Purple Martin House Guide: Plans, Placement, Height and Gourds

Why eastern martins nest where people build

A crescent 3 inches wide and 1 3/16 inches tall is the most important single measurement in purple martin housing. A martin fits through it; a European Starling, in most cases, does not. The Missouri Department of Conservation puts the tolerance bluntly: below 1 3/16 inches martins cannot get in, and any fraction more starts to let starlings through. Almost everything about a purple martin house comes down to details at that scale, because in the East the birds have nowhere else to go.

The Purple Martin Conservation Association (PMCA) states it directly: martins east of the Rocky Mountains rely almost exclusively on human-provided housing. All About Birds, from the Cornell Lab, describes eastern birds that once nested in woodpecker holes along forest edges and rivers and now live almost entirely near towns and cities, while western martins still use natural cavities in mountain forests and Pacific lowlands. In the Southwest, the PMCA’s landlord hints add, martins nest only in old woodpecker cavities.

The habit is old. Native Americans hung empty gourds for martins before Europeans arrived, according to All About Birds, and the Missouri Department of Conservation notes that Native Americans are generally believed to have started what biologists call a “tradition shift”, drawing the birds into gourds. Over one million North Americans put up martin housing, the PMCA says, and many of them are unable to attract breeding martins.

This is the largest swallow in North America: 7.5 inches long and about 1.9 ounces, per the Missouri Department of Conservation. Martins hunt flying insects high in the air, often above 150 feet and sometimes 500 feet or more, according to All About Birds, and spend the winter in Bolivia, Brazil and elsewhere in South America. The North American Breeding Bird Survey shows a decline of about 25 percent between 1966 and 2019.

Pacific Northwest martins are a separate case. The PMCA notes that this subspecies, Progne subis arboricola, has not developed the colonial style of the East, and recommends gourds and single-unit housing in groups of three or four on a pole rather than a multi-room house.

Purple martin houses and purple martin gourds

The choice between a house and a gourd rack matters less than what sits inside either one. The PMCA’s minimum compartment is 6 by 6 inches, but research has shown that larger compartments, 7 by 12 inches, give better protection from predators and weather and keep nestlings more comfortable. An adult martin is about 7.5 inches long, and a compartment has to hold four to six nearly grown young plus both parents. In its FAQ the PMCA asks for cavities of approximately 6 by 11 inches.

Purple martin gourds have their own size rules. The PMCA says no less than 8 inches in diameter, with 10 to 12 inches ideal. The Missouri Department of Conservation is a little stricter: it suggests plastic gourds around 10 inches across and notes that gourds at least 9 inches wide let martins nest far enough back that owls cannot easily see or reach them.

The performance gap is real. PMCA research reported by the Missouri Department of Conservation found that pairs in large gourds laid more eggs and fledged broods of up to seven young, while the classic 6 x 6 inch compartments found in older aluminum houses fledged the fewest, three to four being the norm. On that evidence the 6 x 6 room is the weakest of the widely used designs, and the department notes that many landlords now enlarge old houses to 6 by 12 inch rooms or switch to large gourds.

Materials follow a short list from the PMCA. Aluminum, thick plastic, wood and natural gourds all work, provided the outside is white, since white reflects heat. Wood must be untreated, ideally 3/4 inch thick, in cedar, cypress or redwood. Plastic must be thick and preferably UV-resistant, and no light should show through the walls; a translucent house heats up like a greenhouse, which can kill nestlings. Wood and natural gourds keep an unpainted interior. Slick floors in metal or plastic housing can leave young with permanently splayed legs, a potentially fatal condition prevented by a textured subfloor. Every cavity needs a door, cap or lid so it can be opened during the season.

Natural gourds have one advantage that plastic cannot copy exactly. Their rough inner walls let the female shape a nest without slipping, and the darker interior may help the birds feel secure, according to the Missouri Department of Conservation. The trade-off is upkeep: scrubbing, an antifungal soak and white paint. Plastic purple martin gourds skip that work and last a long time, the department notes, and it advises choosing ones with a crescent entrance and an access lid on the side. The purple martin housing page goes further into house and gourd designs.

Entrance holes: round, crescent and the 1 3/16 inch rule

A traditional round entrance is generally 2 1/8 inches across, with 1 3/4 to 2 1/4 inches acceptable, placed 1 to 1 1/2 inches above the floor or porch, according to the PMCA. That hole lets martins in. It also lets starlings in.

Starling-resistant entrance holes, which the PMCA abbreviates SREH, solve that by trading height for width. The PMCA’s housing page gives 1 3/16 inches tall by 2 3/4 to 3 inches wide, with the bottom of the entrance flush with the compartment floor or no more than 1/4 inch above it. Its older printed standards allow up to 1/2 inch above the floor, and its landlord hints say flush or 1/4 to 1/2 inch above the porch and never higher. The crescent is the most popular shape; the Excluder, Modified Excluder, Excluder II and Conley II are the other designs the PMCA names.

Cutting one by hand is a careful job. The PMCA’s instructions call for a jigsaw, cutting slightly small and then filing or sanding to the exact height, because a hair too big admits starlings and a hair too small keeps martins out. The Missouri Department of Conservation’s version starts from a 3 inch circle drawn with a compass and recommends rounding the corners so martins do not catch their wings.

Martins have to learn the shape. The PMCA notes that some take to starling-resistant entrances at once while others need hours or even days, sticking their heads in and out before committing.

Purple martin house plans: what a plan has to get right

The best-known purple martin house plans come from the PMCA itself. Its T-14 plan, a traditional Amish design according to the Missouri Department of Conservation, has 14 extra-large compartments with access doors and slide-out nest trays, and the house winches up and down. The plan comes as a 44-page booklet that also covers martin attraction. The PMCA also has blueprints for a winch-operated rack that suspends up to 16 gourds.

A warning from the PMCA’s own landlord hints applies to every other set of purple martin house plans: many published plans and some manufactured houses are built to improper dimensions. Before building or buying, check any plan against the standards above. A usable plan gives compartments of at least 6 by 6 inches and preferably 7 by 12, compartment height of 4 1/2 to 6 or 7 inches, an entrance about an inch above the floor (or a crescent at floor level), a door on every compartment, ventilation holes drilled at an upward angle or tucked under the roof overhang, drain holes in each compartment floor, and a white exterior.

Weight belongs in the plan as well. The Missouri Department of Conservation notes that wooden houses are heavy enough to be best mounted on a winch, while smaller wooden houses of six or eight compartments, under about 25 pounds, can go on a telescoping pole. Cypress and cedar last longest; exterior plywood does not last as long as solid boards.

Purple martin house placement: open sky, human company

Purple martin house placement is the major reason people fail to attract martins, the PMCA says: housing in the wrong spot, or a site that is not martin habitat at all. An open site lets the adults spot aerial danger in time to escape, the Missouri Department of Conservation explains. The PMCA’s standards ask for the most open spot available, at least 40 feet and preferably 60 feet from trees or buildings. NestWatch states the same distance more strictly: no trees or buildings within 60 feet in any direction. The Missouri Department of Conservation asks for at least two or three open flyways.

The southern half of the breeding range is more forgiving. The PMCA’s landlord hints note that southern landlords can sometimes attract martins with housing 15 to 20 feet from trees, while also saying that, generally, the farther from trees the better.

The second rule surprises newcomers. Martins prefer to nest near people. The PMCA asks for housing within 100 feet of human housing or activity, and its hints give 30 to 120 feet, on the reasoning that human presence helps protect the colony from predators. The Missouri Department of Conservation suggests the landlord’s most open location, where the birds can be watched from the house.

Wires are a point where the sources read differently. The PMCA’s hints say never to attach wires to a martin house, especially wires leading to trees, buildings or the ground. NestWatch says overhead utility wires nearby are fine as perches, as long as they do not block access to any side of the house. The two do not actually conflict: no wire touches the house, but a wire overhead is welcome.

Water is a special case. NestWatch notes that houses on poles in shallow water, for example along a lakeshore, may have fewer problems with sparrows, starlings, cats, squirrels and raccoons, provided the house sits about 2 feet above the high-water level and the spot is shallow enough to check while standing in the water. As a rough guide to purple martin house placement on an ordinary lot: the open middle of the yard, clear of tall shrubs and vines, within about 100 feet of the house. If you cannot satisfy both rules, the PMCA’s hints point toward open space: relocate the housing to a more open area, mount it higher, or prune.

How high should a purple martin house be?

Height is the second half of purple martin house placement. The sources give four answers to the question of how high should a purple martin house be, and they overlap. The PMCA’s housing page says 12 to 18 feet. Its printed standards say 12 to 20 feet, and its landlord hints 10 to 20. NestWatch lists 10 to 15 feet. The Missouri Department of Conservation says martins are attracted to housing 12 to 20 feet up and notes that most commercial pole systems stand 12 to 15 feet after installation.

Read together, 12 to 15 feet is the range every source accepts, and it is a sensible default. Higher housing is more exposed to wind and storm damage, the PMCA warns. The exception is a lot with trees near the pole, where the landlord hints suggest mounting higher as one of three fixes, alongside moving the housing or pruning.

So how high should a purple martin house be on a typical open lawn? About 12 to 15 feet, on a pole set in concrete 18 to 25 inches deep, per the PMCA. Many manufacturers offer ground sockets or stakes so the pole can be removed or moved later.

Poles, winches and telescoping systems

Housing that cannot come down is not suitable housing. The PMCA’s standards require martin housing to raise and lower vertically on a telescoping pole, a rope and pulley or a winch, because it may need lowering as often as daily to remove sparrow and starling nests and to monitor the colony. The Missouri Department of Conservation names the same three systems and advises against tilt-down poles on a hinge, because they cannot be used once martins start nesting.

Two small additions prevent a big problem. The department recommends orientation marks on the house, so the compartments go back up facing the same compass directions after each check, and a locking mechanism that stops the house spinning in the wind. Martins that find their compartments facing a different way can become disoriented and abandon their nests.

Maintenance is routine. The PMCA asks for winch, pulley and telescoping systems to be oiled regularly and for rope and cable to be checked for wear, with frayed lines replaced. A strong pole is worth the cost: the department advises buying the strongest system available and not overloading it, so wind does not bend it.

Predator guards from the pole up

Predation is the most common reason martins abandon a colony site, according to the PMCA. One trip up the pole by a snake, raccoon or squirrel, or a few visits from an owl, hawk or crow, can make every surviving bird leave.

The PMCA’s rule for climbing predators is that every active house gets a pole guard, because any pole, wood or metal, round or square, can be climbed. The top of the guard must be at least 4 feet above the ground; lower, and large raccoons and snakes can get around it. The Missouri Department of Conservation describes a simple stovepipe baffle made from a 24 inch length of 7 inch round pipe over a mesh floor on L brackets, which can be fitted without taking the house down and should wobble slightly when set correctly. The pole guard page covers guard designs for martin poles.

Aerial predators need a second layer. Conventional 6 x 6 inch rooms let owls and hawks reach the nest easily, the PMCA notes, so deeper compartments help, and every house should also carry an external owl guard. Its do-it-yourself version is a removable cage of 2 x 4 inch hardware cloth fastened outside the house. For houses and gourds alike, the PMCA’s housing page describes owl guards made of vertical rods across the front.

The PMCA’s FAQ reads the signs: feathers or whole wings below the housing, or doors ripped off, point to hawks or owls; missing eggs or young suggest a snake; blood and feathers, a raccoon.

House Sparrows, starlings and the native neighbors

House Sparrows and European Starlings are not protected by the Migratory Bird Treaty Act, and both compete hard for martin housing. The PMCA’s landlord hints describe them filling compartments, chasing off martins, fighting nesting birds, killing nestlings and breaking eggs; the Missouri Department of Conservation adds that starlings can kill adult martins with their long bills. All About Birds notes that the two species often push martins out of whole areas.

Control works at the level the sources describe. Keep entrances closed until martins arrive. Use starling-resistant entrances. Remove sparrow and starling nests, repeatedly if needed, which is where a pole that lowers easily earns its place. The PMCA lists trapping and shooting as the ways to remove the birds themselves, and NestWatch’s guidance adds that any trap must be checked hourly and its operator able to identify the species with certainty. Feeding choices nearby matter too; NestWatch’s list of sparrow-attracting seed is summarized in the bird feeder guide.

Native birds are a different matter, and harming them is illegal. Tree Swallows, Eastern Bluebirds and Great Crested Flycatchers sometimes claim empty martin rooms. The PMCA’s advice is to plug the martin entrances temporarily and put up suitable single-unit housing 30 to 50 feet away; once the natives settle in the new box, reopen the martin house. The birdhouse guide gives the entrance sizes and plans for those boxes.

A season guide: scouts, subadults and opening day

The word “scout” causes more mistakes than any other in martin keeping. Scouts are not searching for new sites on behalf of a flock. All About Birds and the PMCA both describe them as the first martins back in spring, older birds returning to colonies where they bred before. They are unlikely to switch to new housing.

The birds that colonize new sites are the subadults, last year’s young. According to the PMCA’s range map notes, these yearlings start arriving four to six weeks after the established adults and keep arriving for another four to six weeks in the north and ten to twelve in the south. Martins can therefore be attracted to new housing through mid-May in the south and mid-June in the north. Only about 15 percent of young return to nest at the site where they fledged, the PMCA’s banding work has found; the rest settle at new sites nearby.

That timing sets the calendar. Put the housing up a few weeks before martins are due, with the entrances closed. On a new site, open about four weeks after the first martins are expected in the area; opening for the scouts is four to five weeks too early and tends to hand the rooms to sparrows and starlings. Keep the housing open through June, since martins may start nesting as late as the end of June anywhere in North America. As a regional guide to dates, the Missouri Department of Conservation reports first spring arrivals in Florida in January, older males in southern Missouri in early March and subadults there from late April into May.

Established sites run on a different clock. Martins show strong site fidelity, so the same birds return year after year, and the PMCA’s hints say an active site can stay fully closed until the martins come back and land on it. At the end of the season, housing should be taken down and stored or closed up, so wasps, squirrels and other birds do not move in before spring.

One more point on replacement. The returning birds know their housing, and the PMCA’s hints warn that removing or drastically changing an active house can make them abandon the site. Put the new housing next to the old and give the colony a full season to accept it before taking the old one down.

Landlord basics: nest checks, nesting material, weather

Nest checks are part of keeping martins, not a risk to them. The PMCA says martins do not abandon a colony over nest checks and recommends lowering the housing in the afternoon, never in the early morning or late evening. Its FAQ suggests a check every 5 to 7 days; the Missouri Department of Conservation recommends weekly checks and says they can continue until the young are about 20 days old without making them fledge early. Numbered compartments and written records let a landlord project hatching and fledging dates.

For nesting material, the PMCA uses a few handfuls of pine straw, the long dried needles of white pine, in each cavity, or cedar shavings. The Missouri Department of Conservation adds that a flat base of white pine needles helps subadults, which tend to be poor nest builders. Parasites build up fast; the department notes most nests are full of mites after about ten days. The remedy both organizations describe is replacing the nest material with fresh, dry material. Pesticides are never used in martin nests. The PMCA’s hints also describe sprinkling one or two teaspoons of freshwater diatomaceous earth on the floor before the old nest is replaced with clean material.

Weather is the threat a landlord can do least about. Martins eat only flying insects, and cold, rain or wind grounds the insects. The PMCA’s FAQ says trouble starts after more than three days of cold or rain; its hints say martins begin to die of starvation after four or five days. Heat is a danger too: nestlings can die when temperatures stay above 100 degrees Fahrenheit for many days. Landlords help with supplemental food, crickets, mealworms or scrambled eggs, often tossed into the air near the housing, and the page on feeding martins in cold snaps covers that in detail. The main index lists the rest of the site.

One popular reason for keeping martins does not hold up. Martins are day feeders, the Missouri Department of Conservation points out, and contrary to a claim often made for them, they are not consumers of mosquitoes.