xvi. 14 



FLAPPING FLIGHT 



455 



(Figs. 261-2). This arrangement directs the air stream over the upper 

 surface of the wing in such a way as to provide an extra lift by creating 

 a 'suction zone' of reduced pressure. How T ever, high camber, like low 

 aspect ratio, reduces the speed of the bird. 



14. Flapping flight 



Flapping flight involves a complex, 

 screw-like motion of the wing, down- 

 wards and forwards then upwards 

 and backwards, more rapid upwards 

 than downwards (Fig. 264). The 

 action of the wings differs during 

 take-off or landing from that in sus- 

 tained flight (Brown, 1953). In the 

 former conditions, when the speed 

 is slow, forward velocity is provided 

 by backward movement of the wings. 

 During each stroke, beginning with 

 the wings raised, they are first moved 

 downward and then forward, pro- 

 viding lift (Figs. 264, 265). This 

 movement is produced mainly by 

 the pectoralis major. During the 

 upstroke the wing is first adducted, 

 folded and flexed, and supinated at 

 the wrist, by the actions of pectoralis 



minor and other muscles. A very rapid backward flick then follows, 

 produced by upward and forward rotation of the humerus, extension 

 of the wing, and pronation of the manus. The effect of these move- 

 ments, produced largely by the triceps and other extensors, is to 

 provide a forward component. 



This form of flight involves mainly the primary feathers. It is 

 evidently very tiring and can only be continued for a few seconds. In 

 sustained flight the downstroke is as in slow flight but the upstroke is 

 much simpler, with only a slight backward flick of the primary feathers. 

 The action is such that the inner part of the wing provides lift, the 

 tip propulsion. The upstroke in fast flight is thus mainly passive, 

 produced by the pressure of air against the under surface. The major 

 part of the effort needed to provide lift and forward propulsion is thus 

 provided by the pectoralis major. This muscle weighs as much as one- 

 fifth of the body weight in flappers, such as the lapwing, as little as 



Fig. 264. Pathway of the wing tip and 

 wrist joint relative to the body during 

 free flapping flight of a gull. Equal 

 time-intervals are shown. Note the 

 great speed of the upward beat and 

 that the forearm is raised before the 

 wing tip. (After Demoll.) 



