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We measured wing joint mobility in penguins, alcids, diving-petrels, and non-diving fliers. Great reduction in mobility of the intrinsic wing joints was found in penguins, but not in alcids or diving-petrels. This reduction is correlated with simplification of the intrinsic wing musculature. In contrast, alcids and diving-petrels, which use their wings in both air and water, retain the full functional capacities for flight. Movement through the air probably requires a capability for subtle and varied motions, forces, and shape changes that preclude stiffening and simplification of the wing. Hence, the conversion of an aerial wing to a flipper, as in penguins, must be possible only after the evolutionary loss of flight.

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