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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