This is the first time I've had a Mountain Chickadee as a yard bird!
We have several Black-capped Chickadees around our home and I enjoy hearing them calling outside, even when I am indoors. Chickadees just make me happy. Having another kind of Chickadee in my yard is just cool!
Identification of Mountain Chickadees is pretty simple. I just look for that white eyebrow. That is the easiest and quickest characteristic to separate it from all other chickadees. Their calls are similar to B-C Chickadees, but different. Mountain Chickadees seem to me to be a bit smaller too.
Here are a couple other views of the Mountain Chickadee I enjoyed in my backyard the other day.
Here's a post from a few months back about Carolina Chickadees and how they compare to Black-capped Chickadees.