BIEBS AND TUEIR FEATHERS 13 



like structure throughout. They frequently consist of a stem 

 without any rhachis, the barbs forming merely a tuft at the 

 end of the quill. 



Finally, there are among the contour feathers, coming from 

 the same holes in the skin, long, slender, almost hair-like parts, 

 filament feathers, or hair^ feathers. These have little distinction 

 of stem and rhachis, and almost no barbs at all, though some- 

 times there are a few small ones near the end of the rhachis. 



Besides the foregoing, which can be found on nearly all 

 birds, there are peculiar growths which are characteristic of 

 certain groups, distinguishing them from others. Thus the 

 herons and a few other birds have on their breast and hips 

 downy feathers which continue to grow indefinitely ; but as 

 fast as they grow the ends crumble to powder, forming a whit- 

 ish, greasy or dusty spot. These are called powder-doicn tracts, 

 and are covered with poivder-down feathers. 



Very few birds have the feathers equally distributed over 

 the skin. Most birds have the feathers closely placed on cer- 

 tain patches or bands of the body, while other spaces are either 

 entirely bare (as the lower breast and belly of the English 

 sparrow), or merely covered with down. The penguins and 

 toucans have the skin almost entirely and evenly covered with 

 feathers, but the great majority of birds have large open or 

 naked spaces as far as the skin is concerned, though the plum- 

 age as a whole in most cases really covers the body com- 

 pletely. There are a few exceptions ; thus the head and more 

 or less of the neck are naked in such birds as the vultures, 

 buzzards, etc. 



The general marking or coloring of a bird depends upon the 

 changes in the coloring of its individual feathers. 3Iottled 

 plumage is given by margined^ feathers; streaked plumage 

 by striped ^ feathers ; spotted plumage by dotted ^ feathers, and 

 barred plumage by cross-striped^ feathers. 



