4 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 (Maluridz). 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’ other 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 protected 
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 
