Two basic types of man-made nesting structures are attractive to purple martins. Hollowed-out gourds are the simplest, least costly method of providing nest sites for purple martins, but require periodic replacement. Lightly sand any black mold off the outside of the gourd. Remove all pith and seeds. Use light-colored paint to reduce heat accumulation inside the cavity.
Martin houses are usually the most secure man-made nest sites.
In addition, they require considerably less space than gourds do. Many different designs of nest structures are used by mar tins, but some general specifications apply to all. More than one entrance, placed preferably on different sides of the birdhouse, should be provided for each compartment. Floor space should measure at least 6 inches by 6 inches, with a minimum ceiling height of 4 inches. Wide porches, at least as wide as entrance holes, should be provided.