GENERAL HABITS 185 



we should say the upper arm-bone or humerus, 

 moves in an elliptical direction during the down- 

 stroke (which takes a longer time to make than 

 the up-stroke), which describes the front half of 

 the ellipse, the wings moving forward as well as 

 downward. Having made this stroke, the wing 

 moves backwards and upwards, describing the 

 posterior half of the ellipse, and the plane of the 

 wing is altered, so that it looks down and for- 

 ward. During somewhat rapid flight the up- 

 stroke is chiefly a passive movement, the bird 

 continuing to rest on the wings, the velocity of 

 the body slightly, decreasing, but recovering 

 speed again with the next down-stroke. Just 

 as flight commences, however, the up-stroke is 

 an active one, and the long quills in the wing 

 separate from each other, thus reducing the air 

 resistance to the back of the wing as it travels 

 upwards. 



Soaring or sailing flight is much the rarest 

 form of aerial motion indulged in by birds. It 

 primarily differs from gliding flight, in the fact 

 that the bird practising it does not of necessity 

 lose vertical position or velocity as a result of 

 atmospheric resistance. The birds in which 

 soaring or sailing flight is most typical are 

 Vultures, Falcons (notably Eagles), Frigate 



