124 NATURAL SELECTION VI 
according as both sexes, or the males only, are adorned with 
conspicuous colours. 
The sexual differences of colour and plumage in birds are 
very remarkable, and have attracted much attention ; and, in 
the case of polygamous birds, have been explained by Mr. 
Darwin’s principle of sexual selection. We may, perhaps, 
understand how male pheasants and grouse have acquired 
their more brilliant plumage and greater size by the continual 
rivalry of the males both in strength and beauty; but this 
theory does not throw any light on the causes which have 
made the female toucan, bee-eater, parroquet, macaw, and 
tit in almost every case as gay and brilliant as the male, 
while the gorgeous chatterers, manakins, tanagers, and birds 
of paradise, as well as our own blackbird, have mates so dull 
and inconspicuous that they can hardly be recognised as 
belonging to the same species. 
The Law which connects the Colours of Female Birds 
with the mode of Nidification 
The above-stated anomaly can, however, now be explained 
by the influence of the mode of nidification, since, with very 
few exceptions, I find it to be the rule—that when both 
sexes are of strikingly gay and conspicuous colowrs the nest is of the 
Jirst class, or such as to conceal the sitting birds ; while, whenever 
the male is gay and conspicuous and the nest is open so as to expose 
the sitting bird to view, the female is of dull or obscure colours. I 
will now proceed to indicate the chief facts that support this 
statement, and will afterwards explain the manner in which I 
conceive the relation has been brought about. 
We will first consider those groups of birds in which the 
female is gaily or at least conspicuously coloured, and is in 
most cases exactly like the male. 
1. Kingfishers (Alcedinidz). In some of the most brilliant 
species of this family the female exactly resembles the male ; 
in others there is a sexual difference, but it rarely tends to 
make the female less conspicuous. In some the female has a 
coloured band across the breast, which is wanting in the male, 
as in the beautiful blue and white Halcyon diops of Ternate. 
In others the band is rufous in the female, as in several of the 
American species ; while in Dacelo gaudichaudii, and others of 
