170 



MASS. EXPERIMENT STATION BULLETIN 242 



are practically identical with figures based on the number of individual birds. 

 Tiie greatest proportion of high persistency birds occurred in tlie 1922 flock, 

 and the smallest percentage in the 1923 flock. The years 1924 and 1926 show 

 practically a constant proportion of higii and low persistency females. 



No relation is apparent between iiigh persistency and mortality rate. In 

 tiie five-year period 2869 birds are considered, about 73 per cent of which are 

 highly persistent. The total mortality rate for the persistent pullets is 31.89 

 per cent, and for the low persistent group the figure is 32.03 per cent. 

 Therefore no difference in vigor between the groups is apparent, and per- 

 sistency and vigor are independent. 



12. Relation of Winter Egg Production to Mortality Rates in the Laying 

 Houses. 



Harris (1926 and 1927) has shown that the egg production of birds dying 

 during their pullet year is lower than production for the same time interval 

 by birds that survive to the end of the year. His records were obtained on 

 White Leg'horns, Rhode Island Reds and White Wyandottes in the Storrs 

 Egg Laying Contests. Harris also found that the egg records of birds that 

 died were more variable month by month, indicating that both good and in- 

 ferior layers were concerned. This worker concluded from his studies that 

 lower mean monthly production tended to lie associated with lower vigor, and 

 that the relation between egg production and mortality rate is definite but 

 probably highly complex. 



The pullets hatched during the five years covered by tliese studies are tabu- 

 lated to discover whether heavy laying affects mortality in this strain. Only 

 pullets surviving to March first are considered. The individual e^:^ records 

 to March first of all pullets dying between March first and the dlose of the 

 pullet laying year are grouped together, and the individual egg records to 

 March first for aJl pullets living to the close of the year are placed in another 

 group. In this manner it is possible to discover relations between winter egg 

 production and subsequent rates. 



Table 11 — Relation of Winter Egg Production to Vigor 



