Birds have a number of enemies common in the suburbs and exurbs. Cats, squirrels, mice, and even other birds can be a hazard to bird babies. Cats can usually be deterred by some kind of sheet-metal guard around a bird house pole. Squirrels and mice not only threaten nestlings, but they also can harm the house by chewing on the wood during the off-season. There is no effective way to keep squirrels away, but metal shields or baffles mounted under the house on its pole usually deter mice.
House sparrows, English sparrows and starlings are real problems for the smaller songbirds such as wrens, chickadees, and titmice. One way to discourage predator birds is to be sure the house has a hole that is no bigger than 1 1/4 inches. One inch is even better. Do not provide a perch near the entrance hole. Consider installing a block of wood around the entrance hole to discourage predators. Glue the block of wood onto the front piece of the house before drilling the entrance hole with a hole-saw.