250 A THEORY OF BIRDS' NESTS. 



of nidification (dependent on structure and environ- 

 ment) lias 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 incubation, 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 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 situa- 

 tions, 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 fe- 

 male 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 Hang- 

 nests. 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 react on each other and 

 attain their full development together. 



