Making a gourd martin house is inexpensive, but It takes several months of preparation to grow and cure the gourds. Bottle gourds should be at least 8 inches in diameter and 10 to 12 inches tall. Use gourds with 1/2-inch thick walls for good insulation and less likelihood of cracking.
Clean bottle gourds with soap and water, dry them, and apply rubbing alcohol to the surface. Store the gourds in a warm dry place, removing any that begin to rot. Gourds require several months to cure, at which point they weigh very little and the seeds inside rattle.
To create a bird house, determine the location of the entrance by holding the gourd by its stem. Mark a center point that faces straight out in the outermost part of the curved area. Cut a 2 1/2-inch opening using an expansion bit or key hole saw. Drill several drainage holes in the bottom and two holes directly across from each other near the top from which to attach a hanger. Paint the gourd with white exterior enamel paint to reflect heat, or apply a layer of varnish or shellac to increase its strength. Hang the gourd 10 to 15 feet from the ground, with the entrance hole facing an open area and away from prevailing winds. When purple martins migrate in late August or early September, take down the gourd house, clean it, and store it until early spring in a location inaccessible to rodents.