In the mountains of Colorado, we have two nesting species of bluebirds that commonly nest; the Mountain Bluebirds and the Western Bluebirds. We rarely get the Eastern Bluebird nesting here.
What we have found is that it is not the box size that determines what species nests within the box, but rather the location in which the nest box is placed that determines which species has the best chance of nesting within it.
For example; if the nest box is placed out in an open field, there is a good chance Mountain Bluebirds would use it. However, there is an equal chance that Violet-green Swallows may use it too.
If the box is placed near a bush there is a good chance that House Wrens will use the box.
If the box is placed near water, there is a good chance Tree Swallows will use the box.
If the box is placed within open Ponderous Pine forests, you have a good chance of Western Bluebirds nesting in it.
Mountain Bluebirds arrive in the mountains of Colorado in late February-early March and the Western Bluebirds arrive a bit later.
Bluebirds often built a nest within a nest box; then leave for a few days, to a few weeks, before returning to begin nesting. Sometimes that simply build a nest in a box and never return to nest in it.
Occasionally a single male will mate with two or more females and raise two families at the same time.
Bluebirds feed upon such things as grasshoppers, mealworms, earthworms, berries, suet and various flying insects.
Below are photographs of Mountain, Western and Eastern Bluebirds.
Below are the other birds that you may find nesting within your nest boxes. Violet Green Swallow, Tree Swallow and House Wren.