112 CONTROLLED NATURAL SELECTION 
the males desert the females, they become, 
by a post-nuptial moult, cryptically coloured. 
It appears probable, therefore, that the male 
coloration during the breeding season is for 
the purpose of protecting the female. In the 
other birds, the males assist in the rearing of 
the young and do not lose this conspicuous 
plumage, by means of a post-nuptial moult. 
The following conceptions are thus arrived 
at: 
1. The females are cryptically coloured be- 
cause of the open-nesting habits. 
2. The males retain attracting coloration 
for the purpose of protecting the young. 
3. The difference in colour between the 
sexes thus arising, is also used during the 
breeding season, for the purpose of protecting 
the female. 
Three birds which present sexual differences 
in colour, nest in closed nests: the Stonechat, 
Whinchat, and House Sparrow (Table VIII). 
In the case of the Stonechat and the Whin- 
chat, it is probable that this nesting habit is 
a recent acquisition, for they lay blue and 
blue-green eggs, characteristic of open-nesting 
birds, and not the white eggs usually laid by 
closed-nesting birds ; further, although these 
nests are domed, the covering is frequently 
