Bluebirds sometimes build more than one nest, as a diversion to predators.
They will nest in tree cavities.
Not usually, bluebirds normally nest closer to the ground than martins.
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.
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.
Not in one where the bluebirds already are nesting, but wrens are cavity nesters, and will nest in unoccupied bluebird houses.
You can put nest boxes out with bird feeders and hay. The birds will feel safe within a nest box.
Grass, twigs and moss and other materials suitable for a nest.