i84 THE STORY OF THE BIRDS 



these birds gain an initial velocity by running for 

 some distance, or, in the case of aquatic birds, by 

 striking the water with the tips of their wings for 

 the first few strokes. Smaller birds gain an ini- 

 tial velocity by a spring, and this fact explains 

 why so many short-legged birds rise with diffi- 

 culty from a flat surface, or, in some cases, are 

 unable to do so at all. That kind of active flight 

 known as "hovering" (of which the Kestrel is 

 such an excellent and familiar example) can only 

 be indulged in by birds of very powerful flight, 

 great exertion being apparent in the frequency 

 with which the wing strokes succeed each other. 

 As a rule, the wings are moved quickest as the 

 flight commences, the angle through which the 

 pinions move being a large one, in some cases 

 the wings striking each other above the back at 

 the end of the up-stroke, and the tips nearly 

 meeting at the end of the down-stroke. This 

 may often be remarked in the Ring-Dove, the 

 noise made by the wings striking each other 

 being considerable. As the flight becomes 

 quicker the wing-beats become slower, and the 

 angle made by the wings is much smaller. We 

 are indebted to instantaneous photography for a 

 clear demonstration of how the wings of a bird 

 are moved during flight. The wings, or rather 



