Bluebirds usually nest in tree cavities, but these birds don't excavate holes. In fact, they nest in abandoned holes of woodpeckers or use human-made nest boxes. Now the question is what you can do when other birds start using your bluebird nestboxes.
You may notice that house sparrows, aggressive competitors for bluebirds, nest in your bluebird nestbox. In this case, you can destroy their nests because they're not native birds and therefore not protected by any state or federal laws.
If you see European starlings or other birds nesting in your bluebird nestbox, you can remove them without approval. They’re under the Migratory Bird Treaty Act of 1918. To get rid of them, you have to call avian professionals and let them do their jobs.