This is bird note.

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 underwater to catch 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 all its all iterative syllables simply means blinking.

This extra eyelid is hinged on 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 Bird Note, I'm Mary McCann.

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