“ For Eastern Bluebirds, you can use a 1½ inch round hole, a 1⅜ x 2¼ inch vertical oval hole, or 1⅛ inch horizontal slot entrance. Western and Mountain Bluebirds prefer a 1 9/16 inch round opening or 1 3/16 inch slot entrance. Where the ranges of the species overlap, use the larger opening.”
Lots of instructions here:
https://www.nabluebirdsociety.org/PD...box%20Recs.pdf[/QUOTE] Thanks so much