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. As with all our nest boxes, the design includes appropriate ventilation, drainage and opens for easy clean-out. Sustainably grown 1" thick Eastern White Pine provides superior insullation. Hand crafted with pride in Maine. Repair or of any defects in manufacturing or materials upon return of any defective item to Coveside Conservation Products.