Date: 3/7/17 10:15 am
From: Bob Archer <rabican1...>
Subject: [obol] Re: Dipper question
Folks are getting confused about what they are seeing. The dipper has an eyelid that is covered in white feathers. The original question was in regards to why the dipper has white feathers on its eyelid. The membrane is another neat feature birds have.

Bob Archer

> On Mar 7, 2017, at 9:58 AM, Harry Fuller <atowhee...> wrote:
> 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
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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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