A THEORY OF BIRDS' NESTS. 255 



to assist in incubation. It is possible, therefore, that 

 the group may originally have used open nests, and 

 some change of conditions, leading the male bird to 

 sit, may have been followed by the adoption of a 

 domed nest. This is, however, the most serious ex- 

 ception I have yet found to the general rule. 



6. Superb warblers (MaluridaB). The males of these 

 little birds are adorned with the most gorgeous colours, 

 while the females are very plain, yet they make domed 

 nests. It is to be observed, however, that the male 

 plumage is nuptial merely, and is retained for a very 

 short time ; the rest of the year both sexes are plain 

 alike. It is probable, therefore, that the domed nest 

 is for the protection of these delicate little birds against 

 the rain, and that there is some unknown cause which 

 has led to the development of colour in the males only. 



There is one otlier case which at first sight looks 

 like an exception, but which is far from being one 

 in reality, and deserves to be mentioned. In the 

 beautiful Waxwing, (Bombycilla garrula,) the sexes 

 are very nearly alike, and the elegant red wax tips to 

 the wing-feathers are nearly, and sometimes quite, 

 as conspicuous in the female as in the male. Yet it 

 builds an open nest, and a person looking at the bird 

 would say it ought according to my theory to cover 

 its nest. But it is, in reality, as completely protscted 

 by its colouration as the most plainly coloured bird 

 that flies. It breeds only in very high latitudes, and 

 the nest, placed in fir-trees, is formed chiefly of 

 lichens. Now the delicate gray and ashy and purplish 



