Blink of an Eye

July 19, 2013 Posted by: EH - Park Ranger (Yosemite Valley)

Eating lunch at Taft Point (a lovely overlook reached by a trail leaving from Glacier Point Road), my favorite bird paid me a visit. A large, black bird with an inquisitive nature, the common raven (Corvus corax) is a frequent visitor to campgrounds, picnic areas, and picturesque overlooks. While the raven kept an eye on my peanut butter and jelly sandwich, I caught the raven's eye on film.

Like most vertebrates, birds have eyelids that they close while preening or sleeping. But their eyes also have another protective shield, a nictitating membrane! This additional "eyelid" sweeps over the eye sideways, from the corner of the eye closet to the beak across to the outside corner, functioning to keep the eyeball moist and clear of dust. The membrane can also be useful during flying as it is translucent, still allowing the bird to see!

You know the little pink nub in the corner of your eye? It's actually an evolutionary remnant of the nictitating membrane, no longer functioning as it does for birds.

Although dust may get into our eyes, these ravens are set for a patient perch at a blustery overlook, staying on the look out for leftover lunch. So keep your food close, or it may disappear in the blink of an eye! 

Nature Scene, Glacier Point, EH