Here are four bluebird boxes in perfect bluebird habitat - open, mowed grassland. Every one was being used by House Sparrows (HOSP). One was a slot box, the others were standard NABS boxes.
Although some people find that slot boxes (shown in the photo to the left) are HOSP-resistant, this HOSP clearly didn't know that was the case. Shallower slot boxes may be less preferred by HOSP, but also offer easier access to predators. Another problem with wooden slot boxes is warping roofs, which can change the size of the entrance. See pros and cons of various nestbox styles.
In reality, any nestbox suitable for bluebirds can be entered by a HOSP. Even if they do not prefer a certain box style, if nothing else is available, HOSP may still use it. See HOSP Management - Nestbox Style.
This HOSP nest demonstrates the use of seed heads, which are generally not present in bluebird nests. It also shows typical HOSP construction with nesting material curving up the sides of the box. See descriptions of nests and photo album. Also see HOSP photos.
I am going to contact the trail owner to see if they will let me help out. See template of a letter that can be sent to owners of neglected nestboxes that are breeding HOSP.
HOSP nests are typically bulky, and contain feathers and trash.
It's always depressing to see blue feathers incorporated into a HOSP nest. HOSP will even build their nests on top of their victims' corpse. See descriptions and photos (warning - graphic) of HOSP attacks.
Notice trash (cellophane) and overall bulkiness of nest.
As late as 8/11/06, one of these boxes still had an active House Sparrow nest in it with four eggs. This is one reason why HOSP are such effective invaders - their nesting season starts earlier and ends later than most other cavity nesters. They may have 2-4 broods per season.
In addition, the HOSP nest has a tunnel entrance generally found in nests made out in the open air. HOSP nests in nestboxes often lack the tunnel entrance.
You cannot begin to preserve any species of animal unless you preserve the habitat in which it dwells. Disturb or destroy that habitat and you will exterminate the species as surely as if you had shot it. So conservation means that you have to preserve forest and grassland, river and lake, even the sea itself. This is vital not only for the preservation of animal life generally, but for the future existence of man himself—a point that seems to escape many people.
-Gerald Durrell, The Nature Conservancy
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Neglected Nestboxes
other cavity nesters. They may have 
