Snakes, mice, rats, chipmunks and squirrels will all be drawn to decorative birdhouses.  You may also notice an increased number of insects.  Depending on where you live, there may be other types of wild animals that will suddenly develop an interest in your yard.  Some of them can be quite large.  If you have small or young children in your home, you may want to make certain that the presence of decorative birdhouses will not draw wild predators that can harm them.