SHARP-EATMAN
NATURE

Bird photographers love to focus on the eyes of their subjects; if the eye is sharp, the entire photograph feels crisp.  At times, however, you may have examined what you thought was a perfect picture, only to find it “marred” by the appearance of the bird’s eye – instead of the brilliant red or yellow or vibrant brown you were hoping for, the bird’s eye is veiled with a cataract-like covering that makes it appear half-blind. 

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The bird, nonetheless, is fine: what you are looking at is called a nictitating membrane, a pearly film that moves over the front of a healthy bird’s eye in order to protect it.  That membrane may be an important part of your picture – one that adds to it rather than detracting from it, because you may have captured an interesting or dramatic aspect of avian life.

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In 1898, naturalist Elliott Coues wrote a study titled Bird’s-Eye Views, in which he described a bird’s nictitating membrane as a see-through translucent "third eyelid" that closes sideways across the eye, rather than up and down like a bird’s other two conventional eyelids. This third eyelid essentially permits birds to gaze out at the world with hidden eyes.

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Coues believed that a chief function of the nictitating membrane was to regulate the amount of light admitted into the bird’s eye.  “The eagle,” he wrote, “is, probably, able to soar aloft directly in the sun's rays, by drawing this covering over its eyes.”  During the daytime, Coues observed, owls are able to protect their light-sensitive eyes from damage by observing the world through their shuttered nictitating membranes.

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Birds of prey, such as ospreys and eagles, which depend on acute sight to hunt for prey, use their nictitating membranes to counteract wind resistance that can dry out the birds' eyes as they fly at death-defying speeds. The nictitating membrane both serves as a kind of windshield and helps hold in tear-like lubricating fluids produced by the bird's eyes.

 

A bird's third eyelid also serves as a screen that shields the bird's eye from dirt and debris as it soars through the air.  Birds (with the exception of albatrosses, ostriches, hornbills and eagles) tend to lack eyelashes and eyebrows that help shelter their eyes from dust and grit. The nictitating membrane, however, can brush damaging particles away from the eye much like an eyelash.  The word “nictitating” comes from the Latin nictare, which means “to blink”.
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Coues observed that the nictating membrane also functions to protect the bird’s eye from attack. “If we menace a bird's eye with the finger,” Coues wrote, “we see that the nictitating is the first of the lids to rush to its defence.”  Birds often shut their nictitating membranes when threatened.

 

Accordingly, rather than aim for the perfectly-focused iris of a bird's eye, a photographer might consider capturing moments when a bird's nictitating membrane is drawn:  its presence might underscore the melodrama of what's depicted.

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Birds make use of their third eyelids when on the attack themselves:  the little blue heron shown above right closed its nictitating membrane while using its beak to grab the feathers of a tri-colored heron, in apparent anticipation of a counter-attack. The presence of the little blue heron's nictitating membrane gives it an intense, eerie quality, as if the bird is absorbed in the violence of a primordial struggle.

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A photographer is likely to see nictitating membranes when birds are engrossed in the  photogenic but risky activities of hunting and feeding.  Raptors such as owls, hawks and eagles deploy their nictitating membranes to safeguard their eyes when struggling with prey. Eagles use their third eyelids to protect themselves from the pecking of their chicks’ sharp beaks when delivering fish to the nest. Similarly, wading birds with spear-like beaks, such as herons, egrets and cranes, can be observed screwing tight their nictitating membranes when feeding their fledgling offspring.

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Water birds employ nictitating membranes when plunging under the surface to fish.  During such activity, the nictitating membranes serve as kind of snorkeler’s goggles that allow the bird to see underwater while foraging for food and that protect its eyes from particles floating in the water.  The membranes also shield the bird's eyes from unseen counter-attacks by prey.  Some water birds, such as loons, ducks and cormorants, have a window-line pane on the nictitating membrane that allow them to focus better through it when diving underwater.

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Birds also make use of their nictitating membranes when engaging in the most mundane of tasks -- preening.  Such behavior allows them to avoid accidentally scratching an eye with a sharp claw or talon.

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NICTITATING MEMBRANE CHARACTERISTICS

 

A bird's nictitating membrane is made up of blood vessels), nerves and connective tissue.  Because the membrane is translucent, it may transmit the color of the bird's iris when drawn.  Thus, the nictitating membrane of the black-crowned night heron shown at right appears red. In other birds, the membrane may mimic the color of the bird's eyelid.  The nictitating membrane of the hooded vulture shown further below appears blue, like the surrounding eyelid.

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REFERENCES

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Elliott Coues, “Bird’s-Eye Views,” The American Naturalist, Vol. 2, No. 10.  Chicago:  University of Chicago Press for the American Society of Naturalists, 1868 (Dec., 1868), pp. 505-513.  Print.

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Elphick, Birds:  A Complete Guide to Their Biology and Behavior (London:  London Natural History Museum, 2016), pp. 57-62.  Print.

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Burcham, J. S., Fred M. Dille, Walter K. Fisher, Frank Stephens and J. Grinnell, “From Field and Study:  Notes on the Habits of the Water Ousel,” The Condor [a publication of the American Ornithological Society], Vol. 6, No. 2 (Mar. - Apr., 1904), pp. 50-52.

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The Cornell Lab of Ornithology Handbook of Bird Biology, 3rd ed., edited by Irby J. Lovette and John W. Fitzpatrick.  (Chichester, West Sussex: John Wiley & Sons, Inc., 2016), pp. 207-209, 257. [Information about transparent panes on the nictitating membranes of diving birds; and about bird species that have eyelashes.]

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Miller, Paul, "The Mystery of the Inner Eyelid," Scientific American, Fall 2015, p. 81. 

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Wygnanski-Jaffe, T. C., J. Murphy, C. Smith, M. Kubai, P.  Christopherson, C. R. Ethier and A. V. Levin.  “Protective Ocular Mechanisms in Woodpeckers Eyes,” Eye, Vol. 21, No. 2 (Jan. 2007), pp. 83-89.

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CITE THIS PAGE:

Sharp, Paula and Ross Eatman, "Bird Eyes:  Nictitating Membranes.  Birds of the Florida Swamps. May 15, 2018.  Web. [Date accessed. <https://www.florida-ecology.com/nictitating-membranes>

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