24 THE STORY OF BIRD-LIFE. 



are more or less brilliantly and conspicuously 

 coloured, no such change of dress is possible. 

 Having so definitely "assumed the purple" and 

 renounced the plainer garb, it is impossible to 

 revert. As a rule, the quills are moulted in 

 pairs, thus, even when moulting, escape by flight 

 is possible. 



Some birds there are who carry the process of 

 moulting much further, and even periodically 

 shed their claws as in certain grouse, or the 

 sheath of the bill as in our common English 

 puffin. Concerning the colours and coloration 

 of feathers — for these are not the same — there 

 is much to be said, but most of it is of too 

 technical a character to be dealt with here. 



Strange as it may seem it is yet true that the 

 colour which a feather appears to be is often 

 quite other than that which it is, at least the 

 chances are one to three of its being so. The 

 colours of feathers are classed under three heads. 

 Under the first of these come, what are known 

 as chemical or absorption colours. These are due 

 to pigment, or colouring matter, either evenly 

 distributed or difi'used through the substance of 

 the feather, or to little particles of colouring 

 matter * packed closely in between its fibres, if 

 we may so term what would be scientifically 

 known as the cells of the tissue, which combined, 

 form the feather itself. Such colouring matter is 

 not afi'ected by changes of the position of the 

 light by which it is illuminated. Thus, red, 

 yellow, brown and black feathers always remain 

 the same. Orange and yellow generally, green 

 rarely, blue never, belong to this category. 



