MEASUREMENT OF THE WINTER CYCLE IN THE EGG 

 PRODUCTION OF DOMESTIC FOWL^ 



By Raymond Pearl, 

 Biologist, Maine Agricultural Experiment Station 



In a series of papers the writer and his associates (2, 6, 9)^ have shown 

 that there are to be distinguished definite cycles in the egg-laying activi- 

 ties of the fowl. The two most striking and definite of these cycles we 

 have called, respectively, the "winter" and the "spring" cycles, these 

 terms being used because of the seasonal incidence of these periods of 

 laying activity. In the writer's studies on the inheritance of fecundity 

 (4, 5, 7, 8) in the fowl he has used as an index of the innate fecundity of a 

 bird its pullet-year "winter production," defined as the number of eggs 

 produced before March i of the bird's pullet year — i. e., the first March i 

 following the individual's birth. The reasons why this measure of 

 productivity rather than some other was chosen for the work have been 

 fully set forth in earlier papers and need not again be gone into here. It 

 may suffice to say that, by all the tests which it has so far been possible 

 to apply, this index of fecundity has proved very satisfactory in practice. 

 The results which one obtains with it are duplicated in every essential 

 particular if one uses the longer period of one year, but genetic differences 

 in fecundity are more strongly emphasized in the shorter period, with a 

 corresponding gain in the precision and certainty of the Mendelian 

 analysis. 



It has never been contended, however, in any of the writer's work that 

 winter production, as above defined, was anything more than an index 

 or indicator of innate fecundity. It is logically obvious that the only 

 perfect measure of total fecundity would be some direct function of total 

 fecundity. All that the writer's work has shown regarding the point 

 here under discussion is that winter production is a good indicator, all 

 things considered, of a fowl's innate fecundity capacity. It is not a 

 perfect indicator, but that it is a good one is confirmed not only by 

 the experience of this laboratory but also by that of other workers 

 (i, 3. 10). 



In the course of the writer's investigations regarding this character, 

 studies have been made of various other fecundity indicators besides 

 winter production. The thought occurs to one that possibly under other 

 environmental conditions than those prevailing in Maine winter produc- 



' Papers from the Biological Laboratory of the Maine Agricultural Experiment Station, No. 89. 

 2 Reference is made by number to "Literature cited," pp. 436-437. 



Journal of Agricultural Research, Vol. V, No. 10 



Dept. of Agriculture, Washington, D. C. Dec. 6, 19 15 



ay Maine — 6 



(429) 



