Decorative birdhouses should have their entry holes blocked to prevent birds from using them. Use them as garden decor, not for nesting birds. Unless a unit is properly designed for a specific bird species, a bird may use it for nesting but put themselves in danger. If entry holes are too small, their young cannot exit at fledging time. If entry holes are too large, predators get in. If there is a perch by the entry hole, predators have yet another way to attack the brood.