Vo 1. 3, No. 10 
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eggs (based on six nests found in 1966 that were believed to have complete clutches) 
and a hatchability level of 98 percent (based on five clutches), an average of 
10.4 chicks should have left the average successful nest in 1966. Some indication 
of the gradual attrition of brood size is given in Table 5* The average number of 
young per brood declined from 9*3 chicks at 1 week of age to 3*0 chicks at 10 
weeks of age. Although the average brood in 1966 was slightly larger than in 
either 1965 or 1964 (Table 5)> less than one-third of the chicks leaving a 
successful nest in 1966 survived to 10 weeks of age. 
As in past years, hay mowing was a mortality factor acting on prairie chicken 
broods in 1966. At least six members of one brood were killed by hay mowing, and 
on one occasion a member of a 1-week-old brood was killed by a car as the hen 
attempted to move her chicks, when disturbed by mowing, across a public road. On 
two occasions broods were reluctant to leave the unmowed portions of legume fields 
during the mowing operations; fortunately, both farmers delayed their mowing long 
enough to allow the chicks to leave the fields. It is hoped that various means 
can be found to increase the attractiveness of the sanctuaries to broods and 
thereby increase brood survival. 
Table 5* Numbers of young in prairie chicken broods, Bogota Study Area, 1 963 - 66 . 
Sample sizes are in parentheses. 
Estimated Age 
of Young in 
Weeks 
Average Number of 
Young per Brood 
1963 
1964 
1965 
1966 
1 
— ( 0 ) 
— ( 0 ) 
— ( 0 ) 
9-3(3) 
2 
— (0) 
— ( 0 ) 
1 . 0 ( 1 ) 
4.0(1) 
3 
— ( 0 ) 
— ( 0 ) 
6 . 0 ( 2 ) 
— (o) 
4 
9-0(4) 
8 . 5 ( 2 ) 
7 . 0 ( 2 ) 
7-3(4) 
5 
9-0(3) 
5.3(3) 
— ( 0 ) 
4.0(1) 
6 
— ( 0 ) 
5-3(6) 
4.0(1) 
— ( 0 ) 
7 
6 . 0 ( 1 ) 
5-5(2) 
1 . 0 ( 1 ) 
— ( 0 ) 
8 
6 . 5 ( 2 ) 
— ( 0 ) 
13-0(1) 
4.0(2) 
9 
---( 0 ) 
— ( 0 ) 
— ( 0 ) 
3-0(1) 
10 
— ( 0 ) 
— ( 0 ) 
— ( 0 ) 
3 . 0 ( 1 ) 
Avg. No. of Young 
8 . 2 ( 10 ) 
5 . 8 ( 13 ) 
5 . 6 ( 8 ) 
6.1(13) 
Avg. Age of Broods 
in Weeks 
5.4 
5.6 
4.6 
4.7 
6. Rabbit Management J. A. Bailey, J. C. Hanson 
Methods for radio-tracking seven cottontail rabbits on the University of 
Illinois Farms during January and February, 1966, have been described in an 
earlier report (Monthly Wildlife Research Letter, September, 1966). Some 
observations of the kinds of habitat used by these animals are reported here. The 
study area is an intensively managed experimental orchard having we 11-de1ineated 
habitat types arranged in rectangular patterns. Each cover type was classified 
according to the density of its overstory (more than 2 feet tall) of woody 
