Attracting Purple Martins
Martins are colonial nesters and are entirely willing to use birdhouses. Expert flyers and voracious incect eaters.
To attract Purple Martins to a birdhouse, the house should be placed in an open space, at least 40 to 60 feet from any trees taller than it is, preferably within a few hundred feet of human habitation. The birds need a wide space to approach the house from at least two sides, and they seem to know that people will protect them from their natural predators. The activity of people does not seem to bother them at all.
The house should be mounted 10 to 20 feet off the ground, preferably on a telescoping pole so that it can be lowered for easy cleaning and for checking on the young in the nests. A predator guard on the pole is also highly desirable. The house should not be moved, once martins have taken up residence in it, as they will reject it the following year if it is in a new location.
If any other species of bird nests in the martin house before the martins arrive, the house will not attract martins. This may involve frequent, even daily, lowering of the house to evict unwanted tenants, or plugging the entrance holes until the martins arrive.
The houses which are most successful in attracting martins are of a light color. This helps to keep the house cool in the summer's heat and highlights the dark entrance holes. Ventilation holes also keep the birds from perishing on hot days.
The birdhouses should not be opened up until about four weeks after the first martins are scheduled to arrive in the area. The first to arrive, commonly called "scouts," are not really scouts, but are merely the oldest martins. Older martins cannot be attracted to new locations, because they have high loyalty to the exact home where they bred in the past. Usually only the previous year's fledglings can be attracted to unestablished sites, and they begin returning to an area about 4 to 5 weeks after the "scouts."
Bushes and shrubs growing beneath the martin house should be removed, as this attracts predators such as cats, and martins will avoid the house. Similarly, they will avoid houses which can be reached by squirrels, even by a wire.