38 THE BIOLOGY OF BIRDS 



colour being always in the order of the colours in the rain- 

 bow. Such metallic or prismatic colours are well illustrated 

 in Starlings and in Birds of Paradise and in the tail of the 

 Peacock. Abundant pigment, usually dark, is covered by 

 a transparent coating, which seems to act like a number of 

 prisms. These apparently changing colours seem to be 

 restricted to the barbules and usually to those barbules 

 which have no barbicels. 



(b) Physical or structural coloration is also illustrated 

 and in great exuberance by those feathers which do not 

 seem to change their colour with the incidence of light or 

 the movement of the observer. All blue and violet colours, 

 all green colours except in the case of turacoverdin, and 

 some bright yellow colours, are of this nature. In most of 

 these cases there is a layer of pigment of some other colour 

 (e.g. brown), and outside this there is a colourless layer 

 with a peculiar structure, e.g. numerous microscopic ridges. 

 The beautiful iridescence of the pigeon's neck appears to 

 be due (Strong, 1903) to thin-plate interference colours or 

 Nev\i;on's rings effects, produced on the dorsal surfaces of 

 the distal portions of the feathers, where spherical pigment 

 granules are in contact with an outer transparent layer. 

 In some other cases the explanation is slightly different. 

 In many yellow feathers, there is no pigment at all ; the 

 coloration is purely physical, the surface probably acting 

 like a grating that allows of the reflection of the yellow rays 

 only. This may be seen in some Toucans, Macaws, and 

 American Orioles. In a general way it may be noted that 

 non-metallic structural coloration is distinguishable from 

 pigmentary coloration by the fact that it is at once altered if 

 the surface of the feather be injured. If there is a dark 

 pigment enhanced into a blue colour, or a yellow pigment 

 enhanced into green, the pigment colour will be seen when 

 the feather is held up against a bright light. 



Modifications of Colour. — The raw materials of the 

 pigments are brought into the growing feather by the blood 

 which enters the base of the quill ; the pigments get their 

 final form inside the living cells of the growing feather ; 



