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 to
nest 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!
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Mystery Nest Maker 
