Last year we were ready to hook up to go camping and I heard chirping coming from the pin box of our fiver. After looking into the void I discovered a nest with several baby birds, they had feathers and looked like they were about to leave the nest. I reached in and all of the birds came tumbling out and flew/fluttered away. We hooked up and drove 110 miles to a campground in Pa. and when we were unhooking at the campground we heard a chirping sound and one more baby bird then flew out.
When we returned I cut some hardware cloth with small holes and formed it to the opening and it remains there to this day, we also had to cut a piece of foam pipe insulation to stuff in the small gap at the front top of the pin box. Wrens are exploratory and will get into everything they can.