xvi. i 4 FLAPPING FLIGHT 457 



one-ninth in soarers, such as the gull. It does nearly all the work, the 

 other muscles serving to give extra lift when needed. 



The feathers are held by an ela- 

 borate system of tendons and in 

 some birds they are allowed to 

 twist only when the wing is being 

 raised and the barbs of the 

 feathers themselves are so ar- 

 ranged that they open like the 

 vanes of a blind when unaer 

 pressure from above, but close 

 when the pressure is from below 

 (Fig. 268). In other birds, especi- 

 ally those that fly fast with a slow 

 wing beat, such as gulls and 

 swans, the wing is probably rigid 

 on the up as well as on the down 



Fig. 267. Bill-fisher leaving its hole in a 

 bank, showing wings half-way through the 

 upstroke. The upper arm has reached its 

 highest point and the forearm is just starting 

 upwards; its primary feathers have opened 

 on the right wing, reducing resistance. 

 (Drawn from photograph by Aymar, Bird 

 Flight, published by John Lane, The Bod- 

 ley Head, Ltd.) 



strokes, and is twisted so as to 



produce forward and upward components on the up stroke 



vinculum 



PRIMA 



RIBS 



5 6 78 910 II 12 13 #. 

 S £CO/VDARlE S 



Fig. 268. Diagram of wing to show arrangement of the flight feathers. 

 (From Pycraft, A History of Birds, Methuen & Co., Ltd.) 



The whole upward movement is usually faster than the downward 

 one. Before the wing tip has reached its highest point the upper arm 

 is already beginning to descend and in this way the line of flight is 



