Western purple martins use natural cavities in trees and cactus for housing. Eastern ones depend almost entirely on man-made houses. Both types live in colonies, condo style, so a group of hollow gourds hung as a mobile or a multiple family dwelling of apartments under one roof attracts them. Mature male "scouts" arrive ahead to claim the previous year's housing somewhere between late February and April, depending on the region. Colonies return to the same home every year.
Place their house in the sun and in the center of an open area 40 feet from trees; within 100 feet of your house (they like to be around people). Martins want a pond or lake within 3/4 of a mile. Locate the apartment 15 to 20 feet off the ground, paint the building white so it is easily seen from the air. It is best if the homeowner has the ability to temporarily block entrance holes to discourage sparrows and starlings and the apartment is easily raised and lowered for cleaning and ejecting other birds. The compartments should measure 7 by 12 inches, with the entrance holes 2 to 2 1/4 inches in diameter and 1 inch above the floor.
Reasons Housing Fails
Martins residents not automatic. May be slow to arrive, even though conditions are ideal. They are attached to their colonies elsewhere. It may take years and years for tenants. In addition, conditions may not be ideal:
Yard too small. House needs to be at least 30 feet from human housing and 100 feet is much better.
Lack of airspace. Most common mistake is trees compete with flight space around house. No trees taller than the their home within 40-60 feet so they can fly into the apartment with level flight. Raise the house on pole, cut down trees or give up.
House location too remote from people. Beyond 100 feet from humans, the birds lose their sense of protection against their predators (snakes, raccoons, opossums, hawks, crows, owls and others).
Other species enter the apartment first. Other species are permitted to be established in new housing before martins can claim it will develop site tenacity there. Plug holes in martin house and make single unit nesting boxes available to other birds. Once the martins settle, open the martin house doors.
Houses open too early. Competing birds move in. Wait until about 4 weeks after the early martins, the "scouts" are due in the area to open new houses. Do not wait too long, birds need to see dark openings to be tempted to move in. Once used, returning martins will hang around until you see them and then open up the house.
Proximity of house to vines, shrubs, wires. These allow predators, so martins will avoid houses where these situations occur.