THE AVEIGHT OF GROUSE 137 



ance of the hen is at its highest. The fact is sufficiently proved by the com- 

 parison of the weights of the sexes, but if further confirmation be required 

 it would be found in the fact that in June as the cock increases in weight 

 so he becomes less liable to disease, whereas the hen, whose weight is on 

 the downward grade, continues to suffer, and sometimes to die, throughout 

 the summer months. 



The fact that the average weight of the cock is slightly on the upward 

 grade during the months of greatest mortality is somewhat misleading, unless it 

 be remembered that he is still far below his best condition, and was probably 

 about his worst at the time when he first contracted the infection. 



The reason why cocks do not die in September, or hens in November, 

 when their respective weights are again at their lowest, is obvious mere loss 

 of condition is not enough to cause death. It is only where this loss of 

 condition is found in conjunction with a heavy infection of parasites that it 

 becomes a source of serious danger. 



To return, however, once more to the healthy bird, it is to be remembered 

 that in June the hen undergoes a complete post-nuptial moult, 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. 



In changing the winter plumage for a nuptial breeding dress in January 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 carefully investigated and described 

 by Mr Ogilvie-Grant, and have been dealt with in another chapter of this 

 Report. 1 



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

 March 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 Effectg of 

 definite effect upon the weight of the bird. There are, therefore, moult - 

 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. 



1 See chap. iii. p. 34. 



