We were given a talk on bluebirds just last month. She had a bluebird house with a "cage" mounted on the front. It was a piece of chicken wire with about a half inch or so mesh. A piece of that about 6 in. by 26 in. was folded into a square box with the sides being 6 in. and 7 in. There is no top or bottom to this box. The wire box was mounted onto the front of the house so that the bluebird had to fly through it in order to get into the house. She said the birds had no trouble doing this, but coons could not reach in to get the eggs. I'm sure that would keep cats out, too, and maybe even snakes.