The nictitating membrane of a Bateleur Eagle at the San Antonio Zoo. A raptor has three eyelids, an upper, lower and a third called the nictitating membrane. When a raptor closes its eyes it is only the lower lid which moves to cover the eyeball – the upper one remains fixed. The nictitating membrane moves from the front to the back of the eye and is a tough sheath which the bird automatically draws across the eye for protection when there is movement close to its head.
Photo credit: Robert Heischman
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