Not posted for any technical merit, but to show a full coverage
very smooth nictitating membrane. Most have seen this many many
times, but thought it might be of interest to some.
Thanks for opportunity to share....young Red Tailed Hawk
RTH, Full Nictitating Membrane
Not posted for any technical merit, but to show a full coverage
very smooth nictitating membrane. Most have seen this many many
times, but thought it might be of interest to some.
Thanks for opportunity to share....young Red Tailed Hawk
Nice one Dave. Yes I think bird photographers probably see more nictitating membranes than anyone else on the planet!
So here's a question- why didn't evolution produce a totally transparent membrane??
Last edited by John Chardine; 05-02-2009 at 08:10 PM.
transparent, don't know.....but this I found is interesting, especially about the woodpeckers:Originally Posted by John Chardine
In birds of prey, it also serves to protect the parents' eyes from their chicks while they are feeding them, and when peregrine falcons go into their 200 mph dives, they will blink repeatedly with the nictitating membrane to clear debris and spread moisture across the eye. In polar bears it protects the eyes from snow blindness. In sharks it protects the eye while the shark strikes at its prey. Woodpeckers tighten their nictitating membrane a millisecond prior to their beak impacting the trunk of a tree in order to prevent their eyes from leaving their sockets.
Very interesting thread, I didn't know this about polar bears and sharks.
Neat Dave. The eye looks pretty cool, almost iridescent. Peregrine falcons also close the baffles in their nose when diving, so they are more aerodynamic. Thanks for posting, never seen this "frozen".
I am pretty sure that the nictitating membrane isn't clear because that would require it to be moistened, which is what makes the cornea (and other transparent tissues of the body) clear. That would require an additional ocular gland or at least another duct from the existing ocular gland that empties onto the outer surface of the nictitating membrane. Being moist, dust and debris would attach to the outer surface of nictitating membrane, negating its windshield-wiper purpose. Further, the membrane is shut for such a brief period of time, it likely does not impair vision significantly.
Just my thoughts,
Chris
This makes sense Chris. Thanks for this.
Diving birds have to look through the membrane for longer periods and maybe in these species it's clearer.
Another thought Chris- the membrane is retracted most of the time so I don't see any "engineering" problem in keeping is moist. Dust adhering to the moist membrane could be cleaned off each time it is retracted.
When an eyelid (whether upper, lower or third) retracts, the surface tissues fold up, and the cartilage that makes the lid stiff slides back under the folded up tissue. It doesn't slide back like a pocket door does into the door frame, for example. For the surface tissues to fold up and readily open again, the surface has to be dry, hence they are opaque.
How do I know this? I am a veterinary pathologist and my group does post mortem examinations on all of the animals that dies at the Baltimore Zoo and National Aquarium. As a result I have looked at the eye tissues of something over 1000 birds in the last 20 years.
That's great information Chris. Still a little hard to imagine what's actually happening when the nictitating membrane "opens" and "closes" but I'm working on it!
I have noticed differences in the transparency of the membrane between species in my photographs. For example, crows have relatively opaque ones and gannets, relatively transparent. Makes sense in that the gannet really has to see through the membrane when it's diving for food. Have you noticed inter-species differences like this?
this has been extremely interesting.....great thought inspiring info posted
Thanks guys
folds in nictitating membrane?
This an image of a Common Murre (Common Guillemot) nictitating membrane showing what I think are lines on the membrane from when it is folded up. Would this be your interpretation Chris?
BTW, as you know the murre is a pursuit diver but at least in this light the membrane looks opaque.
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