From: Chris Howard
Remote Name: 188.8.131.52
Time: 01:33 PM
Mountain Chickadee is the only chickadee recorded in the Eastern Sierra. (Lower elevations on the west slope of the Sierra get Chestnut-backed.) As far as I know the closest population of Black-capped Chickadees is in eastern Nevada, far from the Sierra Nevada.
The Mountain Chickadees I've seen in the Eastern Sierra so far this summer have all had very narrow superciliaries, like the one you describe. They must be in worn plumage right now like Sibley shows. As for song, Mountains sound to me like "Cheese-burg-er".