We used to have 19 bluebird houses spread out over about 500' of paths in our place in Shoreham.  Never had much luck attracting bluebirds, although they were definitely in the neighborhood.  The problem wasn't house sparrows.  
    The bluebird houses were occupied each year by a few house wrens and a lot of tree swallows.  We tried placing the houses in close pairs which, according to urban legend, discourages nesting by the territorial tree swallows.  Didn't work worth squat    The tree swallows also occupied our expensive purple martin condominium (eight plastic gourds at the top of a pole near the pond), and, most amusingly, our big wood duck house which was on a pole in the pond.  Now there was luxury tree swallow housing!
    It's a good thing that tree swallows are great birds.
        Larry and Mona