82 UNDULATING FLIGHT 



and keep in line with the sides of the ship, 

 however much the latter is tossed and rolled 

 about, so it is with the wings of the bird ; 

 these do not twist in their sockets, but keep the 

 same alignment whatever the emergency. It 

 is not the wings, but the body of the bird 

 which changes position, inclining upwards or 

 downwards, as the case may be. To acquire 

 lifting power the body of the bird must be 

 tilted in an upward direction. When flying 

 on a mean level lifting force is neutrahzed and 

 the wings act as fans. 



Undulating, apart from " gliding," is a style 

 of flight common to many of the smaller 

 birds. Alternately using and closing the wings, 

 the bird sweeps along in wave-like undulations. 

 As the wings are folded the bird falls slantingly 

 and is thus carried on in the same direction 

 until the wings are brought into action, when 

 again it slopes up to the former level, or any 

 level required. This is a speciality of flight 

 exactly adapted to these (perching) birds' 

 requirements ; for it will be noticed that a 

 bird with undulating flight comes to its perch 

 with an upward sweep and alights the moment 

 the energy is spent. Provided the perch is 

 stable the bird comes to rest without a flutter 

 — ^without overbalancing by so much as a 

 hair's breadth. Such wondrous precision and 



