Muscle & Tendons
In mammals, the muscles that move limbs are distributed throughout the body, close to the joints they move. There is ample room on the limbs for a copious blood supply, and even fat storage.
In birds, the muscles are remote from the joints. Limbs are sticks. Even the fleshier wings have only enough tissue to manipulate the feathers in flight. No fat is stored there. Blood vessels and nerves are few.
Bird muscles are collected close to the body’s core, far from the action. Even the muscles that curl the toes are mounted near the hip. Long tendons carry the strength that the muscles provide.
Tendons from muscles along the breastbone flap the wings of birds. Tendons along the backbone twitch the stiff feathers of the tail (rectrices), which are an essential accoutrement in steering and braking during flight.