THE FLIGHT OF BIRDS 307 



out of the tail. Horizontal o-lidino- is often the result. If the 



o o 



wings are folded more than necessary to correct the upward bias 

 the bird will at once begin to crlide downwards. 



Soaring. — A bird is said to sail or soar when with outspread 

 wings it circles up into the sky, describing a course that may 

 broadly be termed spiral. It is only possible in cases where there 

 is a large surface of wing compared to the weight of the body, 

 and is characteristic of many of the Birds of Prey, the larger 

 members of the Crow Family, Pelicans, and Storks. Roy (in 

 Newton s Dictionary of Birds) thus summarizes the chief features 

 of this kind of flight as described by competent observers: — "A 

 certain amount of wind appears to be essential, soaring flight 

 not being observed in a dead calm. Observers seem also to 

 agree in this, that the soaring bird, with motionless outstretched 

 wings (having raised itself some distance from the ground or 

 sea by active wing-strokes), describes in its flight curves or 

 circles which lead it to alternately sail up the wind and down 

 the wind. It describes wide curves, and loses in vertical position 

 while it is directed down the wind, while in going up into the 

 wind it rises higher in the air. The bird may, in describing 

 these curves or circles, rise as high or higher than the point from 

 which it started, and may be as far or farther to windward, and 

 this without any very evident expenditure of mechanical work 

 on the jDart of the bird." 



But it is easier to describe these evolutions than to explain 

 how they are performed, and no entirely satisfactory explanation 

 has so far been advanced. Some authorities have attributed 

 them to the action of upward currents of air, and others have 

 tried to explain it by the fact that an air current moves more 

 rapidly in proportion to its height above the ground. But we 

 know too little about the distribution and force of air-currents in 

 the higher regions of the air, to feel sure that either explanation 

 can account for the fact that Adjutants are able to soar up to an 

 altitude of some two miles from the ground. 



The following view on the subject is advanced by Headley (in 

 The Structure and Life of Birds): " I believe myself that the 

 irregularity of the wind may supply the explanation of soaring. 

 The wind is a 'chartered libertine', and, even when steadiest, blows, 

 as Professor Langley has shown, with great fitfulness. A bird, 

 when soaring, if this explanation be sound, will face a strengthen- 



