Post #32 - A bit of Biology - What is a nictitating membrane?

One of this things that I love about photography is the ability to revisit the birds I saw in the field once I return home. The shot preserves the moment that I experienced and sometimes even augments it. Take for example this shot of a Red-winged blackbird that I took at Great Meadows National Wildlife Refuge in Concord Masssachusetts. 

Red-winged blackbird

Canon 500mm f/4 IS v1 + 1.4x III on EOS 7D1

1/400 at f/ 7.1, ISO 400

Reds are saturated, but I had to give something to get the blacks. ISO kept low to avoid 7D noise

This was a very cold spring morning, so cold that I managed to capture his breath as he belted out his song. I did not notice this artifact in the field; It was only once I got home and started going through my shots that I realized what a unique capture this proved to be. I also realized how lucky I was to have this otherwise odd and admittedly cold-toned background against which the condensed vapor could be visualized.

A similar phenomenon happened last weekend at Bolsa Chica Ecological Reserve in Orange County. Though cloudy for most of of the morning, the sun poked out just long enough for to shoot this cooperative Black-crowned night-heron. He was standing on a pipe, so I decided to crop it as a headshot to keep the man-made (and hideous) pipe out of the frame.

Black-crowned night-heron

Canon 500mm f/4 IS v1 on EOS 7D2

1/1600 at f/8, ISO 800

At one point stretched his neck out and opened his beak. Always one trying to capture action, I was very happy he had given me at least this minimal amount of behavior. When I returned home, I realized that these two shots provided a perfect illustration of the nictitating membrane that protects the eye. It's easy to see the membrane drawn across the eye in the second shot of the bird (below).

Black-crowned night-heron

Canon 500mm f/4 IS v1 on EOS 7D2

1/2000 at f/8, ISO 800

Sun was a bit brighter than in above shot; Shutter dropped to 1/2000 from 1/1600 to keep whites.

In fact, I had no idea that this membrane was called. I actually had to look it up for myself. Beyond the proper name for the structure, I learned that birds are not the only creatures that possesses nictitating membranes. Fish (including sharks), amphibians, reptiles, and some mammals - notably Polar bears, Beavers, Manatees, and Camels - have them as well. In contrast to an eyelid which closes the eye in the vertical direction, the nictitating membrane is drawn across the eye from front to back. The membrane is translucent, providing protection while still permitting some form of visibility. I guess it makes sense that an independent mechanism has evolved to protect the eye while still allowing at least enough vision to, say, escape an unexpected attack from a predator. I am not sure why the bird above drew the membrane across its eye at the time of the shot. Given that he was effectively stretching, it is totally possible he was just going through a routine "system check". Anyway, hopefully this proves at last minimally informative for some of you!