Humerus
Long bone, appendicular (limb) skeleton
Also known as: Upper arm bone, wing bone
Classification
Long bone, appendicular (limb) skeleton
Dimensions
In birds, this bone has a relatively thick, sturdy shaft with wide, flared ends. The length-to-width ratio varies significantly depending on the species' flight style; soaring birds often have long and slender examples, while powerful flappers have shorter, sturdier versions.
Key Features
The identifying hallmark in many birds is the 'pneumatic foramen'—a distinct hole or opening at the upper end that allows the internal air sacs to connect with the bone. It also features a spoon-shaped or rounded head and a prominent, flattened ridge near the top where strong flight muscles once attached.
Similar Bones
This is most often confused with the femur (thigh bone). To tell them apart, look for the deep cavity at the top; the humerus usually has a more complex, sculpted upper end with a noticeable air-sac opening, while the femur typically has a simpler, bulbous neck and head designed for a hip socket.
Across Species
While most mammals have heavy, marrow-filled humeri, the avian versions are pneumatic, meaning they contain air sacs that make them incredibly light for flight. In penguins, this bone is flattened to act like a paddle or flipper, whereas in hummingbirds, it is remarkably short to facilitate rapid wing beats.
Evolutionary History
The humerus traces its lineage back to the pectoral fins of prehistoric lobe-finned fish. Over millions of years, as diverse land-dwelling vertebrates emerged, this bone adapted from a crawling limb to a supportive walking leg, and eventually to the specialized, lightweight structure required for flight in the avian lineage.
Photography Tips
Place the bone on a neutral, solid background with a ruler or common object like a coin for scale. To help with identification, take close-up photos of the upper end from several angles, specifically focusing on the internal cavity and the shape of the joint surface.