They will nest in tree cavities.
According to audubonathome.org: "Bluebirds sleep in nest cavities or on sheltered branches of trees and shrubs"
Bluebirds sometimes build more than one nest, as a diversion to predators.
Not usually, bluebirds normally nest closer to the ground than martins.
In a tree
In a tree
Bluebirds spend much of their time foraging for food. When they have babies in the nest, they spend much of their time caring for them.
They can easily fit out the entrance hole.
From March to June. In the south, they nest earlier, and may raise two broods.
No. They make a new one for each brood but if you take out the old nest after the young have left the nest they may make a new in that birdhouse.
Yes, then clean the box.
Sometimes bluebirds face competition with other birds over nesting sites. Another problem bluebirds have is a lack of natural habitat. In areas that are more developed there are less natural cavities for them to nest in.
They give many bluebirds a place to live at the same time. The population can increase because of the amount of bluebirds that can nest in that area.