Figure 2.

Hindlimb skin variation and standing ability in surface swimming and diving birds. (A) Surface swimming birds can easily stand on land, shown here using a Mallard. The entire lateral and medial surfaces of the limb are covered in skin separate from that overlying the body wall, allowing for a large range of motion at the hip and knee. (B) In contrast, highly diving birds, shown here using a Red‐throated Loon, struggle to stand on land. The medial surface of the proximal leg is not covered by skin and is instead tucked next to the ribs, restricting flexion‐extension mobility at the hip and knee. Only the feet extend away from the body with full skin coverage. Consequently, the body must be upright for the foot to reach under the body's center of mass, often requiring wing flapping to lift the body up to standing or while attempting to walk.