No. 615] 



EGG PRODUCTION 



225 



be, the several types of rhythm shown by the daily records 

 are found to depend very much on the time of day that 

 the eggs are laid. 



None of the various types of rhythm, i. e., one half, 

 three fourths, etc., are characteristic of any one hen, al- 

 though many individuals seem to center about a par- 

 ticular rhythm, e. g., two thirds. A bird with this rhythm 

 may fall to the one half type but does not often, except 

 in the spring, exceed the three fourths type. While little 

 stress can be laid on this point, it is interesting to note 

 this tendency toward a definite rhythm in some individ- 

 uals. But aside from these considerations, birds of the 

 same age, which begin to lay at approximately the same 

 time and which do not become broody, do not lay with 

 the same rhythm. Thus, of two full sisters, hatched the 

 same day, one laid only about every other day, while the 

 second laid about five days out of six. The rhythm, then, 

 is quite an important factor in determining the number 

 of eggs laid. 



Various causes may interfere with the normal rhythm, 

 such as causes that interfere with the formation and 

 growth of the egg, and other causes such as environ- 

 ment, season, method of management and internal factors 

 such as broodiness. In many birds evidences of a rhythm 

 with a period of some length may be noted as shown on 

 page 215. The example given is of course idealized but 

 actual records of nearly the same type may be observed. 



In this connection the question of the nonproductive 

 periods, usually of short duration, that occur in some 

 records and which produce irregularities in the rhythm 

 may be discussed. As will be pointed out later, brood- 

 iness is responsible for some of these periods. A similar 

 period, that may be noted in some hens' records during 

 the winter, may be taken as an index of the existence of a 

 winter cycle. But other individuals may have two or 

 more such periods or may have a single period fairly 

 early in the winter (No. 4529, Fig. 13). Some of these 

 periods may be inherent in the individual's makeup but 



