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One of the great things about photographing wildlife is the opportunity to see aspects of the animals you might not normally see, and then to later research and learn about that aspect of the animal. An example of this is the nictitating membrane on birds. The nictitating membrane is a third eyelid under the typical two eye lids we all have. It’s a translucent eyelid that can be closed while the other eyelids are open to clean and protect the eye. If you own a cat you might have seen an example of this, as cats also have nictitating membranes.
Tim Birkhead, in his fantastic book, Bird Sense: What It’s Like to Be a Bird, says knowledge of this additional eyelid has existed for centuries. People such as Aristotle, Frederick II, and Louis XIV wrote about it. Frederick II wrote in his falconry manual: "for cleaning the eyeball there is provided a peculiar membrane that is quickly drawn across its anterior surface and rapidly withdrawn." John Ray and Francis Willughby wrote in their 1678 encyclopedia of birds, "Most, if not all birds, have a membrane of nictation... where withal they can at their pleasure cover their eyes, though the eyelids be open... and serves to wipe, cleanse, and perchance moisten..."
Tim Birkhead goes on to further describe how the nictitating membrane not only cleans the eye, but helps protect it: “Each time a pigeon puts its head down to peck at something on the ground, the nictitating membrane moves across the eye to protect it from spiky leaves and grasses. In raptors the membrane covers the eye immediately before the bird slams into its prey, and in exactly the same way the membrane covers the eye just before a plunging gannet hits the water.”
In analyzing my images of birds I’ve seen how the nictitating membrane will cover a woodpeckers eyes just before it begins pecking at a tree or stem. I’ve seen instances of the membrane closing just before a bird ruffles its feathers, shaking itself in the process of preening. And in most cases my camera’s shutter just happened to close at exactly the right time to catch the membrane closing or opening while the bird was standing still.
This post contains many examples of these third eyelids in a host of different birds. Most of these images have been heavily cropped to try to show the details around the birds eyes. I hope you find this topic as fascinating as I do. If you’ve never seen the nictitating membrane then keep your eye peeled for it next time you watch a bird up close through a camera or binoculars. Maybe you’ll catch it moving. Perhaps the bird’s eye will appear cloudy for just a moment. That’s the membrane closing and opening. The word nictitating comes from the Latin nictare, which means to blink.
Another interesting tidbit from Bird Sense relates to us humans. Go look in a mirror some time. Look at the corner of your eye nearest your nose. There’s a little pink nub in the corner. That is the remnant of our own nictitating membrane.
If you’re interested in learning more about birds, and what it might be like to be a bird, check out Tim Birkhead’s Bird Sense: What It’s Like to Be a Bird. The book is broken into 7 chapters, each describing a different aspect of how a bird experiences the world. The chapters are:
I found this an absolutely fascinating read. Very educational.