vii FLIGHT 229 



A jerk due to the sudden contraction of this muscle 

 would have exactly the effect which we see when the 

 pigeon in a moment gives his body a downward slant 

 from tail to head. I lawks have, of course, exceptional 

 power of swooping suddenly downwards, and I have 

 found in the kestrel an altogether unusual development 

 of this muscle. It has occurred to me that this power 

 may be also in part due to the pliability of the waist 

 in upward and downward directions, which some birds 

 have in a much greater degree than others. The kestrel, 

 swallow, sand-martin, all three of them able in a 

 moment to put their bodies at any incline, have this 

 qualification to a remarkable extent. Ducks, on the 

 other hand, are very stiff at the waist, and they are, 

 comparatively, but poor performers. The gulls and 

 terns, however, are also remarkable for the same 

 stiffness, and this makes me doubt whether pliability 

 of waist is absolutely essential for these sudden up 

 and down movements. At any rate, much nimbleness 

 can exist without it. 



The wings, as pointed out above, are partly flexed 

 as the bird descends, since it is the front margin which 

 mainly gives support when he is gliding, and if he 

 were to form an extended front, he would travel more 

 slowly and with a less rapid incline downwards, or in 

 a horizontal or upward direction (see p. 190). 



Horizontal Flight and Gliding Flight. 



These have already been described (pp. 180, 190, 

 223)- 



