Who am I?

We saw this small bird briefly when I was out with friends, NW of Calgary two days ago. I know I must know what bird it is, but I just can't think what it might be. The closest I've come is a female Mountain Bluebird, but I can't see any sign of blue in the feathers. Any help with ID would be much appreciated. I suspect I'm going to be feeling like a rookie "birder" if someone does give the name.

Later: thanks, everyone, for identifying this bird as a Townsend's Solitaire female. Really appreciate your help with this.