422 



AVES. 



(herons, some j^arrots, etc.). Nestling downs [neossoptiles) are 

 down feathers with certain characters of their own found on the 

 newly-hatched bird. In the autumn there is usually a complete 

 change of feathers {auUimnal moult), whereas in the spring moult, 

 by which the bird acquires its breeding j)lumage, there is only 

 rarely a complete new formation of the plumage. As a rule, the 

 spring moult is accompanied by a change of colour of the fea- 

 thers (probably due to chemical change in tlie pigment already 



Fig. 234.- — Pterylae and apteiia of Gallus bankiva (after Nitzsch) a ventral, 6 dorsal. 



present), and sometimes by a mechanical breaking off of certain 

 parts of the feathers. The new feathers of the moult are formed 

 in the follicles and from the pulp of the old feathers. 



The plumage is only rarely distributed evenly over the wdiole 

 of the body (Ratitae, penguins). Usuall^^ the contour feathers 

 are arranged in rows — the pterylae, between which there are 

 spaces — the apteria, which are naked or only covered with 

 down (Fig. 23t). The form and distribution of these feather 

 tracts present modifications which can be used in classification. 



