VI 
A THEORY OF BIRDS’ NESTS 
135 
formed chiefly of fir-twigs and lichens. Now the delicate 
gray and ashy and purplish hues of the head and back, to- 
gether with the yellow of the wings and tail, are tints that 
exactly harmonise with the colours of fir leaves, bark, and 
lichens, while the brilliant red wax tips exactly represent the 
crimson fructification of the common lichen, Cladonia cocci- 
fera. When sitting on its nest, therefore, the female bird 
will exhibit no colours that are not common to the materials 
by which it is surrounded ; and the several tints are distri- 
buted in about the same proportions as they occur in nature. 
At a short distance the bird would be undistinguishable from 
the nest it is sitting on, or from a natural clump of lichens, 
and will thus be completely protected. 
I think I have now noticed all exceptions of any import- 
ance to the law of dependence of sexual colour on nidification. 
It will be seen that they are very few in number, compared 
with those which support the generalisation • and in several 
cases there are circumstances in the habits or structure of the 
species that sufficiently explain them. It is remarkable also 
that I have found scarcely any positive exceptions — that is, 
cases of very brilliant or conspicuous female birds in which 
the nest was not concealed. Much less can there be shown 
any group of birds in which the females are all of decidedly 
conspicuous colours on the upper surface, and yet sit in open 
nests. The many cases in which birds of dull colours in both 
sexes make domed or concealed nests do not, of course, affect 
this theory one way or the other ; since its purpose is only 
to account for the fact that brilliant females of brilliant 
males are always found to have covered or hidden nests, while 
obscure females of brilliant males almost always have open and 
exposed nests. The fact that all classes of nests occur with 
birds which are dull coloured in both sexes merely shows 
that these dull colours serve to protect the parents at other 
times than when sitting on the nest, the structure of which is 
determined by the requirements of the offspring. 
If the views here advocated are correct, as to the various 
influences that have determined the specialities of every bird’s 
nest, and the general coloration of female birds, with their 
action and reaction on each other, we can hardly expect to 
find evidence more complete than that here set forth. Nature 
