A bird's bones form rigid girders and platforms. Some are fused (joined) together for rigidity, but others are not, allowing for mobility. The fusion of bones makes the bird's skeleton both lightweight and sturdy. For example, fused hand and finger bones provide strength in the outer wing.
Birds are the only vertebrate animals to have a fused collarbone (the furcula or wishbone) and a keeled breastbone. Formed by fusion of the collarbones at their base, the bird's wishbone offers structural support for the wings. In flying birds, the breast bone is fused to a deep keel (ridge that extends outward) that provides an anchor for the powerful flight muscles.
A bird's backbone is very rigid because most of the vertebrae are fused. The thoracic (chest area) vertebrae and vertebrae other neck vertebrae are fused to keep the bird's trunk stiff. Because of this rigidity, the backbone provides the strong support of the back and wings the bird needs during flight. It also allows the bird to maintain an upright posture while standing.
The vertebrae in the bird's lower back are joined. So are the bones of the hip girdle. This forms a light but strong plate that rests on the thigh bones and supports the bird when it's on the ground.
The backbone ends in a structure called the pygostyle, or tailbone, which supports the tail feathers. Birds can maneuver a fan-shaped tail in a rudder-like fashion to slow or change direction during flight. Don't let the tail feathers fool you – a bird's tailbone is much shorter than is seen in many other vertebrates, such as lizards.
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