Date: 3/9/17 9:07 am
From: Linda Fink <linda...>
Subject: [obol] dipper question redux
Thanks to all who submitted guesses (because that's all they can be).
The white eyelid is not a nictitating membrane. It's an eyelid. Other
birds have dark eyelids. Only Dippers have white eyelids. Some birds
have nictitating membranes (and also eyelids). Humans have eyelids but
not nictitating membranes. A good way to distinguish eyelids from
nictitating membranes is by which direction they close across the eye:
eyelids top to bottom, nictitating membranes side to side. (I have lots
of photos of Dippers showing their white eyelid half-closed if anyone
wants to see them... they close top to bottom.) Here's a good site:
http://www.10000birds.com/what-is-a-nictitating-membrane.htm

I wanted to know why Dippers are unique in the color of their eyelid.
The Sibley Guides online source several folks quoted is the one I found
originally that made me ask OBOL.

I am leaning toward communication as a possible explanation. But I don't
know how to test it out since if they blink more when something that
might be dangerous (like a human) is watching, how does said human
figure out if they blink less when no one is watching? (Call it
curiosity... or too much rain on the brain...)
--

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