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 colours the nest is of the 

 first 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 (Alcedinidse). 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 



