20 



THE BIOLOGY OF BIRDS 



In powder- downs the calamus forms no rhachis, but 

 splits into barbs and barbules which disintegrate into ap- 

 parently greasy powder. They occur in many birds, some- 

 times in patches as in herons and bitterns, sometimes in 

 scattered tracts and tufts as in parrots, the roller, and the 

 immature Lammergeier (Gypaetus). In the bittern the 

 tufted calamus is about half an inch long ; in the more-pork 



Fig. 3. — Different kinds of feathers 

 down-feather, with aftershaft (a.s) ; 3, 



feather with free barbs : 

 shaft (a.s.)- 



I, filoplume ; 2, a permanent 

 nestling down-feather ; 4, 



5, an emeu's feather with very long after- 



(Podargus) the maximum of two inches is reached ; but in 

 most cases the powder- downs are very small. Their use 

 is obscure. It has been suggested that they help to keep 

 the plumage clean and in good condition — a sort of feather- 

 powder. 



Powder-down feathers are peculiarly specialised down- 

 feathers, but they do not seem to be the same in every case. 

 The powder is formed by the degeneration of the cellular 

 sheath covering the barbs, and Schaub has shown (1907) 



