vi A THEORY OF BIRDS' NESTS 131 



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 kingfishers, the protection the female thus 

 obtained, during the important and dangerous time of incuba- 

 tion, placed the two sexes on an equality as regards exposure 

 to attack, and allowed " sexual selection," or any other cause, 

 to act unchecked in the development of gay colours and con- 

 spicuous 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, by whatever cause, 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. This, however, was not perhaps universally 

 the case ; for 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. When this occurred, a 

 special protection to the female would be no longer necessary; 

 so that the acquisition of colour and the modification of the 

 nest might in some cases act and reactj on each other and 

 attain their full development together. 



Exceptional Cases confirmatory of the above Explanation 

 There exist a few very curious and anomalous facts in the 

 natural history of birds, which fortunately serve as crucial 

 tests of the truth of this mode of explaining the inequalities 

 of sexual coloration. 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, the female more gay 

 and often larger. I am not, however, aware that these two 

 anomalies had ever been supposed to stand to each other in 

 the relation of cause and effect, till I adduced them in support 



