250 \4 THEORY OF BIRDS’ NESTS. 
of nidification (dependent on structure and environ- 
ment) 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 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, asin 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. 
