132 BIRDS' NESTS 



universally prevail. This peculiarity necessitates these 

 birds seeking dark retreats during daylight. Hence 

 they may possibly have excavated burrow^s or sought 

 hiding places in caves and crevices or under rocks 

 and stones, not for a nest in the first place (as seems 

 proved by the spotted eggs), but for a refuge from the 

 light. We can then readily understand how the eggs 

 became to be laid in such spots, and the incessant 

 period of incubation (day and night without ceasing) 

 passed in the only comfortable manner. Then as 

 regards many other species which nest in covered 

 sites we may fairly assume that the habit has been 

 acquired to evade special enemies rather than to 

 conceal a showy plumage, as, for instance, in the 

 Chats. This is more particularly the case in groups 

 where the nest is not universally concealed, as in the 

 Flycatchers, the elusion of some danger, or the con- 

 cealment of some exceptional bright or conspicuous 

 plumage in families or genera where dull colours 

 generally prevail, being the ruling motive. Whether 

 the eggs in such cases are spotted or white and 

 colourless is a good and reliable indication of these 

 isolated instances of a changed method of repro- 

 duction to escape certain dangers or better to 

 conform to some altered condition of existence. 



Bearing these facts in mind I do not think that 

 we are justified in considering holes (with the possible 

 exception of the Ratit^e and the Mound Birds) as an 

 archaic method of nesting, but rather as the best 



