Well, that was interesting, thank you. I learned that the bluebird is a member of the thrush family and yet it likes nesting in birdhouses – something UK native thrush family members would never do. We have the blue-tit (and maybe you do too?), which looks very similar to your bluebird but is about half the size (that US/UK size difference again!) and likes to nest in birdhouses.

We have squirrels that raid the bird feed we put out in the garden (ie: yard), including the fat-balls and peanuts that the blue-tits like. But there's never more than three of them at a time and it would take more than a few squirrels to kill off our oak trees. Any road up, they have plenty of bird feed to eat!