Notice anything odd about this bird house?
Some squirrel–not any of us!–has decided he’d rather renovate a birdhouse by gnawing at the small hole opening to enlarge it. Probably a young one, who would rather chew than construct his own leaf nest. True, a good old fashioned hollow tree, or a appropriately sized squirrel nest box, keep the cold and wind out better.
Perhaps you have one lying around?