House Sparrows should not be allowed to use nest boxes intended for bluebirds or other native cavity nesters. A reader recently asked about a suspicious nest he found in a box he tends.
The nest described sounded like intrusion by House Sparrows, an invasive non-native species. If the nest is a mess of grass and feathers and paper and whatever, it's House Sparrows. I remove nest and eggs, if any.
What do other nests look like, so you don't trash the wrong one? Chickadees build with moss. Tree Swallows always incorporate feathers. Bluebirds make a nest nest of grass and/or pine needles, rarely other material. House Wrens use sticks and twigs exclusively.
Don't hesitate to open the box to check on the nest and possible occupants unless you suspect native birds are close to fledging. (You don't want to provoke early departure by opening a box containing young birds near fledging.) Knock first -- from the side, not in front of the opening-- to warn any adult birds in the box, then open. (I've watched people knock as they're tempted to peer into the box, as if to see exactly what is going to fly into their face. It will sharpen your reaction time.)
Birds will tolerate occasional quick looks into their nest box. Some people who provide boxes for bluebirds open the box daily to check on chick progress once the eggs hatch, not that I recommend that. The birds will not abandon the nest because of occasional, brief, discrete looks. If sparrows persist, find a new location for the box, or trap and dispose of the sparrows. It is the male you want to catch. Google "sparrow nest-box traps" for more information.
If you have nest boxes, they should be cleaned once nesting is complete. Open the box cautiously in case wasps or bumblebees are nesting there. I've found both inside boxes, and wasps also in nests attached to the outside bottom of the box. If there are wasps or bees, prop the door open if you can, and leave. Remove all nesting materials from non-occupied boxes. Wear gloves. Avoid the dust that will come from the box; do not stand downwind. I leave my boxes open over the winter, cleaning and closing in the spring, usually early April.
Below, a typical House Sparrow nest, a jammed collection of almost any material the bird could carry.