NATURE-STUDY 



They consist of a hollow quill stem and a solid elonga- 

 tion, the shaft. On the shaft at each side are fine branches 

 called barbs. These are again divided, and sometimes 

 subdivided into smaller branches called barbules and proc- 

 esses . These barbules and processes interlock to form 



a stiff, resisting 

 surface. The 

 shaft and barbs 

 form the vane. 



Besides these 

 feathers there 

 are various mod- 

 ifications in dif- 

 ferent birds,such 

 as the beau- 

 tiful plumes on 

 the heads, chest, 

 shoulders, hips, 

 etc., of ostriches, 

 herons, and 

 other birds; the 

 hair-hke tuft on 

 the breast of the turkey cock, etc. These are used mainly 

 for decoration. 



Birds raise and maintain themselves in the air not by 

 their relative lightness (as is the case with a balloon), but by 

 the reaction of the air against the powerful wing strokes, 

 or else by the force of winds against the wing and tail 

 surfaces. Most birds that flap their wings in flight use 

 the first principle, while the hawk, which circles around high 

 in the air without apparently a stroke of the wing, and only 



Fig. i6. Chimney Swift. 



(Not a swallow.) 



