WINTER CYCLE AND AVINTKH I'Al'Si: 



KIJ) 



both upon en\ ironiiu'iit aiul inlKTilaiuT. I'lic tiiiii- when a f:riiii|i of pullets 

 will begin to lay depends in jnirt ujxm liateliin}!: date, method of feeding an«l 

 management, and upon weatiur eondilions — all of whieii may l)e elassed as 

 rn\ ironmental. The dependence of date of first egg uj)on age when hegin- 

 lung to lay, however, is a relation to a heritable trait, since Hays (1!»'24-) has 

 shown age at first egg to be inherited. 



/. Correlation Between Jlatchiiuj Date and Lenyth of Winter Cycle. 



lime 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, the possibility exists of a relation- 

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

 1 flow tends to substantiate a relation l)etween date of hatching and length 

 (I winter cycle for the total population of birds actually manifesting a 

 winter cycle terminated by a pause: 



Hatches Number of Birds Mean Length of Winter Cycle 



1 32i> 68.33 Days 



2 267 62.54 " 



3 286 59.56 " 

 4. 281 51.91 " 



5 258 47.05 " 



6 237 42.00 " 



7 225 36.21 " 



8 195 38.78 " 

 Grand Average 52.26 " 



The mean length of winter cycle is shown to consistently decrease as the 

 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, 

 while 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, J924) [irobably 

 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 the onset of winter pause. In 

 this connection, the reader should 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 length 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 u.sed for winter cycle 

 in calculating the following con.stants: 



Number of birds 2078 



Mean hatching date (Apr. 17) 4.18 



Hatching date standard deviation .... ±2.26 



Mean length of winter cycle ..... 52.26 



Winter cycle standard deviation ..... ±34.23 



CoeflScient of correlation ...... — .3174 ± .0133 



Regression of hatching date on winter cycle length . — .021 



Regression of winter cycle length on hatching date . — 4.811 



