EXTERNAL FEATURES 35 



dimensions ? If the colours are due to interference, the 

 first supposition must be true ; but if selective reflection is 

 the agent, a comparatively small group of molecules may 

 cause selective reflection. Mallock found that the colour 

 disappeared from feathers, butterfly scales, beetles, and 

 flies when these were subjected to compression, and was 

 thus led to the conclusion that interference of one 

 kind or another is the true cause of natural iridescent 

 colours. 



The pigments of birds are mainly deposited in the 

 feathers, but they also occur on the bare skin, on internal 

 parts such as the roof of the mouth, in the fat of the body, 

 in the yolk of the egg, and so on. Those on egg-shells will 

 be discussed separately. Most of the pigments of birds, 

 apart from egg-shells, fall into two groups — the lipochromes 

 and the melanins. See Krukenberg, Newbigin (1898), 

 Gadow (1893). 



(i) The lipochromes or fatty pigments usually occur 

 in solution in fats, but they are common in feathers and bill, 

 as well as in fat and yolk. A very good example is 

 zoonerythrin (animal red), seen in the " rose " around the 

 eyes of Grouse, and in the red feathers of the Flamingo, 

 the Scarlet Ibis, some Cockatoos, the Cardinal Bird, and 

 others. It is of wide occurrence among animals and is 

 familiar in the higher Crustaceans, such as lobster and 

 crab, shrimp and prawn. It occurs along with fat or oil, 

 and is soluble in alcohol, ether, and chloroform. 



Related to zoonerythrin is zooxanthin, a yellow pigment, 

 occurring in fat or oil difl^used in many feathers. The 

 bright yellow feet and bills of ducks and related birds and 

 of some birds of prey probably owe their colour to 

 zooxanthin. 



(2) The second great group of pigments is that of the 

 melanins, which produce most of the dark colours of birds. 

 The pigment occurs in the form of small particles, not 

 soluble in alcohol or ether. It is probable that the group 

 of melanins is rather heterogeneous, but they have not been 

 much studied either chemically or physiologically. In 



