When I stuck a ruler in the middle of the nest to determine it’s depth, a mouse stuck it’s head out of the middle, and then leaped out of the box. It was probably a white-footed mouse (Peromyscus leucopus), as they are better climbers than deer mice, and are known tonest in woodpecker holes and bird houses.
I was thrown off because the nest was entirely constructed of grass clippings, and mice usually make their nests from a variety of materials, such as grass, leaves, hair, feathers, milkweed silk, shredded bark, moss, cotton, or shredded cloth. A clue I didn’t pick up on was that the nest did not appear to be tamped down – since the mouse was living inside the grass pile. Another problem is the baffle – it does not have a cap on it (notice end of PVC pipe not quite butting up against nestbox floor), and does not wobble. I’ll have to correct that!
See more information on mice and rats on the bluebird trail, hazards, and how to deter them.