10 MASS. EXPERIMENT STATION BULLETIN 416 



far from optimum as far as temperature is concerned. During the season the 

 reproductive system is often depleted following the long period of winter and 

 spring production. In general, birds that show the ability to lay at a high rate 

 in suanmer are superior for breeding purposes. It is desirable to know whether 

 winter clutch size is a reliable criterion from which to judge the probable summer 

 rate of laying. The coefficient of correlation furnishes a good measure of associa- 

 tion and this was determined between winter clutch size and summer clutch size. 

 The following constants appeared: 



Number of birds 1458 



Mean winter clutch size — eggs 3.23 



Winter clutch standard deviation ± 1.42 



Mean summer clutch size — eggs 3.06 



Summer clutch standard deviation ± 1.68 



Coefficient of correlation + .3218 + .0158 



Correlation ratio .3539 



The above means indicate that the rate of laying declined in the summer as 

 compared with the winter season. There is also evidence that the variability 

 in clutch size is greater during the summer months. Regression of summer clutch 

 size on winter clutch size was not strictly linear so that the correlation ratio .3539 

 expresses the association. This is a rather intimate correlation and indicates 

 that large clutch size in winter is likely to persist through the summer. 



16. Whiter Clutch Size and Fall Clutch Size 



Birds hatched in March and April usually complete their first 365 days of 

 laying in September or October. In this study the period including September 

 and October is designated as the fall period. Knox, Jull and Quinn (1935) 

 pointed out that the number of eggs laid in August and September at the close 

 of the first laying year was the best measure of persistence in White Leghorns 

 and Rhode Island Reds. They reported the partial correlation between produc- 

 tion during these two months and annual production to be + .733 for Leghorns 

 and + .772 for Rhode Island Reds. A high rate of laying in September and 

 October is maintained only by superior birds. It is therefore very important to 

 know whether the mean clutch size of the first winter is a criterion of possible 

 clutch size in September and October at the end of the laying year. The coeffi- 

 cient of correlation was therefore calculated between winter clutch size and fall 

 clutch size, giving the following constants: 



Number of birds 1297 



Mean winter clutch size — eggs 3.26 



Winter clutch standard deviation. ..... ± 1.46 



Mean fall clutch size — eggs 2.21 



Fall clutch standard deviation + 1.24 



Coefficient of correlation + .2646 ± .0174 



Correlation ratio .3306 



The above records show that intensity was at a low level in September and 

 October. The variability in clutch size was also considerably greater in the fall 

 season. Regression of fall clutch size on winter clutch size was non-linear, but 

 the correlation ratio, .3306, showed an important association between the two. 



