House Sparrows are exotic, introduced species from Europe. They have no legal protection from the USFWS in North America.
They have a strong, seed crushing beak that easily out-competes the slender/weaker insect-eating beak of other.
The best way to prevent these pests from occupying your nest boxes is to keep them away from livestock (especially cattle) or homes with feeders using millet and/or cracked corn as these locations attract large numbers.
But these pests can fly up to ½ mile to occupy a box, so considerable habitat is lost nest boxes unless another alternate is used.
And the best alternative is a Van Ert trap (VanErt Sparrow traps: 22684 US 69, Leon, Iowa 50144, 641-446-6471, fvanert@grm. net, www.vanerttraps.com (cost is about $10 + S&H) Van Ert are easy to insert and highly effective when used properly. should be set only when the sparrow is building a nest or has already constructed one.
Then, all nesting materials should be removed from the box (including eggs). Usually, the male is trapped and that is a good thing because he is the “enticer” of the female.
Trapping a female is not as effective as the male will attract another female. Rarely, both are trapped together.
Dispose of the sparrow, put a clear, plastic sack (held tight at the bottom) over the nest box. Open the door and the sparrow will fly out and can be penned against the side of the box and disposed of.
In 60-75% of the time, only one trapping is necessary to clear up the nest box for protected songbirds.
In two years of ALAS studies, almost 100% of boxes with removed sparrows have produced young from other songbirds, mainly bluebirds.
The major problem faced by trapping sparrows is how long to leave the traps in place. I use two approaches:
Set trap and return after monitoring the rest of my boxes or
Set trap and return the next day.
By that time, the sparrows are dead, but if you have trapped a swallow or bluebird, they will still be alive and can be released unharmed.
My experience is this: “Leaving traps in nest boxes rarely leads to the deaths of accidentally trapped but protected songbirds, if the trap is left no longer than 12 hours”.
See also this PDF made by a bluebird lover.