CLASSIFICATION AND LIFE HISTORY 



33 



From a practical point of view the weight of Grouse is a 

 most useful indication of the health of a moor, this aspect of 

 the subject is fully dealt with in a later chapter, 1 and in the 

 present chapter it is proposed to describe only the seasonal 

 fluctuations in the weight of healthy adult birds. 



It is found that sex is a primary factor in determining the 

 weight of an individual bird. An adult cock Grouse is as a 

 rule heavier than an adult hen when both are well grown and 

 in really good condition. This is true all the year round, except 

 in spring, for at this time when the hen begins to sit she is 

 heavier and in better condition than at any other time of the 

 year, while the cock is not at his best. There is, therefore, 

 at this season, a tendency for the average weight of both sexes 

 to approximate, and even for the advantage to be on the side 

 of the hen. The difference in the fluctuations of weight between 

 the cock and the hen bird is shown in the Table given below, 



SEASONAL VARIATION IN AVERAGE WEIGHT OF HEALTHY GROUSE. 



OUNCES 



JAN 



FEB 



MAR 



APR MAY 



JUNE JULY 



AUG SEP OCf NOV DEC 



C0C/TS 



HENS 



\ 



The immediate reason for this difference in spring is prob- 

 ably the one which naturally suggests itself ; viz., that the 

 exigencies of courtship have a precisely opposite effect upon 

 the male and female. 



In December, the adult cock Grouse's weight averages 

 24'22 ounces compared to 21 '07 ounces for the hen, while in 



1 Chap. viii. p. 215. 



