This simple PVC hole guard was invented by Don Hutchings of TX.
Instructions:
Cut a 6" long section of 4" diameter PVC pipe that the birds will enter through. (If raccoons continue to prey on nestbox contents, try a 9" length.)
Drill a 1/5" hole (1 9/16" for Mountain Bluebirds) in a PVC sewer and drain cap. Some folks also put a chickadee size hole on it, so you can alternate and use it as a hole restrictor.
Drill 3 holes in the cap for screws (to attach the cap to the box.)
Place the drilled hole over the entrance to the box (with the open end of the cap facing out, so you can connect the tube), and attach it to the box with 3 screws. (Don't glue them together as then you will need a really long screwdriver, or one with an extension, to attach and remove the guard.)
I don't have any experience with its effectiveness, or on acceptance by cavity nesters, but Hutchings found it was successful. Some bluebirds nest in mailbox tubes made of 4" PVC.
