Date: 3/7/17 9:58 am
From: Harry Fuller <atowhee...>
Subject: [obol] Re: Dipper question
Jules is correct and it is believed the transparent membrane both protects
the eye when the bird is swimming underwater and protects the necessary
lubrication to stay on the eyeball and not get dissolved ...the white you
see is the leading edge of the membrane which moves across the eye like a
shutter or sliding door...we notice it most on Dippers because we can get
close enough; been a long time since I was ten feet from a loon

On Tue, Mar 7, 2017 at 9:15 AM, Jules Evens <avocetra...> wrote:

> That white "third eyelid" is a nictitating membrane and many
> waterbirds-alcids, loons, etc- have it as a means to clean and protect the
> eye's surface. (Reptiles and sharks have them as well.) It may also serve
> some function in communication in some landbirds (e.g. crows and ravens).
> There is a thorough discussion of its function and anatomy Cornell's
> "Handbook of Bird Biology (3rd ed.)
> Jules Evens
> Portland
> Sent by my iPhone
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Harry Fuller
author of *Great Gray Owls of CA-OR-WA*, see:
author of *Freeway Birding*, see: *
birding website:
my birding blog:

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