and 
gray squirrels, Vermilion County), and 1.5-1.8 (both species. Pope County) 
squirrels per acre in Illinois in mixed hardwood stands 40+ years old. These 
densities indicate that a loss of 8-10 nestlings per 100 acres represents less 
than 10 percent and probably nearer 5 percent of the fall squirrel population. 
At present harvest levels, such losses seem easily sustainable and have not 
appeared to affect the year-to-year squirrel harvest. 
There is evidence for gray squirrel populations, both hunted and protected, 
that the postweaning survival of summer-born young is substantially less than 
for spring-born young. Most of these summer-born squirrels become active after 
15 September, when there are fewer young of buffer species available to absorb 
predator pressures. They must spend much of their time on the ground hunting 
food, which is mostly eaten or cached by other squirrels, and must defend them¬ 
selves against adult squirrels and spring-born young who are usually quite 
intolerant of these recently weaned juveniles. In most years, these summer- 
born young are not recruited into the population as permanent residents, but 
disperse or die. It seems likely that a similar situation also occurs with fox 
squirrels. 
NATURAL HISTORY SURVEY 
