Nictitating Membranes
Sometimes referred to as a “third eyelid,” the translucent nictitating membrane visible across this juvenile Black-crowned Night Heron’s eye serves to protect it from foreign objects and to moisten the eye while at the same time allowing the bird to retain some degree of visibility. It extends from the inner corner of the eye to the outer corner, and is drawn across the eye much like a windshield wiper. The membrane is thinner and more transparent than the fleshier upper and lower eyelids and is used periodically by birds when foraging, flying, diving, feeding young, gathering nesting material, etc. In this case, the heron’s nictitating membrane was drawn across its eye seconds before it plunged beneath the surface of the water to capture a crayfish.
Birds aren’t the only animals that possess nictitating membranes – it’s relatively common in fish, amphibians, reptiles, and mammals (but they are rare in primates).
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Owls & Humans Share Trait
Birds have three eyelids – an upper eyelid, lower eyelid and a third semitransparent membrane called a nictitating membrane that sweeps across the eye much like a windshield wiper. This membrane keeps their eyes moist, and protects their corneas from being scratched.
In most birds, including owls, the upper and lower eyelids are used to close the eyes when sleeping, and the nictitating membrane is used for blinking. Humans close their eyes mainly by lowering the upper eyelid, where most birds do so by raising the lower lid. Owls (and a few other birds such as parrots, toucans, wrens and ostriches) are more human-like in that their upper lids are usually lowered to close their eyes. Owls also usually close their eyes, partly or entirely, when capturing and transferring prey, scratching their face, preening another owl and copulating. (Note the rows of feathers on this barred owl’s upper eyelids.)
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Nictitating Membrane Provides Moisture & Protection to Eyes
You and I have two opaque eyelids, one above the eye and one beneath. When we blink, they meet in the middle of our eyes. Some birds, amphibians, reptiles, fish and mammals have three eyelids – two similar to ours, and a third translucent or transparent eyelid, called a nictitating membrane. This membrane moves horizontally across the eye from the inside corner to the outer edge of the eye, much like a windshield wiper, when needed for protection, to clear debris or to moisten the eye. Although this American Crow’s nictitating membrane looks as if it was blinding the crow, it isn’t. Because of the membrane’s translucency, the bird can still see when the membrane is covering its eye.
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