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...) --