Native to North America, purple martins have come under intense pressure by proliferating introduced bird species such as the English house sparrow. Male sparrows are extremely territorial. They have been known to sneak into the nests of purple martins and destroy their eggs or kill the young. Using it's strong beak, a sparrow will even kill an adult martin unable to escape its house. A colony of purple martins will gradually disappear if sparrows nest near or in their houses.
Monitor nests. Every week take the time to lower the pole and inspect the purple martin houses. Remove any and all sparrow nests. Sparrow nests are distinct from martins in that they are made of straw and grass with a center cavity. Martins’ nests are made of leaves and mud and lay flat.
Trap sparrows. Many companies make sparrow traps that resemble common nesting sites. These traps do not kill the bird so that you can check to make sure a sparrow has been trapped and not a native species.
Kill sparrows. Unfortunately, sparrows have become a pest species in the United States and threaten countless native species. The Audubon Naturalist Society of the Central Atlantic States recommends killing sparrows.
Board up the birdhouse until sparrows leave. This is a simple method to deter sparrows from using your purple martin houses. If they start building nests before the martins arrive or continually try to usurp the martin house, board up the entrances for a few days until the sparrows leave to search for new nesting sites.