202 



ANIMAL LOCOMOTION-. 



surface of the wings makes with the horizon. "When a bird 

 wishes to fly in a horizontal direction, it causes the under 

 surface of its wings to make a slight forward angle with the 

 horizon. When it wishes to ascend, the angle is increased. 

 When it wishes to descend, it causes the under surface of the 

 wings to make a slight bachvard angle with the horizon. 

 When a bird flies up, its wings strike downwards and forwards. 

 When it flies down, its wings strike downwards and backwards. 

 When a sufiicient altitude has been attained, the length of 

 the downward stroke is generally curtailed, the mere exten- 

 sion and flexion of the wing, assisted by the weight of the 

 body, in such instances, sufiicing. This is especially the case 

 if the bird is advancing against a slight breeze, the effort 

 required under such circumstances being nominal in amount. 

 That little power is expended is proved by the endless 

 gyrations of rooks and other birds; these being continued 

 for hours together. In birds which glide or skim, it has 

 appeared to me that the wing is recovered much more 

 quickly, and the down stroke delivered more slowly, than 

 in ordinary flight — in fact, that the rapidity with which the 

 wing acts in an upward and downward direction is, in some 

 instances, reversed; and this is what we should naturally 

 expect when we recollect that in gliding, the wings require 

 to be, for the most part, in the expanded condition. If 

 this observation be correct, it follows that birds have the 

 power of modifying the duration of the up and down strokes 

 at pleasure. Although the wing of the bird usually strikes 

 the air at an angle which varies from 15° to 30°, the angle 

 may be increased to such an extent as to subvert the position 

 of the bird. The tumbler pigeon, e.g. can, by slewing its 

 wings forwards and suddenly throwing back its head, turn 

 a somersault. When birds are fairly on the wing they have 

 the air, unless when that is greatly agitated by a storm, 

 completely under control. This arises from their greater 

 specific gravity, and because they are possessed of independent 

 motion. If they want to turn, they have simply to tilt their 

 bodies laterally, as a railway carriage would be tilted in 

 taking a curve,^ or to increase the number of beats given by 



1 If the albatross desires to turn to the right he bends his head and tail 



