But in spite of the fact that sailing flight calls for the exercise of comparatively little muscular power, the structure of the skeleton suggests 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 tendency 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 bracing 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 albatrosses 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 buzzard and birds of prey, some of which both soar and flap, have the wishbone 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