To ensure full houses along a bluebird trail, place cleaned-out boxes by March 15, when adult male bluebirds return to Wisconsin and stake out their territories. Remove early season nests after the residents fledge, to encourage a second nest in the same nest box. Gaard suggests keeping a few spare houses to install in newly mowed areas, or where tree swallows take over a box after an early season bluebird nest fledges. In southern Wisconsin, the first brood of bluebird young will fledge around the end of April. The adult pair, with the assistance of the first brood, will often nest again and raise a second brood.
Younger adults nest later in the year and raise one brood.
Wrens and tree swallows compete for nest sites at this time, and these more aggressive species may evict bluebirds.