Bluebirds Building Nest in Birdhouse

Regular readers of the Colvin Run Habitat Blog know that Eastern Bluebirds are rare in the Habitat. While a bluebird house or box was erected last spring, a pair of chickadees immediately moved in, only to be evicted later in the season by a pair of house wrens. A bluebird was observed perched on top of the bluebird box last year, but did not build a nest.
At the beginning of March, we began observing a bluebird (never seen as a pair) in the Habitat about once a week. In the last few days, a bluebird has been observed not only perched on top of the box, but actually entering the box with nesting materials. I took these three photos after sneaking through the shrubs to get as close to the bluebird as possible.
I am guessing that this bluebird is the male, as the male supplies the nest materials while the female does the actual building of the nest.