canberrabirds

Re: Nictitating membranes

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Subject: Re: Nictitating membranes
From: "Geoffrey Dabb" <>
Date: Sun, 29 Jan 2006 14:43:45 +1100
I was interested in Philip's comment about nictitating membranes (NBs), because one does sometimes notice them while doing a little quiet birdwatching.  In my earlier message I was not able to reproduce the eye action as seen on video as clearly I would have liked, but it does seem clear to me that each time the bird (or that particular individual) called it momentarily closed its eyelids - not the ring, of course, which is the same colour.  I suspect other pigeons might do the same thing.  The action takes less than 0.5 sec, perhaps only 0.1 sec.
 
  
 
On the other hand when the NB is used, the eye can remain open, although partly obscured, because the NB is semi-opaque - or semi-transparent depending on how you look at it.  I have many video frames that illustrate this phenomenon, which might sometimes take less than 0.1 sec.  If it were much faster the video would be unlikely to record it at all, let alone the human eye.  Digital video (as used in Aust) records at 25 frames per second, but the individual frame might be shot at say one thousandth sec, so the bird could blink without the camera seeing it.  When it does record it, it is usually as a clear eye momentarily becoming milky.
 
At this point I might throw in a little shot of  a Barking (aka Winking) Owl taken some years ago.  The second frame shows the eyelids closing and simultaneously the NB crossing the eye.  In the bird's left eye the NB is starting across as usual.  I cannot explain why in the right eye the NB is coming from a different direction and is straight-edged when it should be movement-blurred -  perhaps the NB on the left eye was being activated and the one on the left came up partially in sympathy.
 
 
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