So what can we do to help? Well, if you live in a conjested suburban neighborhood then your options are going to be limited. It may be that putting out bluebird boxes simply isn't prudent because there are too many problems. This can be discouraging, but there is no way around the facts. There is no point in putting out bluebird boxes if all you are going to produce are more and more house sparrows. That won't be good for anybody.
If you live further out in the country, however, there may be more you can do to discourage house sparrows. First, you should be sure to put out you nest boxes in open areas away from trees and shrubs. This will discourage house sparrows and wrens. Secondly, you should only put out nest boxes that can be opened. Finally, you should open these boxes frequently and check to see what's going on in them.
Bluebirds build neat nests of delicate grasses that cover the bottom of a nest box. Tree swallows make more elaborate nests that almost always include feathers of some sort. House wrens build nests out of twigs and these nests tend to completely fill the nest box. In contrast, house sparrows build big, sloppy-looking nests out of coarse grasses (below left) . If you find such a nest then you should remove it from the box.
If you are unsure what kind of nest you are looking at you'll want to look at the eggs. Bluebirds lay blue eggs, tree swallows lay white eggs, and house sparrows lay white eggs with gray speckles. (below center). If you have a twig nest with small brown eggs that are spotted with darker brown speckles (below right), then you have a house wren (a nice bird to have around!). Be sure to faithfully remove any house sparrow nests whenever they are found!