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Nictitating Membrane Provides Moisture & Protection to Eyes

12-4-14  crow-nictitating membrance 109You and I have two opaque eyelids, one above the eye and one beneath. When we blink, they meet in the middle of our eyes. Some birds, amphibians, reptiles, fish and mammals have three eyelids – two similar to ours, and a third translucent or transparent eyelid, called a nictitating membrane. This membrane moves horizontally across the eye from the inside corner to the outer edge of the eye, much like a windshield wiper, when needed for protection, to clear debris or to moisten the eye. Although this American Crow’s nictitating membrane looks as if it was blinding the crow, it isn’t. Because of the membrane’s translucency, the bird can still see when the membrane is covering its eye.

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5 responses

  1. Dr Jo

    Hi Mary,

    I guess years ago my father misunderstood how they actually work, but when we were adolescents he told us he thought we had nictitating membranes bec when there were tasks to be done, it was as if a membrane crossed over our eyes so it still looked like we were paying attention, but we weren’t. At least he introduced us to the concept. Good to have the correct info now, and thank goodness (most) kids eventually grow up!

    Cheers, JoAnn Berns

    Date: Tue, 2 Dec 2014 12:57:40 +0000 To: dr_jo@hotmail.com

    December 2, 2014 at 6:23 pm

    • That is so funny. Your father obviously had a tremendous sense of humor! We do have the remnants, I guess you’d call it, of a nictitating membrane — in the corner of each eye, next to your nose, that little fleshy thing, I believe, is what’s left of it.

      December 2, 2014 at 9:13 pm

  2. Jean Harrison

    Spectacular photo, Mary.

    December 3, 2014 at 1:13 am

  3. Sandra Chivers

    Mary, has anyone mentioned concern over lack of winter songbirds at their feeders, possibly due to the large crow population in the Upper Valley? Particular areas in Lebanon and Hanover, near the white pine stands? I have witnessed hundreds in the air during my walks around Lebanon in the last couple years. I live in Hanover and have noticed a huge decline in birds at my feeders. Crows are ever present in my area. Anything that can be done about the crow over population?

    December 28, 2014 at 2:20 pm

    • Hi Sandra,
      I am afraid there’s not much that can be done about the crows, and I haven’t heard of their having an effect on birds at feeders…you might contact the Vermont Center for Ecostudies in Norwich — ornithologists there would be able to answer you much more confidently! Do let me know if they say other than that which I said!

      December 28, 2014 at 4:57 pm

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