Bluebirds sing to attract mates and warn other bluebirds not to enter their territory.
Many bluebird don't migrate. As long as there is enough food available they will stay in the same area.
They lay their eggs in spring and summer.
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.
Bluebirds sometimes build more than one nest, as a diversion to predators.
They will nest in tree cavities.
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 usually, bluebirds normally nest closer to the ground than martins.
It depends on the kind of bird. Some that have long lifespans (eagles, penguins, albatrosses, large parrots) have only one or two young at a time and nest only once a year (or less often), while others that have short lifespans usually have many young at a time (quail, turkeys) and/or nest more than once a year (doves, some songbirds).
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.
You can put nest boxes out with bird feeders and hay. The birds will feel safe within a nest box.
Not in one where the bluebirds already are nesting, but wrens are cavity nesters, and will nest in unoccupied bluebird houses.