For most birds, keen eyesight is critical for survival. But many birds lead lives that can be very hard on the eyes — like flying at breakneck speed, racing for cover into a dense thicket, or diving under water to capture prey. Imagine how the chips fly as this Pileated Woodpecker chisels a cavity. Fortunately, birds have evolved a structure for protecting their eyes. Beneath the outer eyelids lies an extra eyelid, called the nictitating membrane. It helps keep the eye moist and clean while guarding it from wind, dust, and hazards.More info and transcript at BirdNote.org. Want more BirdNote? Subscribe to our weekly newsletter. Sign up for BirdNote+ to get ad-free listening and other perks. BirdNote is a nonprofit. Your tax-deductible gift makes these shows possible.