Often, a commercial birdhouse is decorative object that has been designed with garden aesthetics in mind, rather than functional and safe use for birds.
Those with larger holes put cavity-nesting birds at risk of predators (like rodents, squirrels, and larger birds) who can easily enter.
Bob Mulvihill, ornithologist at The National Aviary, notes you even have to take into account the specific birds you want to attract when choosing a birdhouse:
‘Avoid perches for bluebird boxes. Perches give wrens and sparrows easier access, and not only do these small birds compete for bluebird boxes, they will attack a bluebird, her eggs, and her young,’ he explains.