xvii. ia EGGS 507 



either accidental or, in kingfishers, made by the bird itself. Similarly, 

 holes already in trees may be used (tit-mice), but the woodpeckers 

 drill their own holes. The female hornbill walls herself into a hole 

 in a tree with the help of mud brought by the male and for the three 

 months of incubation she is fed by the male through a small aperture. 

 In the brush turkeys (Megapodes) the eggs are not incubated by the 

 bird but are buried in a mound of decaying material, which provides 

 the necessary heat; the young bird is independent from the moment 

 it hatches. 



The bower-birds of Australia and New Guinea are passerines in 

 which the male builds an elaborate structure of plants, ornamented 

 with bright objects. Here he displays to a female and sings. A bower 

 that is left may be destroyed by another male and its decorations 

 stolen. The male constantly refurbishes his bower and gyrates round 

 it, tossing the decorations violently. This seems to be a form of dis- 

 placement activity. When the female ultimately becomes receptive 

 copulation occurs, the bower often being demolished in the act. The 

 eggs are laid in a nest nearby. 



11. Shape and colour of the eggs 



Eggs are as varied as the rest of bird structures. The shape is 

 determined by the pressure of the oviducal wall and the blunt end 

 always emerges first. With lesser pressure from behind the egg 

 approaches a spherical shape. The pointed end may serve to prevent 

 the eggs rolling away (guillemot) or to help the eggs to fit together 

 (plover). The pigment is laid down at the end of the travel along the 

 oviduct and is derived from the bile pigments. The coloration is 

 usually procryptic in eggs laid on the ground, whereas those laid in 

 holes are white and birds like the pigeons that do not attempt to 

 protect themselves or their nests by concealment also have light- 

 coloured eggs. The significance of the varied colours of eggs that are 

 not procryptic is obscure. It is likely that they serve as a stimulus to 

 the brooding bird, who will sometimes leave a nest when a wrong- 

 coloured egg is inserted. A further sign of this is that there are 

 various races of cuckoo, each laying eggs appropriate to the nest it 

 parasitizes; but the genetics and behaviour of cuckoos are still very 

 obscure subjects. 



12. Brooding and care of the young 



Usually it is the hen who broods the eggs, but the cock may assist 

 and in a very few species he does all the brooding. Brooding is not a 

 mere sitting on the eggs, but depends on the development of a vascular 



