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
Pdx
On Mar 7, 2017, at 9:58 AM, Harry Fuller <atowhee@xxxxxxxxx> 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@xxxxxxxxx> 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
http://birding.aba.org/mobiledigest/OR01#1250500
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--
Harry Fuller
author of Great Gray Owls of CA-OR-WA, see:
https://ecowise.wordpress.com/2015/05/08/the-great-gray-owl-book/
author of Freeway Birding, see: freewaybirding.com
birding website: http://www.towhee.net
my birding blog: atowhee.wordpress.com
