The nest boxes need to have sufficient depth below the entrance hole to keep these birds from reaching eggs or chicks by sticking their heads into the nest box. Cleaning out old nests after the young fledge will also help keep the next nest that is built low enough to protect eggs and young. There are two other avian predators that are small enough to enter a bluebird nest box. A house wren may peck holes in the eggs of other birds that are nesting in its territory. The best defense against house wrens is to place the nest box in an open area far away from brush and shrubs.
House wrens are native birds that are protected by law.
Their nests and eggs should not be disturbed. House sparrows may peck holes in the eggs of other birds, kill nestlings by pecking their skulls or eyes and even kill adult birds if they find them in the nest box. The best defense against house sparrows is to place the nest box in an open area away from barns and other buildings where house sparrows are present. This non-native bird is so common that it is difficult to avoid them. You next line of defense is to remove house sparrow nests and eggs wherever you encounter them.