436 



THE BIRDS 



xvi. 4- 



4. Colours of birds 



Birds possess colour patterns more vivid than those of any other 

 vertebrates, using them not only for concealment but also as the chief 

 means of recognition and sexual stimulation and hence as the basis of 

 their social life. Like other animals that live far from the ground and 

 move fast (primates) the birds have a poor sense of smell, often none 



at all, but they have very good 

 vision, and in many species the 

 turning of discriminating eyes 

 by one sex upon the other has 

 led to the development of a very 

 gorgeous covering. The feathers 

 alter the appearance of the bird 

 so completely that it is not fan- 

 tastic to compare their effect with 

 that of clothing in man. 



As in other animal groups the 

 colours are produced partly by 

 pigments and partly by reflection 

 and diffraction effects (structural 

 coloration). The most common 

 pigments are melanins, ranging 

 from black through brown to 

 yellow, and laid down in the 

 feathers by special cells in the 

 papilla. The processes of these 

 amoeboid chromatophores con- 

 vey pigment to the epidermal cells 

 (Fig. 251). Carotenoid pigments 

 (soluble in organic solvents) are also found, such as the yellow xantho- 

 phyll of the duck's bill and feet and the red astaxanthin of pheasant 

 wattles. White is usually given by reflection. In blue colours incident 

 light is reflected from a turbid porous layer overlying a deposit of 

 melanin pigment. In iridescent feathers interference of light in thin 

 surface films gives colours like those of soap bubbles. The more 

 specialized iridescent feathers produce Newton's rings, with colours 

 of the second and even third orders. The turacos or plantain-eaters of 

 Africa contain two very peculiar pigments, a copper-containing red 

 porphyrin turacin, which is soluble in weakly alkaline water and dis- 

 solves out in the rain, and the green, iron-containing turacoverdin. 



Fig. 251. Deposition of pigment in feather 

 germ. Transverse section through a develop- 

 ing arm feather. 



c.b. cell body of pigment cell; p. process of pig- 

 ment cell; r. cells forming a radius; sh. sheath of 

 feather germ. (After Strong, from Streseman.) 



