Purple Martins (Progne subis) are the largest of the swallow family who have been known to even be seen on radar as large groups take flight. Martins east of the Rocky Mountains solely dependent on human-provided housing. Martins started using gourds originally provided by Native Americans as housing dating back to the colonial times. They have shown an affinity to nesting in natural gourds, man-made gourds, and aluminum houses.
Martins will return to the same breeding site year after year so if you have lost your Martins there is a good reason why. Make sure you control House Sparrows and European Starling populations in and around your colony. Like many birds, the Purple Martin does have a relatively short time you may enjoy their song each year so preparation is paramount in having a successful season.