You can get metal plates that are exactly the right size for a bluebird entry hole. You add them to the outside of the box over the hole. No squirrel can chew through that. It can take years for bluebirds to catch on to a new house. Sometimes they find one they like and get evicted by a mob of tree swallows. But if you have enough houses, eventually the tree swallows will allow one bluebird family to nest in their least favorite house. Bluebirds like short grass (grass that is mowed) rather than tall grass (like in a hayfield). And they like to hunt from fences. I finally have one house (out of nine) that gets bluebirds every year. It's a joy to see that flash of blue.
You can get metal plates that are exactly the right size for a bluebird entry hole. You add them to the outside of the box over the hole. No squirrel can chew through that. It can take years for bluebirds to catch on to a new house. Sometimes they find one they like and get evicted by a mob of tree swallows. But if you have enough houses, eventually the tree swallows will allow one bluebird family to nest in their least favorite house. Bluebirds like short grass (grass that is mowed) rather than tall grass (like in a hayfield). And they like to hunt from fences. I finally have one house (out of nine) that gets bluebirds every year. It's a joy to see that flash of blue.