A Theory of Birds' Nests 83 



rendered her inconspicuous, by assimilating her to surrounding 

 objects, as the earth or the foliage, would, on the whole, be pre- 

 served the longest ; and thus lead to the attainment of those brown 

 or gi-een and inconspicuous tints, which form the colouring (of the 

 upper surface at least,) of the vast majority of female birds which 

 sit upon open nests. 



At the commencement of this article I have endeavoured to 

 prove that the characteristic differences and the essential features 

 of birds' nests are dependent on the structure of the species, and 

 upon the present and past conditions of their existence. Both these 

 factors are more important and less variable than colour ; and we 

 must therefore conclude that in most cases the mode of nidification 

 (dependent on structure and environment) has been the cause and 

 not the effect of the similarity or differences of the sexes as regards 

 colour. When the confirmed habit of a group of birds was to 

 build their nests in holes of trees, like the toucans, or in holes in 

 the ground, like the kingfisliers, the protection the female thus ob- 

 tained, during the important and dangerous time of incubation, 

 placed the two sexes on an equality as regards exposure to attack, 

 and allowed " sexual selection " to act unchecked in the develop- 

 ment of gay colours and conspicuous markings in both sexes. 



When, on the other hand, (as in the tanagers and flycatchers) 

 the habit of the whole group was to build open cup-shaped nests, 

 in more or less exposed situations, the production of colour and 

 marking in the female was continually checked by its rendering her 

 too conspicuous, while in the male it had free play, and developed 

 in him the most gorgeous hues. In cases, however, where there 

 was more than usual intelligence and capacity for change of habits, 

 the danger the female was exposed to by a partial brightness of 

 colour or marking, might lead to the construction of a concealed or 

 covered nest, as in the case of the tits and hangnests ; so that the 

 acquisition of colour and the modification of the nest might in 

 some cases act and react on each other, and attain their full de- 

 velopment together. 



There exist a few very curious and anomalous facts in the 

 natural history of birds which fortunately serve as a crucial test of 

 the truth of this mode of explaining the inequalities of sexual 

 colouration. It has been long known that in some species the 

 males either assisted in or wholly performed the act of incubation. 

 It has also been often noticed that in certain birds the usual sexual 

 differences were reversed, the male being the more plainly coloured, 



