Seems like broken apart egg shells to me. We have had up to 3 sets of bluebirds in Spring and after the young birds fly away I empty the nest from the birdhouse and there are never any broken shells left in the house, so the bluebirds must be taking them out ( perhaps with some poop inside). BTW when the young birds are ready to come out the parents become very aggressive and if you approach the house they dive at you to chase you away. Imagine the guts those young birds must have when they come out of that birdhouse hole, jump and fly for the very first time.
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