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. Hawks 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). 
