28o THE BIOLOGY OF BIRDS 



utilised for secondaiy advantages in certain cases. That it 

 is not always utilised in this particular way, proves nothing. 



Professor Newton referred to the pigeons' white eggs in 

 very exposed nests, to the whiteness of owls' eggs, whether 

 laid in a hole, or on the bare ground, or in an open nest on a 

 tree. The gos-hawk has a white egg, and that of its relative 

 the sparrow-hawk is blotched, yet the nests of the two birds 

 are built in precisely the same kind of position. Such cases 

 prove that it is not a matter of life and death for an egg in an 

 exposed nest to be coloured like a crow's, but they do not 

 rule out the suggestion that pigmentation may sometimes be 

 a useful screen. 



The eggs of the snow-bunting are richly and delicately 

 coloured, but they are sedulously concealed beyond the 

 reach of reflected light. This proves that the type of pig- 

 mentation has not always any significance as a screen, but 

 it does not prove that the screen theory may not sometimes 

 apply. 



(4) The eggs of cuckoos are very diverse in coloration, 

 but it is likely that one and the same cuckoo lays only one 

 type of egg. This is placed in a foster-parent's clutch, and 

 in some cases the resemblance is so close that it needs a 

 practised eye to detect the cuckoo's egg. In other cases, 

 the intruded egg is strikingly conspicuous and challenges 

 attention. Thus there is a very interesting occurrence of 

 resemblance on the one hand and contrast on the other. It 

 would require prolonged investigation to make sure that the 

 similar eggs are more successful than the dissimilar ones. 

 Some foster-parents are more dupable than others, but we 

 do not know whether colour-resemblance in the eggs makes 

 the duping easier. The mother-cuckoo is often very 

 deliberate in her choice of a foster-nest, but we do not know 

 whether this is in any way determined by the colour of the 

 egg she lays. We do not know whether she appreciates the 

 colour at all. It is certain, of course, that she cannot change 

 the colour at will, and that she is not " maternally impressed." 

 But our ignorance far exceeds our knowledge of the cuckoo, 

 and most other things. 



