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Question about nictitating membranes

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I (think I) know what a nictitating membrane in birds is, and what it's for.  In raptors, as far as I know, it's practical purpose is to protect the eye, but it also helps maintain (some) vision when searching for and/or actively attacking its prey, but the bird still needs to blink.  My question here is related to both pics posted below.  They're both of the same immature Eastern Red-tailed Hawk taken in two consecutive frames.  In the first photo the membrane is translucent.  But in the second photo, it appears opaque.  Are there two layers to this membrane?  Are there two separate membranes, specifically in predatory birds?  There is no ID required here.  I just need to satisfy my curiosity.



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I think the first photo is without the membrane, or maybe it’s starting to come out of the corner. Birds have three eyelids: Upper, lower, and nictitating. Diurnal birds close the lower one, and nocturnal birds close the upper one. The nictitating comes from the inner side (closing away from the beak) perpendicular to the other eyelids. I think the first photo is just a little blurry in the eye area, and/or the membrane is starting to slide over, making the whole eye seem a little opaque, as opposed to the second photo where it is fully closed. 

Edited by Avery
Had a fact backwards
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I should've included this "eye wide open" shot for comparison.  Comparing this one to the first shot I originally posted, there's clearly a noticeable difference.  In the first two pics, the eye is definitely obscured by something.  What confuses me is the big difference between those two.  The first is semi-transparent, while the second one is definitely not. 


Edited by smittyone@cox.net
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10 hours ago, smittyone@cox.net said:

Anyone else want to field this one?  


14 hours ago, gpoole said:

The nictitating eyelid in the first photo doesn't look fully closed, perhaps allowing in a little more reflective light resulting in a clearer eye image. Just a guess.

I agree with @gpoole. If the nictating membrane is being backlit by the light that is hitting the eye, that would explain the variance in transparency. The only other plausible idea I can come up with is the bird having the ability to control the thickness of the membrane, perhaps stretching it into thicker/thinner segments at different stages as it closes. That's my creative mind at work trying to stretch things to fit the 'plausible' category though.  I like the plausibility of the backlit idea better.

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I don't think we should overthink this. I believe we simply have two photos of the same membrane at various stages of opening/closing.

Over the years I have lots of shots of different bird species at various stages of opening and closing the nictitating membrane. This can cause it to look quite variable.

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