Not usually, bluebirds normally nest closer to the ground than martins.
Generally house sparrows and house wrens. Both are cavity nesters and will compete with bluebirds for nesting spots.
Why of course!!!
No, that is not common. Robins don't use nestboxes so perhaps it's a bluebird. Bluebirds have a orange chest and a blue back.
The Eastern bluebird is common there.
Yes. They live in New York where the environment is suitable.
They eat common garden insect pests.
Yes. The Eastern bluebird is common there.
House sparrows and starlings often compete with bluebirds for places to build nests.
Both are thrushes, members of the family "Turdidae".
Bluebirds are generally colored blue.
That would be for chickadees and titmice. Too small for bluebirds.
Bluebirds do sometimes just go in half way. They may be arranging nesting materials or doing something else. Entrance holes for bluebirds should be 1.5 inches.