A cat will occasionally leap up to boxes if they are located too near ground level. If cats are a problem, locate the box to 6 feet from the ground. Also, attach a bell to the cat's collar to warn the birds of its presence. Keep cats indoors when fledglings are about to leave the nest. With raccoon populations at high levels it is important to provide protection to nesting bluebirds.
Raccoons often ravage nesting boxes.
A greased pipe appears to deter raccoons from climbing to the box. If you are not able to use a greased pipe mount, a nesting box with a roof overhanging the entry hole by 5 inches should help. On standard boxes, the raccoon can sit on the box and reach in to attack the nest. The longer overhang appears to retard this activity. After fledglings hatch, check every 3 to 4 days for fly larvae.