An 8" diameter gourd will provide enough room for most birds. A 1 3/8" hole is suitable for wrens, titmouses and nuthatches. A 1 5/8" hole is suitable for bluebirds, swallows and some woodpeckers. A 2" hole is suitable for purple martins and most woodpeckers. If the gourd is thick, you will need to get through the shell gradually with a less aggressive, more frequent pass. Two coats of spray polyurethane each year will help protect your gourd birdhouse from the elements.