Cavity nesters divide into excavators, such as woodpeckers, and secondary users that occupy holes already made. Bluebirds are secondary users, and the supply of holes is their binding constraint. Removal of standing dead wood, replacement of wooden fence posts with metal, and competition from introduced house sparrows and European starlings all tightened that supply through the mid-twentieth century.
The response was structural rather than legislative. Volunteers installed nest boxes at spaced intervals along routes, monitored them through the season and removed sparrow and starling nests. Because a box is a directly countable nesting attempt, the effort produced its own measurement. Continental survey data show eastern bluebird numbers increasing over recent decades, and the species is listed as Least Concern.
The simplification to avoid is treating boxes as a cure. They shift where the birds nest without replacing dead-wood habitat, and an unmonitored box in the wrong place can favour house sparrows, which kill bluebirds in cavity disputes. The bird responded to sustained management, not to hardware alone.