Bluebirds are Up to Something, Somewhere

It looks very much like our Eastern Bluebirds are well into a brood cycle.  I haven't seen as much activity from the bluebirds and what I do is of a male only.  This makes sense: we're in the right time and others are also working on a first brood.  My only question is where the heck the nest is of the resident bluebirds?  It is not on this property and I don't see nearby candidates that strike me as good bets.  Obviously they ignored my post box for the early part of spring.  But one never knows if the later part of spring will find that site more attractive.  We should get results of this brood cycle soon.