BIBDS 



<57 



ward wing stroke, on the other hand, the resistance of the 

 air is diminished since the feathers are separated more or 



JfiG. 49. Wing of Tern. (Photographed by E. R. Sanborn, N. l. 

 Zoological Park.) 



less like the slats of a Venetian blind, thus allowing the air 

 to pass between them. 



barbules / 

 without " ~ ' 

 hooks 



shaft 

 FIG. 50. Structure of a feather. 



barb 



An examination of a single feather l shows that it consists 

 in the first place of a shaft running through its length (Fig. 

 1 See footnote, p. 66. 



