Humerus
Long Bone, part of the appendicular (limb) skeleton
Also known as: Upper Arm Bone / Wing Bone
Classification
Long Bone, part of the appendicular (limb) skeleton
Dimensions
In many medium-to-large birds, this is a relatively long, straight-to-slightly-curved bone. It is typically wider at both ends and narrower in the middle shaft.
Key Features
The top end usually has a large, rounded head that fits into the shoulder socket, and a prominent ridge for muscle attachment. The bottom end has a spool-shaped area that forms the elbow joint. Many bird humeri have a visible small hole on the inner side of the top end where air sacs enter the bone.
Similar Bones
The humerus can be confused with the femur (thigh bone). To tell them apart, look at the top: the femur usually has a very distinct, ball-like neck and head that sticks out to the side, whereas the humerus has a more integrated, rounded top that sits more directly on the shaft.
Across Species
In birds, the humerus is light and often hollow to facilitate flight. In soaring birds like vultures, it is exceptionally long, while in flightless birds like penguins, it is flattened and dense to act like a paddle or flipper. In mammals, it tends to be thicker and heavier to support the animal's weight against gravity.
Evolutionary History
The humerus is one of the most ancient bones in the vertebrate limb, tracing its roots back to the lobe-finned fish of the Devonian period. Over millions of years, it transitioned from a sturdy fin-support to the primary anchor for powerful flight muscles in birds and digging muscles in terrestrial burrowers.
Photography Tips
Lay the bone flat on a neutral surface and take a photo from directly above. Always include a standard ruler or a common object for scale. Getting a clear close-up of the broad, lumpy ends is especially helpful for telling different species apart.