BIRDS. 



231 



urally be assumed that under the vigorous beating of the 

 wing they would form little, if any, resistance. There is, 

 however, a special arrangement, shown in Fig. 273, by 

 which each barb is locked with its fellow by little second- 

 ary branches called barbules, that are generally serrated 

 and provided with hooks, and in this way a strong surface 



is presented to the wind. Plumes, as those 



of the ostrich, are 



feathers without 



barbules. Down 



is feathers in 



which the barbs 



are extremely 



soft and free. 



The feathers 



shed water by 



being oiled with 



a secretion the 



birds take from 



an oil-gland near 



the tail. After 



the reproductive 



season, birds gen- 

 erally moult or 



shed their feath- 

 ers. Some birds, 



as the ptarmigan, do this two or three 



times a year. 



Senses. The eyes of some birds are remarkably pierc- 

 ing, and their vision extremely acute. All possess a third 

 eyelid or nictitating membrane (Fig. 270, 2) that covers and 

 protects the ball of the eye, so that eagles can gaze directly 

 at the sun without blinking. The eyes of these birds and 

 their allies are also provided with a ring of hard plates that 

 forms an apparatus by which the bird can adjust its sight 

 to objects near at hand or at a distance. The penguin, 



FIG. 272. Parts 



of a feather. 

 i, quill ; 2, 

 shaft ; 3, 3, 

 vane or barbs ; 

 4, accessory 

 plume. 



FIG. 273. Barb from a 

 goose - quill, showing 

 the booklets highly 

 magnified. 



