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Wednesday, August 16, 2006

Nictitating membrane

Nictitating membrane

In April 2006 Johnny Wee spotted a White-throated Kingfisher (Halcyon smyrnensis) perching on a branch of a tree, eying a lizard on the ground nearby. He managed to take a few images with his digital camera as the bird launched at the prey. Back at his computer, he processed the images and found a number of them had birds whose eyes were white. Thinking that these were photographic artifacts, he erased them all.

In July 2006 Johnny observed a Pied Fantail (Rhipidura javanica) eying an insect. While intently concentrating on the prey, its eyes turned white and its tail feathers fanned out. This time he did not erase his white-eyed bird images but kept them as comparisons. The eyes returned to normal after feeding.

Two different observations on two different species of birds, both with eyes turning white just before the birds lunged on their preys must mean something.Birds have three eyelids – one upper, one lower and a nictitating membrane. The third is between the two other eyelids and the cornea and moves sideways. It is used in cleaning and protecting the eye.

It is believed that birds cover their eyes with the nictitating membrane when under water. This has been disputed by some, as the membrane, being translucent and not transparent, would obscure the sight of the bird in its search for food. Others question the necessity of covering the eyes in water, based on our experience of seeing under water.

The nictitating membrane is also believed to comes in useful during flying. The bird cannot afford to close its eyelids often when in the air. Loss of vision, even momentary, caused by closed eyelids can throw the bird off balance.

Ornithologist Geoffrey Davison believes that the whiteness of the eye in the image is due to the nictitating membrane coming across the eye just at the moment the image was taken. This may or may not be related to the bird’s intended movement.

Birders-photographers, please keep a look out for this phenomenon in your images.

Latest: My copy of Handbook of Bird Ornithology (2nd ed.) (2004) by S. Podulka, R.W. Rohrbaugh, Jr. & R. Bonney (eds.), New York: Cornell Lab of Ornithology, arrived today. On page 1.7 it is said that "In raptors and other predatory birds, the nictitating membrane protects the eyes as the bird pursues prey through heavy cover, such as a blackberry thicket."

Input by YC and Geoffrey Davison, images of Pied Fantail by Johnny Wee.

2 Comments:

At 12:34 am, Blogger Corax said...

I came across your blog entry while searching for information on nictitating membranes in various animals. I have often seen pictures of birds with their nictitating membrane covering the eye, and never thought that someone not familiar with the way birds blink might think it was a defect in the photograph! But I suppose it could easily happen that way.

Most birds only close their outer eyelids when they sleep, or at times while preening or scratching. Normally they keep their eyes open constantly, and blink frequently with the nictitating membrane only, for the same reason we blink our eyes - to keep them moist. And just as we stand a good chance of catching a person blinking when taking their picture, the same thing happens frequently when taking pictures of birds!

 
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