Good point about the nestbox entry hole size. I checked with some
North American Bluebird Society (yes Virginia, there is a society for
everything) information: "The most important dimension of a nesting
box is the entrance hole. Eastern Bluebirds can easily manage to
enter a nesting box with a 1 1/2 inch opening. Mountain Bluebirds
are a slightly larger species and require an opening of 1 9/16
inches. Great care must be taken when making the 1 9/16 opening,
since anything larger will permit the European Starling to use the
box....Not only does the Mountain Bluebird require a larger entrance,
but they also prefer a larger nesting box. They usually have larger
broods and the bigger box provides more room for the nestlings."
They give dimensions of 6 1/2 by 9 3/4 inches for the front board of
a slant-top box, with sides 5 inches deep. Most of my boxes have 1
1/2 inch openings. I get lots of Westerns, but only a few Mountain
Bluebirds. Funny thing.

Dana Visalli

"Population stabilization is for the birds."