If you keep checking them, you can kick out the un invited guests as Woodies will renest if their first clutch gets destroyed. They may choose your box if it is clean. I always had a problem with flickers just going in and poking holes in the eggs before the hen went to setting. rough cut lumber works great and the ducklings do not even need a ladder to get out. I used pl 400 or liquid nails as well as screws. Make them as strong as you can!!!!
Good advice. We evicted a couple of mergansers from a box last year and the woodies used it.
