Birds: Masters of the air
To provide enough length and space for these muscles, the shoulder joint on each side has moved upward, to a pos-ition level with the backbone. And to resist the powerful forces produced when the flight muscles contract, the sternum is braced away from the shoulder joints by enlarged coracoid bones, and away from the backbone by strong bony ribs.
For a bird to run on 2 legs and to launch itself into the air from the ground, the hindlegs must be long and powerful. This has been achieved by elongating the bones of the lower leg, and by adding a new section to the ankle (the tarso-metatarsus), formed by elongating some of its bones.
The powerful muscles needed to swing the legs forward and backward are attached to the modified pelvic, or hip, girdle, the various bones of which have become elongated. The ilium projects far forward from the hip joint, and is strongly attached to the backbone by between 11 and 23 vertebrae (compared with only 2 or 3 in most reptiles). The pubis and ischium both project backward from the hip joint.
To prevent itself falling forward while walking, a bird's body has been shortened, and also moved backward between the legs to bring the center of gravity to a point above the feet. The enlarged sternum, therefore, ends up lying between the knees.
Because the powerful, flexible wings can both propel and guide the bird, a long tail is no longer needed for stability. It has been reduced to a short "pygostyle," the feathers of which can be erected to form a fan that acts as an air-brake when the bird alights.
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