Avian mechanisms for vision in air and water

Suggested mechanisms by which aquatic birds can compensate for the refractive loss of the cornea underwater include an exaggerated underwater accommodative increase in refractive power, the development of a flattened cornea which is non-refractive in either medium, and use of the nictitating membrane as an underwater refractive goggle. Recent research indicates that certain waterfowl are capable of altering the shape of the lens dramatically by means of the ciliary and iris muscles and that penguins have flattened corneas. The nictitating membrane does not play a refractive role.