PLUMAGE CHANGES OF THE COCK GROUSE 39 



the pigment in sucli feathers is probably taken by shining air globules, as 

 it is in the hair and feathers of the majority of white animals and birds. 



It is thus easier to believe that a sudden check, either by a change of 

 temperature, or by wet and cold, or by want of sunshine, or by change ia 

 food, has for the time so far affected the tissue metabolism of the bird 

 that a feather which began to grow upon a circulation lacking pigment 

 particles, and which was therefore originally planned for the paler plumage, 

 may, by a sudden increase in the metabolism of the bird, and so in the 

 output of waste products to the blood, be completed as a feather of the 

 more deeply pigmented plumage, thus producing a feather with the characters- 

 of both. 



This is a plausible explanation, but is still open to some doubt, for the 

 difierence between the broad - banded bufl' and black flaukfeather of the 

 nesting hen, and the dark red-brown finely cross-lined feather of the same 

 bird in winter, is obviously greater as regards pigment distribution than as 

 regards the actual quantity of pigment deposited in the feathers. 



If there are, as has been held, distinct pigments, such, for example, as 

 buft', black, and orange-red, in the various colour-tones of the Red Grouse, 

 it becomes easier to see that the loss of the red pigment, which is utilised 

 for the eggs, leaves the buff and the black in greater quantity for the 

 nesting season plumage. In the winter all three would once more be available. 



The fat of the nesting hen is distinctly rich in colour, but in no case 

 that we have seen has it amounted to the orange-coloured fat which is 

 often seen in overfed Pheasants, and quite commonly in Gulls and Terns- 

 which have been feeding on red crustaceans. In these birds the orange-red 

 fat or oil, tints not merely the fat beneath the skin, but even the white 

 feathers of the breast and body often present a very beautiful rosy flush. 



The whole question of pigment production and pigment distribution, 

 intimately connected as it is with the question of the excretion of waste products 

 and the deposition of fat, both in health and in disease, has not reached a 

 stage which admits of dogmatic statement upon the subject of pattern change 

 in feathers without moult. 



One recognised method of changing a colour-pattern in feathers without 

 moult is to be seen in the male of the familiar House Sparrow, which 

 produces a handsome jet-black cravat in the breeding season, where before 

 was a nondescript greyish throat ; and this it does by the simple process of 



