510 THE PIGEONS 



dove. In short, it is because the wood-pigeon and turtle-dove are 

 descended from hole-nesting ancestors that their eggs are white, the 

 fact that they continue to lay white eggs shows that the lack of colour 

 is no serious handicap in the struggle for existence. 



These birds, like the parrots, kingfishers, and the host of 

 other species which lay white eggs in holes, do so, not as Wallace 

 contended, to conceal their eggs, but because they lay in holes. 

 Such were probably the chosen sites of the primitive birds, and 

 whiteness was absolutely necessary in order that the eggs should be 

 visible, even if imperfectly, in the dim light which found entrance 

 to the nest-chamber from the entrance hole. Without such guide 

 the eggs would almost inevitably get broken before the incubation 

 period was over, when the bird entered the nest to brood. Coloured 

 shells appeared when such brood-chambers were exchanged for open 

 nests, either to serve as a protective mantle, as in the case of the 

 plovers, for instance, or possibly, as I have already suggested, to 

 serve as a protection against excessive light. The fact, however, that 

 ostriches and rheas, among struthious birds, lay whitish or creamy eggs 

 which are often exposed for some time to the burning sun, militates, 

 it must be admitted, against this idea that a light-screen is necessary. 



A point in regard to the feathers of pigeons demands notice 

 here. Some time ago I drew attention to the fact that if a feather 

 of one of these birds be pressed upon, or squeezed between plates 

 of glass, a perfect image of the feather, in the form of a pale blue film, 

 remains. This film is formed by a waxy substance akin to that given 

 off by "powder-down" feathers or " filo-plumes," such as are met 

 with in the herons, for instance. So far, however, no one has 

 succeeded in discovering the origin of this filmy matter, or what 

 purpose it serves, but to it must be ascribed the "bloom" which 

 pervades the plumage of these birds when alive. 



