146 THE STRUCTURE AND LIFE OF BIRDS CHAP. 



Rhea— may be found primitive feathers that are not 

 very different from the scales on the bird's own legs 

 or on a lizard. Birds in general have down feathers 

 among the large ones, and these down feathers are 

 often merely a little fluff at the top of a quill, though 

 sometimes they are almost perfect miniatures of a 

 typical feather. Besides these there are thread- 

 feathers, filoplumes, always growing close to the 

 base of one of the large feathers. Sometimes, like 

 hair, the thread-feathers are perfectly simple and un- 

 branched ; the branches are never more than a very few. 

 The Nightjar has, bordering the mouth, a number of 

 bristles that look like filoplumes, but are really ordinary 

 feathers of which only the shaft remains. There are also 

 found on some birds, notably on some Parrots and on 

 the Heron, powder-down feathers, so called because 

 they shed a fine powder. They continue to grow, and 

 the ends of their branches give off a whitish dust which 

 is at once greasy and dry. What purpose they may 

 serve is quite uncertain. 



By the help of these simpler specimens we must 

 try to realise that the most elaborate feather is only 

 a much-divided scale. Such a feather I must now 

 describe, and then try to show how it has grown from 

 a skin papilla. Take a large one from the wing or 

 tail of any common bird. The semi-transparent base 

 is the quill (0, fig. 38) ; it has two small apertures, 

 one at the bottom, the other at the top, where the 

 branches begin, on the under-surface (U 1 and 2). 

 At the lower one the papilla entered to give the need- 

 ful nourishment, and if a young feather be taken, the 

 quill will be found full of blood (P). 



