I am only working with I read and what Myron has been offering, but trees are a no-no. Any critter that can get thru a 3" x 4" hole will have breakfast. One of my above posts pointed out mink as a popular offendor. I think you will hard pressed to provide enough predator protection on a tree to provide safe haven. I won't do it. I cannot encourage nesting to provide a neighborhood buffet.
