The key to a birdhouse is not a manufacturer’s custom design but a simple box of the proper size.
If you put up nest boxes of the proper dimensions, you will typically get a wren or a chickadee; a flicker or a red-bellied woodpecker; a screech owl; and perhaps a titmouse, for a usual maximum of four species a back yard.
But bird authorities urgently warn that if a box larger than one for a chickadee or a wren is put up, it must be opened periodically to make sure it houses the birds you invited.
European immigrants, house sparrows and starlings, will get in if they can, kill the birds inside and build their own nests.
Contrary to myth, the invited tenants won’t mind a quick peek.
To create a demilitarized zone in this war, try building a nesting box that is attractive only to the smaller birds.
It should have a floor 4 inches by 4 inches, inside dimensions; walls 8 to 10 inches high; and a hole 1 inch in diameter, 6 to 8 inches above the floor.
It should be placed about 4 feet off the ground for easy observation, and should have holes near the top of the walls, to let hot air escape, and in the bottom, to let rain out.