Vol. 13, No. 11 
Page 3 
5 . Responses of Prai ri e Ch ickens to Habi tat Manipulation R. L. l/estemeier 
The means for clutch size, fertile eggs per clutch, and hatched eggs per 
clutch of prairie chicken eggs were higher in 1970 than in any of the preceding 
7 years on the Bogota Study Area (MWRL 13(10):2). However, the mean hatchability 
of 89.5 percent (349 of 390 eggs hatched) for 30 incubated clutches in 1970 was 
the lowest for the 8-year period. 
A correlation analysis between percent hatchability and clutch size showed 
an inverse relationship (£ = -0-933, P<0.01). When the mean clutch size was 
relatively high, the number of prairie chicken eggs that hatched tended to be 
correspondingly lower. There was no significant correlation between percent 
fertility and clutch size (£ = 0.029). Thus, decreasing hatching efficiency with 
increasing clutch size occurs independently of the level of fertility. These 
data suggest the possibility that clutch size in the prairie chicken is limited 
by the number of eggs that a hen can cover and successfully incubate. 
6. Rabbit Management 0. B. Rose 
The annual fall censusing of cottontail rabbits on the 100-acre (40-hectare) 
4-H Area at Robert Allerton Park is being continued this fall, the 15 th 
successive year. During the 10-day November trapping period, 104 cottontails 
were trapped and ear-tagged, 10 more than the 94 captured during the October 
trapping period. Estimates of the number of rabbits in the population, calculated 
from the trapping data, were l42 by the Schnabel method, 284 from the geometric 
maximum likelihood estimate (MLE-G), and 307 using the maximum likelihood 
estimate from a regression of the logarithm of the number of individuals in a 
frequency class on frequency of capture (MLE-R). The MLE-R is considered the 
best method of estimation. All three estimates are biased because, during the 
trapping, predators removed approximately 20 rabbits from the traps. These 
losses not only prevented the recording of some captures (both new captures and 
recaptures) but also precluded the possible recapture of the same 20 cottontails, 
thus reducing the proportion of recaptures in the total and thereby increasing 
the size of all three estimates (Schnabel, MLE-G, and MLE-R). Estimates 
adjusted for the trap losses were 137 (Schnabel estimate, 5 less than the 
unadjusted estimate) and 265 (MLE-G, 19 less than the unadjusted estimate). An 
adjusted MLE-R estimate was not possible. Thus, our population estimates, even 
when adjusted, show a definite increase in numbers of cottontails on the study 
area, which must be attributed to immigration. The data do not show any 
significant losses, from mortality or emigration, in the animals trapped in 
October. Consequently, the increase in population represents a movement of 
animals from outside the area into it. This movement may be a shift toward 
greater use of those portions of home ranges within the 4-H Area and less use 
of those portions in the adjoining cropland. This same pattern of increase in 
the cottontail population from October to November existed in I 968 but not in 
1967 or 1969- 
