Chickadees Nesting in the Bluebird House

I have been keeping an eye out for visitors to the new Bluebird house. As of three weeks ago, there had been no sign of nest building or visitors entering the house.
So imagine my surprise, when I checked the house today and found not only a fully built nest, but a female incubating. I was really surprised that it was not a Bluebird, but a Carolina Chickadee, who are extremely common year-round in the Colvin Run Habitat. Given that the Chickadees are known as early nesters, hatching may be right around the corner.
The photos are quite marginal in that the side door to the house was intended only to clean out old nests, not for photographing.
And here is a photo of a chickadee not sitting on the nest.