Last year we placed six birdhouses around our yard. Two hang from branches, three are on tree trunks, and one bluebird house is on a post in the ground. We observed bird families in the two hanging houses, in one of the three on trees, and in the bluebird box.
This year we cleaned out all of the birdhouses by mid-March and it was great to see a Bluebird claim their house two weeks later.
Detailed information about bluebirds: All About Birds (The Cornell Lab of Ornithology)
How to build a bluebird box: NestWatch (The Cornell Lab of Ornithology)