This creates a light, but strong, support for the legs. In the same manner the tail vertebrae are fused together, creating a strong location to attach the tail feathers. Since birds need great strength to fly, the flight muscles need to be firmly attached at the shoulders. Here again, the collarbones are fused together to form a wishbone. This provides the strong support needed by the flight muscles at the shoulders.
Birds have other adaptations that keep their weight low.
Birds do not have teeth or the heavy jaws that are needed to support teeth. Instead, birds have an internal organ called the gizzard. The gizzard holds small stones or gravel that have been swallowed by the bird. This gravel is used by the bird to grind its food into pieces small enough to be digested.