what size should the entrance hole be for a bluebird box
The most important element is getting the entrance hole size correct, especially to eliminate larger pest birds such as starlings from using the box. For a round entrance hole, eastern bluebird boxes should be 1 1/2 inches in diameter and 1 9/16 inches for mountain and western bluebirds. Eastern bluebird houses can also have a 1 3/8 x 2 1/4 inch vertical oval hole or a 1 1/8 inch horizontal sparrow-resistant slot entrance at the top front of the box.