Martins traditionally used tree cavities for nesting, but now houses supplied by people are their primary form of residence. Purple Martins are named for the purplish sheen on their bodies. Expect Martins to return from their Brazil wintering grounds around March 15th. Providing housing for Martins is certainly more of a challenge than for other birds. Many varieties and considerations need to be considered to see if it is right for you.
Location, location, location
Martins actually prefer being close to humans, perhaps because they know it will help them stay safe from predators. The Martin house should be at most 100 feet from your house, but not any closer than 30 feet. They also like the house to be in an open area with the fewest trees in your yard. Air space at the house should be void of trees in at least a couple of directions for 40-60 feet, and the entire open area should be no less than 80 x 80. Although not required, they seem to prefer housing with a proximity to water. Placing plastic purple martin decoys on the house will also get their attention.
The two main varieties of homes are multi-room and gourd systems. They can be combined on the same pole system. The best material for multi-room units is aluminum, because aluminum stays cooler in the summer. Wooden houses are better insulated and will retain more heat than aluminum houses. They should also be almost white to reflect the sun and stay cool inside. An advantage to hanging gourd nests is that they swing in the wind, which does not bother Martins but will discourage Starlings and House Sparrows from using them. Telescoping poles are nice since they can be easily raised and lowered for maintenance.
Once you raise your housing unit on the pole, you can leave it alone until you take it down for fall cleaning. However, if you engage in more active maintenance you can increase the nesting success and the number of Martins. For example, checking nests regularly can help to discover problems like unwanted nesters, predators and blowfly infestations. Accurate record keeping can help your success over time. After lowering and inspecting the house, it is VERY IMPORTANT that the house be returned to the exact height and orientation. To facilitate this, use a permanent marker to mark your pole so you can return it to the same vertical and horizontal position. The house should never be tilted after nesting begins as it may damage the nest and eggs. The best times to inspect are mid afternoon, but avoid doing it during cold, rainy or windy weather. Keep in mind that not all of the eggs will hatch and not all of the nestlings will fledge. This is normal. Placing platforms around the house can aid in nesting success. You can also put crushed eggshells and mealworms on the platform. Eggshells will facilitate calcium and egg production, and the mealworms are for eating. Perching spots around the house also make it more desirable.
Enjoy your adventures with Martins and if you have additional questions, do not hesitate to ask.