Purple Martins are a little different from the other birds that frequent people’s backyards. This is because they actually prefer to see human activity around their site. Aside from this, they also have very specific space requirements for them to find the man-made nest boxes attractive.
There are various suitable materials for housing of Purple Martins including aluminum, thick plastic, wood and natural gourds. However, the exterior of the houses should be white as it reflects heat to keep the house cool in hot temperatures. Cedar, cypress, pine or redwood is fine for wooden housing but it should be untreated and at least ¾” thick to provide the proper insulation. Plastic housing should be of thick materials which are preferably UV-resistant and does not allow the light to filter through. During periods of extreme temperature, a layer of insulation in the attic of the plastic or metal housing will provide the needed protection.
The minimum compartment size in Purple Martin houses is 6 x 6 inches although research results tend to favor deeper compartments measuring about 7 x 12 inches from front to back. This size offers greater protection from predators and the elements while keeping the nestlings comfortable. Generally, round entrance holes in Purple Martin houses measure 2 1/8” in diameter. However, a range between 1 ¾” and 2 ¼” is acceptable. Entrance holes should be placed about 1 – 1 ½” above the floor. An entrance hole that seeks to be Starling-resistant should be positioned no higher than ½” above the floor. Door plugs will prevent occupation by other birds during the non-breeding season.
Regular monitoring of houses of Purple Martin is necessary thus the house should provide access to individual compartments during the nesting season. Houses need to have hinged doors, access caps or other means of accessibility. It should also raise and lower vertically either on a telescoping pole, lanyard or winch system.
Since Purple Martin houses are placed in the open, materials and construction methods should be chosen based on their ability to withstand the environmental elements. It should also have good ventilation, drainage and insulation. An extended roof overhang protects the compartments from the rain. Ventilation holes drilled at an angle or placed under the roof overhang prevents water from funneling into the compartments while enabling air to circulate. Drain holes at the bottom of each compartment allow wet nests to dry faster. Raised sub-floors and larger compartments offer additional protection from the rain.
The recommended height of the mounting pole for Purple Martin houses should be from 12 – 20 feet. Poles of any type are easily climbed by predators such as snakes, raccoons and squirrel thus the need for pole guards. Larger and deeper compartments provides the nesting birds a way to avoid a predator’s reach as nests can be placed further from the entrance. A hardware cloth fastened to the outside of the Purple Martin house creates a protective cage that makes predator access a little more difficult although it is easily removed for nest checks and monitoring.
Trees and Purple Martins do not go together. There should be no trees taller than the houses of Purple Martin within 60 feet. The air space immediately surrounding their house should be unobstructed in at least a couple of directions so they can fly to and fro in nearly all flight levels. If the 60 foot rule cannot be followed, the house should be mounted higher relative to the trees. Tall bushes and shrubs around the bases of the pole and vines growing up the pole are undesirable environmental elements for Purple Martin houses. A Purple Martin enthusiast should be able to offer multiple houses or at least 24 total nesting compartments to ensure a long term colony site survival.