Sibley has illustrations on facing pages of what he calls the "Gray Adult" female Mountain Bluebird and the "Drab Adult" female Western Bluebird; they are quite similar. I originally thought this was a Western, and the bird shown at http://www.pbase.com/image/88608233 -- photographed same place, same time -- was a Mountain. Note the thicker bill on this bird, and the pale chin and line across the forehead on the other one, as shown by Sibley.

When I posted this for discussion, Joe Morlan pointed out that this bird has (barely visible) white undertail coverts; this establishes it as a Mountain Bluebird, despite the other apparent distinguishing features. Clay Kempf also noted the treeless habitat and the white linings of the wing coverts as indicating Mountain Bluebird.