We cut down most of a big holly tree (a clump with several large trunks) in our backyard but left one trunk so we could put a birdhouse on it...and here it is, finally. I used scraps of wood from our studio, ceramic tile mosaic for the front and a recycled roof from another old birdhouse. Left-over paint was also used and in no time birds were occupying the new place. As the house is pretty large (to visually fit the size of the trunk) I built a smaller walled off "room" inside the house so the birds would have a better nest building space. The size of the opening to the house helps determine what type of bird will be attracted to the house. Wrens and smaller birds go for holes around 1 1/4" to 1 3/8".