278 THE BIOLOGY OF BIRDS 



the same pigments, similar proportions of parts, similar 

 secretory activity, the same rate of oviducal movement, 

 and so on — therefore similar results. There is extraordinary 

 variability in the colour of cuckoo's eggs, but there is very 

 cogent evidence that when a cuckoo has begun to lay eggs 

 of a certain colour, she keeps to this. 



The third point is that the pigment is often hidden from 

 the outside, being deposited in the deeper parts of the spongy 

 layer of the calcareous shell. In such cases it cannot be of 

 much direct use unless as a screen protecting the develop- 

 ment of the embryo from certain rays of the sun. 



Our argument is that there is no need to search too 

 diligently for the utilitarian significance of the distinctive 

 coloration of birds' eggs. The pigmentation may be a 

 by-play of metabolism and the constancy of pattern an 

 expression of an orderly constitution. And that may be all. 



Yet it is not in the least inconsistent with this to inquire 

 whether in particular cases a particular coloration may not 

 have a use and may not have come under the sifting action of 

 natural selection. 



§ 10. Possible Uses of Egg- Coloration 



(i) The eggs of birds that lay their eggs in ground-nests 

 or in exposed nests are often very inconspicuous. About 

 ninety British birds nest on the ground, and in many cases 

 the eggs (and the birds too) harmonise protectively with their 

 surroundings. This is plain in the case of Ringed Plover, 

 Tern, Stone Curlew, and Lapwing. When danger threatens, 

 the alert cock-bird often gives the alarm and the female 

 leaves the eggs, runs along the ground for some distance 

 and then takes wing. But the exposed eggs are not con- 

 spicuous ; their colouring and patterns harmonise with the 

 ground-scenery ; sometimes the gloss reflects the light in a 

 way that makes them still more difficult to discover. We have 

 seen a woodcock's nest with white eggs and the conspicuous- 

 ness of these was enough to convince us of this at least, that 



