Reader John Chardine is a felicitious combination of a biologist and a professional photographer. He’s posted here before, but I recently saw some of his photos on Facebook and begged him for more. (His photography website is here.) I received a nice collection of themed pictures that imparted a biology lesson, and I’ll share both today. John is off to Antarctica (he goes every year to lecture about Antarctic birds on cruise ships), so I’ve importuned him to send penguin pictures on his return—and also to give me advice about visiting Antarctica, since that’s one of my dreams.
Here are his notes:
One of my little bird photography projects is to collect images of birds showing their nictitating membrane (another is to photograph pooping birds, but that’s for another time). Most bird photographers throw away images if they show the membrane. I keep them.
The membrane is also referred to as the third eyelid and is flicked across the eyeball in a blink or for more prolonged periods. As you are photographing a bird, the blinks are so quick that you rarely see them by eye. The camera, however, often catches the blinks. This is one of many aspects of wildlife photography I really love—revealing biology through still photography.
The membrane is stored in the inner corner of the eye and folds up in there. You often see the pleats when the membrane is extended. It functions to clean the cornea and also protect it when needed. Birds often cover the cornea with the membrane for more than an instant when they are doing anything that potentially may harm the delicate corneal tissues, such as landing, fighting, or even preening. The general wisdom is that nictitating membranes are clear when they need to be—e.g., in diving birds that feed underwater—and show various degrees if opacity otherwise. The apparent opacity of the membrane seems to be dependent in some species on the angle of view, as the image of the Sandhill Crane clearly shows. Maybe the membrane changes with age? Some species like the White Ibis seem to have a particularly thick membrane. In evolutionary terms, perhaps there is a trade-off between the robustness of the membrane and its transparency, and a robust membrane is generally better option if you do not need to see through it. Very little work has been done on this subject.Anyway, here are a few images of this interesting part of birds’ eyes (many other vertebrates have vestigial to full membranes).The clear, diaphanous membrane of a Common Goldeneye—a diving species of duck:
A landbird, the Ostrich, with a full opaque membrane. I assume the squiggly lines on the membrane are blood vessels:
A Virginia Rail, also showing the possible blood vessels. This membrane is quite clear but the rail is not a diving bird. Rather, it moves quickly through thick marsh vegetation (as thin as a rail) and as it does so it probably needs to protect its eye while at the same time seeing where it’s going:
A Sandhill Crane showing variation in the apparent opacity of the membrane due to view angle and maybe light direction. This is the same bird:
White Ibis, showing a particularly thick membrane. The pleats are easy to see here:
Thanks for sharing these amazing photos and the biological explanations, John Chardine! I read somewhere that owls always have their nictitating membranes ‘deployed’ whilst in flight, otherwise the eyes would dry out too quickly.
I had no idea there was this much variance among birds, and I imagine there must be similar variances among reptiles too, from lizards to crocs.
Sub
I wonder why humans have only a vestigial membrane. A clear membrane would certainly be helpful to us, providing protection from pollution and small foreign bodies. Did early humans have them?
Maybe our reptile ancestors, since it can occur in reptiles. I do not know of any mammal today with a functioning form of this membrane. I do not know why since it seems pretty useful and low in energy cost.
Cats have one for sure!
Okay two.
Yes, cats, d*gs, and horses have them too. When they’re visible, it often means a health problem in those animals.
Fantastic shots! Since “nictitate” (I looked it up!) means “to blink,” don’t eyelids also nictitate? I love it when there’s a fancy science word for familiar things: I knew about “inner eyelids,” but I like “nictitating membrane” better. And I’m pleased to find out they are so varied and lovely.
Nictitate. Nictitate. It’s a tinny word, but I love it!
Eyelid is woody.
Very interesting pictures! I especially enjoy the combination of great photography and biology.
Perhaps the directional opacity of the membrane in Sandhill Cranes is simply a matter of its being slightly opaque. So seen straight on (or looking out straight through), it is translucent. But the viewer is through more membrane when viewed at a steep angle so it seems more opaque.
Thoroughly enjoyed the themed photographs, with some science included. I do hope you follow through with your poop theme as well, a little humor never hurts! Reminds me of a photo on the Tetrapod Zoology bl*g of a surreptitiously pooping tapir which was quite amusing.
Wonderfully themed photos! Especially love the zombie ostrich. And looking much forward to poop photos, too.
Gorgeous photos!
On my bird forum we have a long thread, “Zombies; they live among us,” of bird pics that show nictitating membranes. Here’s one of mine, an American Dipper ( Cinclus mexicanus):
https://flic.kr/p/oMY3Lr