It's very competitive. Predatory, too, since the smallest, sweetest bird doesn't hesitate to chase or destroy competing species. It's why the birdhouses are widely scattered and precisely designed not only to welcome one species and repel another. He's added features like a faceplate over the frontpiece of the box. Also holed in the exact diameter as the bird house entrance, it makes sure a bigger bird can't reach far enough in to peck at eggs or hatchlings.
Snakes are the prime predators, though.
They're able to slip up a post, pluck out an egg and not disturb a blade of grass in the nest. The reptilian threat was frustrating, until an idea came to add a 4-inch plastic tube over the post holding up the birdhouse. It's too thick for the rat snakes and hognose snakes of the area to negotiate. Many birds, tree swallows and wrens tend to leave the area in winter. But the forest holds others.