Ecology and Management of Squirrels 
Vd1» 19 , Noi & 
Page 2 
_ C.M. Nixon, 
S. P. Havera 
Monitoring the reproductive status of squirrel populations in late winter 
and early spring often indicates the successfu 1 ness of the approaching squirrel 
season. Squirrels born in February and March, from the first or winter breeding 
season, are available to hunters at the beginning of the season in August and 
September. However, squirrels born in July and August, from the second or 
summer breeding season, do not become self-dependent and available to hunters 
until October, when interest in squirrel hunting is waning. 
The reproductive condition of squirrels in east-central Illinois has been 
monitored for six consecutive springs. Two study areas in Vermilion County have 
been 1ivetrapped for 10 days in May since 1971- The numbers of adult female 
squirrels in breeding condition were noted. Based on sample sizes of between 20 
to 40 adult female fox and gray squirrels, the proportions that had spring litters 
S.^X-^.^Te 13 percent ’ 96 —' - 79 
One key to spring squirrel reproduction is the abundance of nuts in the fall 
If mast crops are normal or abundant, squirrels feed well in the fall enter the 
consumotio^d 9 °°h C ° nditi ° n ’ and have an am P ,e supply of nuts stored for 
consumption during the winter months. When the breeding season arrives in late 
December January, and February, the adult female squirrels that have had an 
enterT/ W 'T r diet are in sufficlent, V good physical condition to 
enter estrus. Then, depending on the size of the population, reproduction is 
verage or above If, however, the nut crop is low, then reproduction may be 
below average. A poor mast crop coupled with a severe winter can drastically 
reduce spr.ng reproduction as well as squirrel populations. 
valuahlI° r T ati0n C °' ,ected ° ver a P eriod of y ea rs at the same locality provides 
c ' aes to the interpretation of population changes of animals. For 
instance, the_squirrel breeding data previously presented document the response 
! « I hITn reproduction, and therefore squirrel population size, to the 
size or tne tall nut crop. 
from AQ U tr>°R4 the 5 '* Va '“f S for the s P r ' n 9 breeding rate from 1971 to 1976 range 
fn 84 perce " t ' The two other values, 33 percent in 197^ and 96 percent 
wprl i UnU ! U ' However ’ the nut cr °P s preceding both these breeding seasons 
"t * ? ™ h 3St WaS "? mely Scarce ln the faH of 1 973 and the winter of 
973 74 brought heavy snowfall and below average temperatures. The result was 
w squirrel reproduction (33 percent) in the spring of 1974. The next year was 
abounded ° P ?° 5 ' te - f f" ° f ' 974 ' “^-Particularly the white oak crop-! 
abounded. The winter of 1974-75 was mild. Hence, the spring reproduction rate 
the 97 waaremarkabi y hl 9 h (96 percent). Population densities during both of 
these breeding seasons were similar; 24 adult females were examined in I974 and 
26 were examined in 1975 . yi 
ExmiS!L re !f h « t T, S ^ Uld have an avera 9 e or better season this coming fall. 
2 ' - 7 adul ‘ fox and 9 ray female squirrels throughout central Illinois 
^T '" 9 braed ' n 9 rate of approximately 78 percent, thus reflecting an 
1976 ?a IK h 6 r T Cr ? P throu 9 hout central Illinois last fall. The value for 
Illinois, 1 974 W anS ^75 5 ° f 5 ' PerCent ^ 89 perCent in southern and central 
This spring, freezing temperatures in late April and early May throughout 
' ,s may have ° ccur red when many of the nut trees were flowering. Although it 
areas ^ Y t0 855655 the dama 9 e ’ the mast cro P may be reduced this fall in some 
