forelimb wing

The avian forelimb wing contains homologs of the tetrapod forelimb: humerus (upper arm), radius and ulna (forearm), fused carpometacarpus (wrist/hand), and reduced digit bones (I, II, III). Primary flight feathers attach to the hand region, secondaries to the ulna.

In bats, the forelimb wing retains all five digit rays (though digit I bears a claw), with digits II-V greatly elongated to support the patagium (wing membrane). Birds, by contrast, have fused and reduced hand bones without a patagium, relying on feathers for the wing surface.

Forelimb wings evolved in response to selective pressures favoring arboreal locomotion, gliding, and eventually powered flight. The forelimb gradually transformed from a weight-bearing terrestrial limb into an aerodynamic structure optimized for lift, maneuverability, and efficiency.