Bird Sternum
Flat bone; part of the axial (core) skeleton
Also known as: Keel bone, breastbone, brisket
Classification
Flat bone; part of the axial (core) skeleton
Dimensions
Widely variable; in flighted birds, it is typically a thin, plate-like structure with a tall central ridge and a length that spans much of the torso.
Key Features
It looks like the hull of a sailboat with a high, thin central fin (the keel) rising from a flat or curved base. The bone is extremely lightweight, often translucent, and contains small pores. The front end usually has a pointed projection, while the back end can be wide with various notches or holes along the edges.
Similar Bones
At first glance, it might be confused with a pelvic bone (synsacrum) or a large fish scale. To tell them apart, look for the 'sail' or 'keel' structure on the midline; bird pelvises are usually more boxy or tubular with leg-socket holes, whereas the sternum is dominated by that singular central ridge.
Across Species
In humans, the sternum is a flat, narrow tie-shaped bone. In birds, however, it is dramatically expanded to accommodate massive wing muscles. While flightless birds like ostriches have a flat, shield-like sternum, flying birds feature a deep vertical 'carina' or keel that acts like a mountain range on the bone surface for muscle attachment. Hummingbirds have enormous keels relative to their size, while swimming birds often have broader, paddle-shaped breastbones.
Evolutionary History
The bird sternum evolved from the simple bony plate found in early reptilian ancestors. As dinosaurs transitioned into birds and developed powered flight, the sternum expanded and developed its signature keel. This structure allowed for the specialized muscle anchorage required to pull wings downward through the air, marking a major turning point in avian evolution about 150 million years ago.
Photography Tips
Lay the bone on its side to show the height and shape of the central keel. Take a second 'top-down' photo to show the overall width of the plate. Including a common object like a coin or a ruler helps show the bird's relative size, and back-lighting can reveal the delicate internal grain of the bone.