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Transcript: 

This is BirdNote.

For most birds, keen eyesight is absolutely critical — for finding food, spotting predators, and generally staying alive. But many birds lead lives that could be very hard on the eyes: like flying at breakneck speed, racing for cover into a dense thicket, or diving under water to capture a spiny, struggling fish. 

Fortunately, birds have evolved a structure for protecting their eyes. Like humans, they have upper and lower outer eyelids. But beneath the outer eyelids lies an extra eyelid, called the nictitating membrane. Nictitating, for all its alliterative syllables, simply means “blinking”. This extra eyelid is hinged at the inner side of the eye and sweeps horizontally across the cornea. The nictitating membrane is largely transparent, and it helps keep the eye moist and clean while guarding it from wind, dust, and hazards. 

Birds aren’t the only animals gifted with nature’s goggles. Reptiles and some mammals have the extra eyelid, too. A Polar Bear’s nictitating membrane helps protect its eyes against the intense glare of arctic light, preventing snow blindness. 

Nature’s goggles become nature’s sunglasses.

For BirdNote, I’m Mary McCann.

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