WINTER CYCLE AND WINTER PAUSE 



169 



hotli upon environment and inlieritance. The time wlien a group of jiullcts 

 will begin to lay depends in part upon hatching date, method of feeding and 

 management, and upon weather conditions — all of which may be classed as 

 environmental. The dependence of date of first egg upon age wlien begin- 

 ning to lay, however, is a relation to a heritable trait, since Hays (1924) has 

 shown age at first egg to be inherited. 



1. Correlation Between Hatching Date and Length of Winter Ci/cle. 



Time of hatching is generally believed to hold an important relation to the 

 time of appearance of winter pause. Since the appearance of winter pause 

 marks the termination of the winter cycle, tJie I'ossibility exists of a relation- 

 ship between hatching date and length of winter cycle. The table presented 

 below tends to substantiate a relation between date of hatching and length 

 of winter cycle for the total population of birds actually manifcting a 

 winter cycle terminated by a pause: 



Hatches 

 1 

 2 

 3 



4 



5 



6 



7 



8 



Grand Average 



Tlie mean length of winter cycle is sliown to consistently decrease as tlie 

 hatching date advances, with but a single exception in the last hatch. There 

 are eight hatches each year at one week intervals from March 25 to May 15. 

 The total difference in age between the first and last hatches is 49 days, 

 wliile the difference in mean winter cycle length is 30 days. The ability of 

 the later hatched pullets to reach sexual maturity at a slightly earlier age 

 than do early hatched pullets (Hays, Sanborn, and James, 1924) i)robably 

 accounts for the minor inconsistencies in the above table. The means of the 

 eight different hatches for the nine-year period covered by the table indicate 

 a rather important relationship between date of hatching and length of 

 winter laying cycle, which is determined by t!ie onset of winter pause. In 

 this connection, the reader siioidd bear in mind that only winter pause birds 

 are included in the tabulation because no winter cycle can be ascertained 

 in non-pause birds. 



The absolute relation between hatching date and lengtli of winter cycle 

 may be discovered by means of the coefficient of correlation. Available for 

 study are 2078 birds. Class intervals of ten days are used for winter cycle 

 in calculating the following constants: 



Number of birds .... 



Mean hatching date (Apr. 17) . 



Hatching date standard deviation 



Mean length of winter cycle 



Winter cycle standard deviation . 



Coefficient of correlation ...... — .3174 ± .0133 



Regression of hatching date on winter 



Regression of winter cycle length on hatching dale 



