This Kentucky style, slot entrance bluebird house is designed to thwart sparrows. According to research at the University of Kentucky, bluebirds prefer a slot entrance to the standard circular hole — sparrows prefer the opposite. In addition, sparrows don’t like a shallow box and bluebirds don’t seem to mind. This box is only 5″ deep but if sparrows still continue to be a problem, a wooden insert is included to reduce the depth even more. Sparrows rarely use the 3-1/2″ depth, but bluebirds will still occupy it.