It happens very quickly and I needed a fast shutter speed to freeze it. Here the membrane is almost completely closed. It begins at the inside corner of the eye closest to the beak and moves across to the outside corner. In adult birds, eye color is typically some shade of yellow with a distinctly visible pupil (though juvenile birds have much darker eyes).īut every few seconds the bird swipes the nictitating membrane across the eye to moisten and lubricate it. This is the normal appearance of the Bald Eagle’s eye. It warmed my heart that the eagle was so comfortable with me this close and that was a very good thing because Mia and I almost froze to death over the many hours of photographing this magnificent bird. The pole was so close to the road that I sometimes had to take off my teleconverter to get the entire bird in the frame. But for several days in 2009, beginning on a bitterly cold Christmas morning, this adult adopted a favorite perch (a relatively short pole) that looked out over a pond with fish in it. This nearly adult (head not yet completely white) Bald Eagle is neither tame or captive. However camels, polar bears, seals, aardvarks, marsupials and monotremes do have fully functional membranes. Birds, sharks and some reptiles have a fully functional nictitating membrane while in most mammals there is only a small vestigial remnant in the corner of the eye. It is either translucent or semi-transparent so the animal maintains sight while it covers the eye. In fact, if you cover an eagle's eyes with a hood, the eagle will instantly go unconscious, its brain shutting down due to sensory deprivation.The nictitating membrane (nictitan) is a membrane or “third eyelid” found in some vertebrates that can be drawn across the eye for protection and to moisten and lubricate the surface of the eye. This leaves little left over for a sense of touch or taste or smell. What's more, scientists estimate that up to eighty percent of an eagle's sensory input comes from its eyes. In order to change its point of view, a bird has to move its whole head. One drawback is that the eyes of a bird are fixed immovably in its skull. This sensitivity helps a soaring bird find its prey up to a mile away. Their eyes are more sensitive to movement than they are to color. While we have some two hundred thousand vision cells at the most concentrated part of our eye, eagles have one point five million. The actual eyeballs, lying within an eagle's skull, are huge by our standards, taking up more room than the brain itself! Within these eyes, eagles have more light sensitive vision cells than we do. The eyes on an eagle's face may seem relatively small, but the part you can see is only the tip of the iceberg. A bald eagle, gliding high above a meadow, searching for the telltale rustle of a passing mouse, has eyesight that is eight times more powerful than ours. You might say that someone with a keen sense of sight is "eagle eyed," but, unfortunately, that's wishful thinking.
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