i86 THE STORY OF THE BIRDS 



Birds, Gannets, and so forth. These aerial 

 soarers are mostly big birds, with an extensive 

 wing or sail area in proportion to their total 

 weight. More or less wind appears to be abso- 

 lutely essential to this mode of flight, and one 

 of its most marked characteristics is the curved 

 or circular course which it describes. Various 

 theories have been propounded to explain how 

 these soaring flights are performed, but as yet 

 their explanation is by no means complete or 

 satisfactory. They have been attributed to up- 

 ward currents of air, as well as to the varying 

 velocity of the wind at different heights above 

 the earth. By whatever means they are per- 

 formed they are wonderful exhibitions of volant 

 force ; and the observer may well be profoundly 

 impressed as he watches the soaring flight of 

 Vulture or Eagle high up in the blue sky, con- 

 ducted, as it were, with no visible effort. 



From the aerial progress of birds we will now 

 turn to a brief notice of their terrestrial and 

 aquatic motion. There is probably a greater 

 amount of variation in the latter methods of 

 progress than there is in aerial locomotion. 

 There are, for instance, far more birds unable 

 to walk than there are unable to fly. It is, 

 as we know, very exceptional to find birds 



