Turkey Vulture — In The Blink Of An Eye
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Today, it’s back to posting about my beloved desert. I’ve enjoyed my little digression into Yellowstone and the Tetons and it is my hope to make similar side trips in the months and years to come. But, this is primarily a blog about desert life, so let’s get back to it.
A couple of weeks ago I photographed a photogenic and rather handsome Turkey Vulture (at least I think so!) and as I reviewed my images I found one that was different. In this image, my subject looks altogether weird, and, frankly, creepy.
This vulture looks like something from a zombie film. What’s with that blue-gray membrane across its eye?
Well, the vulture isn’t one of the undead. The membrane is something called a “nictitating membrane” and all birds and many reptiles have it in addition to their upper and lower eyelids. So do some mammals including dogs and cats. It’s also known as a “third eyelid.” It works exactly as our eyelids do, to protect the vulture’s eyes from dust and debris. The image shows the vulture while it is blinking.
The nictitating membrane closes horizontally, unlike regular eyelids, which operate vertically. In some species it is transparent, allowing the animal to see through the membrane. Obviously, that is not the case with this vulture. A lot of other raptors have opaque nictitating membranes as well. I have no idea why it is transparent for some, opaque for others.
This membrane may be particularly useful for a Turkey Vulture. Feeding on carrion, these birds often stick their heads into the body cavities of their food. The membrane allows the bird to do that while protecting its eyes.
Here’s a second image of the same bird, taken a fraction of a second after the first image.
Everything’s identical except that the vulture has retracted its nictitating membrane and, no longer blinking, it has its eyes wide open. It is as cute as ever!!
Images made with a Canon 5Diii, 100-400 mm f4-5.6 ISII zoom lens+1.4x telextender, aperture priority setting, ISO 800, f8 @ 1/4000.