XIII 
THE COLOURS OF BIRDS 
267 
of the quills, and no specialisation of particular areas. 
Therefore any tendency to the development of 
brilliant colouring in the wing-quills would be checked 
by the resulting injury to flight, and so to the well¬ 
being of the species. 
It will be noted that in the following descriptions 
red and yellow colours are always ascribed to the 
presence of lipochrome. We have already seen that, 
according to Gadow, yellow may at times be an 
optical colour ; in the cases discussed, however, the 
presence of yellow lipochrome has either been 
directly proved, or is assumed from the simultaneous 
occurrence of a red colour, which is always due to 
lipochrome pigment. The thesis here put forward 
as to the relation existing between brilliant colouring 
and variation in feather structure, we propose to 
develop by a consideration of the colour phenomena 
in sun-birds, humming-birds, and birds of Paradise. 
Colours of Sun-birds 
The Nectariniidse or sun-birds are a family of 
mostly small birds often with brilliant colours, 
inhabiting Africa, and India, and the Malay, where 
they seem to replace the American humming-birds. 
Their beauty and their habit of frequenting flowers 
have caused them to be frequently confounded with 
true humming-birds, but they are not in any way 
related to the latter. The bright colours are almost 
entirely confined to the males, and are by them 
acquired with extreme slowness, so that birds are 
said to be not infrequently seen mated while the 
male is still in a sort of hybrid plumage. Birds in 
