Bald Eagle Nictitating Membrane
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. It is either translucent or semi-transparent so the animal maintains sight while it covers the eye. 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. However camels, polar bears, seals, aardvarks, marsupials and monotremes do have fully functional membranes. 1/2000, f/8. ISO 500, 500 f/4, 1.4 tc This nearly adult (head not yet completely white) Bald Eagle is neither tame or captive. 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. 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. 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. This is the normal appearance of the Bald Eagle’s eye. In adult birds, eye color is typically some shade of yellow with a distinctly visible pupil (though juvenile birds have much darker eyes). 1/2000, f/8. ISO 500, 500 f/4, 1.4 tc But every few seconds the bird swipes the nictitating membrane across the…