Mountain bluebirds are cavity nesters and are pretty creative at finding nesting spots. It is pretty common to find them nesting in the holes that you find in many of the park signs. This pair was nesting in the middle of Mammoth. Don't be fooled the male mountain bluebird looks like he might be helping to make the nest but the truth is that the female does all of the work.
The mountain bluebird shares much of its range with the western bluebird, although the mountain bluebirds range continues farther north then the western bluebird, and there is often competition between the two for territory and nesting sites. It is easy to distinguish between the two because the western bluebird has an orange chest where the mountain bluebird is all blue.
Since they do not seem to mind the cold very much they are usually one of the first birds that return to Yellowstone in the spring. That is why we have managed to see them every year that we have headed out, even the years where there is still several feet of snow on the ground and it manages to snow a couple of times while we are out there.