124 The Study of Animal Life part i 



a downward as well as a backward direction. When there 

 is more of the downward direction the bird rises, when there 

 is more of the backward direction it speeds forward ; but 

 usually the stroke is both downwards and backwards, for 

 the lightest bird has to keep itself from faUing as it flies. 

 The hoUowness and sponginess of many of the bones com- 

 bine strength of material with lightness, and the balloon- 

 like air-sacs connected with the lungs perhaps help the 

 birds in rising from the ground; but, buoyant as many birds 

 are, all have to keep themselves up by an effort. But the 

 possibility of flight also depends upon the fact that the 

 raising of the wing in preparation for each stroke can be 

 accompUshed with very little effort ; the whole wing and 

 its individual feathers are adjusted to present a maximum 

 surface during the down-stroke, a minimum surface during 

 the elevation of the wing. There are many different kinds of 

 flight, which require special explanation — the fluttering of 

 humming-birds, the soaring of the lark, the masterful 

 hovering of the kestrel, the sailing of the albatross. The 

 effortless sailing motion of many birds is comparable to 

 that of a kite, " the weight of the bird corresponding to the 

 tail of the kite ; " it is possible only when there is wind or 

 when great velocity has been previously attained. 



Fig. 29.— From St. John's Wild Sfioris. 



