Based on the head shape, and paws as well as what appeared to be a mask, we deduced this is a coon-bird (i.e. baby racoon) who took up residence in our bluebird house. When the weather was especially warm, he'd stick his head and paws out, going back in the box once he cooled off.
A friend with some experience in fostering orphaned or injured critters came over to effect a rescue. The "coon-bird" - took off running and turned out to be a squirrel-bird instead. He then kindly posed for us:
this link for info about these fellas.