EFFECT OF SEX RATIO 355 



and all sources of loss were removed. 



In actuality, however, this re]3roductive potential is progressively diminished by a host of 

 influences which translate it into productivity and populations of mature birds. These are con- 

 sidered in detail in chapter XII. Only the inherent factors of reproduction are covered here. 



The subject is opened with a brief discussion of the sex ratio. While the proportion of 

 males to females is primarily important as a prelude to productivity and will be discussed 

 fully under that topic, its inherent tendencies are a factor of reproductive capacity. 



EFFECT OF SEX RATIO 



Among newly-hatched grouse chicks the proportion of males to females is normally about 

 equal. Laboratory examinations of 464 grouse chicks under three months old, revealed 51.5 

 per cent to be males and 48.5 per cent females. In younger age classes the balance was even 

 more perfect. 



As the summer wanes there seems to be a consistent tendency to a slight preponderance of 

 males among the birds of the year. This often results in a few more males than females in 

 the whole population through the fall and winter. Then the balance changes in the spring as 

 the males suffer a somewhat higher loss as a result of courtship activities. In later s|)ring and 

 through the summer, the po|)ulalion of females among the older birds generally exceeds that 

 of the males*. 



There is no indication in the data gathered tliat the capacity for reproduction has at any 

 time been adversely affected by the sex ratio. It is still conceivable, however, especially in 

 marginal coverts, that the ratio of a scattered population might he so unbalanced as to prevent 

 sufficient breeding to niairitain ibc |i(i|iulali(in level. 



PROPORTION OF NON-BREEDI\G BIRDS 



The proportion of a population that fails to breed during the spring season lowers the 

 reproductive potential by that niiK li. Failure to breed may result from inunaturity (although 

 not in the grouse), phvsiological dilliculties, or lack of contact with birds of the other sex. 



The evidence on this factor for each sex will be taken up separately. 



Males 



Determination of the proportion of male grouse that breed is difiicult for it is imjiossible 

 to obtain enough direct observations. Hence one must rely upon the results of the spring 

 mating season as evidenced by the degree of fertility of the females. This is not entirely 

 satisfactory since it is apparent that, being promiscuous, even if every female nested and all 

 their eggs were fertile, it would still he possible that some male grouse failed to mate. 



The data indicate that no appreciable failure in breeding among male birds has occurred. 

 Only three nests of all that have been observed have contained complete clutches of infer- 

 tile eggs. Conceivably, these could have resulted from lack of contact or from inability of a 

 male bird to mate. They more likely resulted from physiological deficiencies in the females. 



The captive birds at the New York State Research Center have shown the same evidence of 

 promiscuity that ajipears to be the habit in the wild''. Five females placed in a pen with a 

 single male all laid fertile eggs. 



* Sci! Chapter XU. p. .SI 1. 

 A See Chapter V, p. 1107. 



