Hi Patsy:
Ugh, this is definitely one of my pet peeves: Using nails or screws to fasten a birdhouse to a tree! As I've stated several times via this forum, please DO NOT use screws or nails to fasten ANYTHING to a tree!
Please consider ratchet straps, bungie cords, rubber tie downs, or industrial plastic zip ties. GPS the location of your birdhouses so that you can occasional change-out the straps so they don't grow into the tree.