BIRDS AS FLYING MACHINES. 477 



But in spite of the fact that sailing flight calls for the exercise of 

 comparatively little muscular power, the structure of the skeleton sug- 

 gests that the wing of a soaring or sailing bird needs a particularly 

 strong point of support, for birds which sail or soar have the bones 

 which sustain the direct pull of the wing strengthened or braced as 

 other birds do not. The shoulder joint of a bird is formed by the 

 shoulder blade and coracoid, this last being the bone which is attached 

 to the breast bone and on which comes the direct pull of the wing, and 

 in front of the coracoids, running downwards towards the sternum, is 

 the wishbone or furcula, corresponding to oar collar bones or clavicles. 

 It is evident that the greater the length of the coracoid the less able 

 would it be to resist the strain brought upon it, and it is also evident 

 that the simultaneous downward stroke of the wings must have a tend- 

 ency to force the coracoids inwards, or towards one another. Obviously 

 the greater the strain the greater the need of strengthening or bracing 

 the coracoid to resist it, and there are in the shoulder girdle of a bird 

 various devices looking towards this end. In some birds, the albatross, 

 for example, the coracoid is short and stout, while in others extra brac- 

 ing is obtained from the wishbone. 



In the humming-bird the wishbone is light and weak and so short 

 that it does not come near the sternum; the pigeon, a bird of powerful 

 flight, is little better off, for the wishbone is so long and slender that 

 it does little or nothing towards strengthening the shoulder joint, and 

 in both these birds which fly by rapid wing strokes the entire pull of the 

 wing is taken by the coracoid. In the frigate bird, on the contrary, the 

 wishbone is not only strong, but it rests upon and is firmly soldered to 

 the breastbone, while at its upper end it fuses with the coracoid, thus 

 making the firmest possible support to the wing. The cranes, which 

 soar well, also have the wishbone united with sternum, and in the alba- 

 trosses and petrels the wishbone touches the breastbone and is so curved 

 forward as to gain strength in this way while, as previously noted, the 

 strength of the coracoid is increased by its shortness. The turkey buz- 

 zard and birds of prey, some of which both soar and flap, have the wish- 

 bone strengthened by having more material added to make the furcula 

 thick and strong while at the same time it is shaped like a wide U 

 instead of a V. 



Either there is more force exerted in sailing than is at first sight 

 apparent or else extra strength is called for in making sudden turns, or 

 when it becomes necessary, as it does more or less frequently, to take a 

 sudden wing stroke. As wings are levers of the third order the longer 

 the wing the more force is required to move it and more strength is 

 needed at the fulcrum or shoulder joint, and since sailing birds have 

 long wings the need of strength is evident. 



Neither birds nor any creatures that live or have lived afford us 



