CLASSIFICATION AND LIFE HISTORY 37 



changing from the now faded breeding or nuptial dress, to the 

 autumn or summer plumage, and this she cannot do without 

 an appreciable drain upon her resources, which is clearly reflected 

 in her weight. 



In changing the winter plumage for a nuptial breeding 

 dress in April she differs radically from the cock, who retains 

 his winter plumage until the breeding season is over. The 

 two sexes moult at different seasons, and each twice within 

 the year. The details of the changes have been dealt with 

 in another chapter. 1 



It has been pointed out that the cock bird begins to grow 

 new feather in April and in August, whereas the hen bird 

 begins to grow new feather in February and July ; and each 

 of these moults appears to have a definite effect upon the 

 weight of the bird. There are, therefore, fluctuations in the 

 weight of the healthy Grouse which are partly due to the 

 moult, and are therefore seasonal, while others are purely sexual ; 

 it must be noted that the seasonal fluctuations differ as to date 

 in each sex. Both seasonal and sexual changes occur in normal 

 healthy birds. These fluctuations must be fully recognised 

 before any useful deductions can be drawn regarding the 

 changes of weight in birds that are or have been diseased. 



The following list includes most of the conditions which 

 commonly affect the weight of Grouse : 



I. IN HEALTH. 



(a) Sex, generally in favour of the male, but in April 



and May rather to the advantage of the 

 female. 



(b) Late hatching, producing birds of both sexes unready 



for the winter ; birds which have missed the best 

 growing months of summer, and which therefore 

 remain permanently undersized and of a poor 

 physique though not actually diseased. 



(c) Moult, in the male taking most effect in March 



and in September; in the female in July and 



1 Vide chap. ii. p. 42. 



