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MONTH Uf WILD LITE RESEARCH LETTER 
department of Conservation and Natural Hiatory Surrey, Cooperating 
T. 0. Scott, Editor 
' rl>ana, Illinois April 1959 Vol. 2, No. U 
W-30-R.12 R. F. Labisky 
The breeding activities among pheasants on the Sibley area were re* 
tarried during March, 1959* The frequency of crowing by cooks was only 2 calls 
per ?-minute period in mid-March; comparable counts for 1957 and 1958 were about 
10 arid A cock calls per 2-minut«? period, respectively. Because the photoperiod 
i3 important in triggering the sexual activity in birds, the unusually low amount 
of rrunlight during early March probably caused the lag in the breeding cycle of 
pheasants. 
The occurrence of warm, sunny weather during April caused an inereeee 
in the sexual activities among pheasants. Crowing by cocks increased from s 
rate of calls per 2-minute period in early April to 3b cells per Mlcsta. 
period in late April. Antagonistic behavior among cocks caused a break-up of 
groups of cocks early in the month; the break-up of groups of hens did not 
occur until mid-April. The establishment (and defense) of territories by oooks, 
and the formation of harems by hens occurred shortly after mid-April. Hens 
reached the threshold of copulatory response on April 21; comparable dates for 
attainment of this threshold in 1957 and 1958 were April 18 and April 20, re¬ 
spectively. Thus, regardless of the variations in early spring westhsr that 
occurred during 1957, 1958, and 1959, the threshold of copuletory response of 
hens was reached at nearly the seme time each year. 
W-L2-R-8 R. D. Lord 
Examination of 277 rabbits shears that the closure of the epiphysis in 
cottontails takes place from 5 to 12 months of age; by tbs 8th month nearly all 
have closed epiphyses. Use of the epiphysis to determine Juvenile-adult retioe 
therefore becomes unreliable as early as August; by October most rabbits born in 
March will be classed ea adults. 
Examination of 36} rabbits aged from 1 to 13 months shows that Illinois 
cottontails apparently grow faster the first U months of their life then do Ohio 
cottontails, for which considerable data are available. 
Calculations based on information about growth-rate of rabbit eyes, 
more refined than those of lest year, indicate that 28.5 per cent of the rabbit 
crop of 1958 resulted from Juvenile breeding. The somewhat cruder estimate made 
tiMliftAL hi&iw'rt! iijrivc.l 
1954 
