i82 THE STORY OF THE BIRDS 



correlated with the general habits of the bird. 

 Gliding Flight is classed by Professor Roy as the 

 simplest. An example of this in its commonest 

 form is presented when a bird, having reached a 

 certain velocity by rapid wing-beats, extends them 

 and expands its tail, and skims forward for some 

 distance. Sometimes the necessary speed for the 

 performance of this kind of flight is attained by 

 descending from some eminence. This move- 

 ment cannot last very long, because it is made 

 either at a loss of vertical height, or continued 

 forward movement. The direction, however, may 

 be upwards or downwards, in the former illus- 

 trated by the swoop of a Falcon with closed 

 wings to the ground, the momentum thus gained 

 being sufficient to send the bird gliding upwards 

 again on extended wings. Most observers must 

 often have been impressed by the consummate 

 ease with which a gliding bird can guide its 

 motion, but how this is done is not at all gene- 

 rally known. Any increase of velocity tends to 

 push upwards the anterior part of a gliding bird's 

 wing, and cause the bird to rise higher in the air. 

 But this a bird can prevent in various ways, 

 either by changing the position of its centre of 

 gravity forwards by extending its neck (in a 

 long-necked bird like a Crane or a Heron), or 



