Over one million North Americans put up housing for Purple Martins. Unfortunately, many of these folks are unable to attract breeding martins. The advice given here will increase your chances of attracting martins. Once martins nest at your location, they will come back every year if you manage the site properly. Landlords who lose their entire colony from one year to the next often suspect their 'flock' died in a storm during migration or was poisoned by pesticides on their wintering grounds. These scenarios are unlikely; the martins that share a breeding site do not migrate or overwinter as a group. The reason for total colony loss is most often the result of something that happened in the landlord's own back yard during the nesting season. Good management practices can prevent or minimize most of these problems.
Purple Martins (Progne subis) are the largest member of the swallow family in North America, measuring 7 1/2 inches (19 cm) long and weighing 1.9 ounces (55 grams). Taxonomically they are placed in the Kingdom: Animalia; Phylum: Chordata; Subphylum: Vertebrata; Class: Aves; Order: Passeriformes; and Family: Hirundinidae. Three races (subspecies) are recognized: Progne subis subis breeding in eastern North America and eastern Mexico; Progne subis hesperia breeding in the deserts of Arizona, western Mexico, and Baja California; and Progne subis arboricola breeding along the Pacific coast of the United States and Canada, and in the Rocky Mountains.
Purple Martins spend the non-breeding season in Brazil then migrate to North America to nest. East of the Rockies they are totally dependent on human-supplied housing. West of the Rockies and in the deserts they largely nest in their ancestral ways, in abandoned woodpecker nest cavities. In the Pacific northwest, Martins are beginning to use gourds and clusters of single-unit boxes for nesting.
The pair-bond of the Purple Martin is monogamous. The male and female cooperate equally in building the nest out of mud, grass and twigs. The female lays two to seven pure-white eggs at a rate of one egg per day. The female incubates the clutch for approximately fifteen days, then the young hatch. The parents both feed the young continuously for a period of 26-32 days until the young fledge. The young continue to be dependent on their parents for food and training for an additional one to two weeks after fledging. It's not uncommon for the fledglings to return to their human-supplied housing at night to sleep during this period. (Click to see an animation of the growth of a nestling purple martin.)
Martins, like all swallows, are aerial insectivores. They eat only flying insects, which they catch in flight. Their diet is diverse, including dragonflies, damselflies, flies, midges, mayflies, stinkbugs, leafhoppers, Japanese beetles, June bugs, butterflies, moths, grasshoppers, cicadas, bees, wasps, flying ants, and ballooning spiders. Martins are not, however, prodigious consumers of mosquitoes as is so often claimed by companies that manufacture martin housing. An intensive 7-year diet study conducted at PMCA headquarters in Edinboro, PA, failed to find a single mosquito among the 500 diet samples collected from parent martins bringing beakfuls of insects to their young. The samples were collected from martins during all hours of the day, all season long, and in numerous habitats, including mosquito-infested ones. Purple Martins and freshwater mosquitoes rarely ever cross paths. Martins are daytime feeders, and feed high in the sky; mosquitoes, on the other hand, stay low in damp places during daylight hours, or only come out at night. Since Purple Martins feed only on flying insects, they are extremely vulnerable to starvation during extended periods of cool and/or rainy weather.
The major reason people fail to attract martins is that they place their martin housing incorrectly, or their site is inappropriate martin habitat to begin with. Martins have very specific aerial space requirements. Housing should be placed in the center of the most open spot available, about 30-120 feet from human housing. There should be no trees taller than the martin housing within 40 feet, preferably 60 feet. Generally, the farther the housing is placed from trees, the better. See site Diagram A. In the southern half of their breeding range, martins are less particular about house placement. Southern landlords can sometimes place housing within 15-20 feet of trees and still attract martins. Height of the housing can be anywhere from 10-20 feet. Keep tall bushes, shrubs and vines away from the pole. Do not attach wires to a martin house, especially if they lead to trees, buildings, or to the ground. If your yard has too many trees near the martin housing, relocate the housing to a more open area, mount the housing higher, or prune (or remove) trees to create a more open site. If you have a wooded lot, but live near a body of water, refer to site Diagram B. Boat docks make ideal locations for mounting a martin house or gourd rack.
At active sites, the first martins usually return within a week or two of the previous years’ arrival dates. (See adult migration-timing map). Landlords should have housing ready, but keep it closed until some martins are back. Adult martins can sometimes be attracted to new sites, especially if their nesting attempt failed the previous year, or if the new site offers superior housing or location.
At uncolonized sites, you can open housing when the “scouts” are due in hopes of attracting adult martins, but be prepared to keep your site free of House Sparrows and starlings (see tips on next page) through both adult and subadult arrival periods. Be ready to supply single boxes or gourds for any native nest-site competitors (Eastern Bluebirds, Tree Swallows & Great Crested Flycatchers) that try to claim martin housing.
Don’t be discouraged if you are unsuccessful at attracting adults; keep trying while subadults are arriving. Subadult martins (last year’s young) will colonize new sites or join existing sites; they begin arriving 4 weeks after the first adults in the northern third of the breeding range (see migration map), 6 weeks after adults in the center third, or 8 weeks after adults in the southern third of the breeding range. Martin migration is a drawn-out affair, and martins can begin nesting up through the end of June, range-wide. Keep your housing ready; don’t close it up or let other birds use it.
Houses and gourds should be painted white, or a light pastel color; trim can be any color. White housing seems to attract martins best. White housing reflects the heat of the sun, keeping nestlings cooler. Compartment floor dimensions should measure at least 6"x6," but 7" x 12" offers better protection against predators and weather, if starlings are controlled. Compartment height can be from 5" to 7" high. Place entrance holes about 1" above the floor. Hole size can range from 1-3/4" up to 2-1/4", but 2-1/8" is recommended. Many published plans for martin housing (and some manufactured houses) are made to improper dimensions. If your housing is unsuccessful, check the dimensions and modify where needed.
Look for housing designed to raise and lower vertically, with easy access to compartments. Landlords may need to lower housing daily to evict nest-site competitors, or to check on martin nestlings. Systems that telescope up and down, or raise and lower with a pulley and winch, are the most practical. Nest checks will not cause martins to abandon their nests or their colony site. Number the compartments and keep written records.
Replacing Active Housing
The same martins return each year and may abandon the site if the housing they are used to is gone, or drastically altered. To safely replace a single active house, place the new housing near the housing you plan to remove, and give the martins an entire season to get used to it. Do not remove the active housing until some of the martins have accepted and bred in the new housing for at least one season. Once martins have nested in the new housing, you can remove the old house, or put an additional new house in its place. Landlords with several active houses can replace a house between seasons without risk of colony loss.
The most common reason martins abandon their colony site is because predators have raided their nests. It only takes one foray up a martin pole by a snake, raccoon, or squirrel, or a few visits by an owl, hawk or crow, to cause all the surviving birds to abandon the site. Landlords who don't conduct weekly nest checks may never know martins, nestlings, or eggs are disappearing. All martin poles (wooden or metal) can easily be climbed by predators and should be equipped with pole-guards. Martin houses that have become regular targets for hawks, owls, or crows should be equipped with owl guards. Landlords should be alert for evidence of predation (e.g. dropped owl feathers, plucked martin feathers, chewed-off martin wings, etc.) under martin housing.
Since martins feed solely on flying insects, they are extremely vulnerable to weather conditions that affect insect availability. Prolonged bad weather, such as rain, snow, cool temperatures, and/or heavy winds, all reduce or eliminate insect flight. If poor weather persists for more than 2 or 3 days, martins begin to die of starvation. Heat waves and droughts can also be a problem. When air temperatures go above 100º F. for many days, nestlings can perish from overheating. Prolonged drought can also adversely affect insect numbers. Some weather conditions may contribute to a population explosion of external parasites normally found in martin nests, including fleas, nest mites, and blowfly larvae. Never use pesticides in nests or boxes. The safest way to reduce the number of nest parasites is to conduct a "nest replacement." First, remove the nestlings to a temporary container. Throw out the old parasite-infested nest. Then replace the old nest material with clean, dry wood shavings, pine straw (dried pine needles), or dry straw. Shape a shallow bowl in the new material and place the nestlings back in the nest.
Range & Migration Map
The purple background on the map below shows the winter (non-breeding) range of the Purple Martin in South America and the breeding range in North America. Colored sections mark average arrival dates of older martins at established colony sites. For example, there is a 95% likelihood that a martin spotted in the light blue section was reported arriving around around February 1st. Yearling martins (subadults), the age-group that typically colonize new breeding sites, don't begin arriving until 4-6 weeks for the northern third of the continent, 6-8 weeks for the middle third of the continent, and 8-10 weeks in the southern third of the continent after these dates, and continue arriving for an additional 4-6 weeks in the north, 10-12 weeks in the south. This means martins can be attracted to new housing through late-May in the south, late-June in the north. West of the Rocky Mountains, Purple Martins have different nesting habits. In the southwest, martins nest only in old woodpecker cavities excavated in giant cacti. In the Pacific Northwest, martins use gourds and single-unit boxes, but not multiple-room houses.
This arrival information does not take into account two-year-old (adult) birds that didn't breed as subadults, OR, birds of any age that had reproductive failure the previous year. Both of these categories of adult-plumaged returning birds will (or might be) looking for new breeding sites, AND will be arriving before subadult birds. Birds that lost their housing because it was removed or destroyed between nesting seasons will also be looking for new sites. Sometimes landlords with optimal habitat may be able to "steal" martins from housing that is in marginal habitat or that is neglected and overrun with starlings and House Sparrows.
Learning to be both patient (in waiting to attract martins) and persistent (in controlling undesirable birds) during the lengthy time window that runs from the return of adults, most of which are not looking for new breeding sites, through the period when prospecting subadults would begin arriving, is a skill that not all prospective landlords have mastered. As a result of this lapse in efforts, their martin housing can end up full of aggressive nest-site competitors that drive off timid subadult martins. Most housing today lowers easily and opens up for trapping and control of starlings and House Sparrows. In addition, there are now several kinds of starling-resistant entrance holes widely available, and numerous kinds of traps for sparrows and starlings.
The PMCA recommends to open your housing up around the dates adult martins are first scheduled to begin arriving in your area, BUT ONLY IF you are willing to follow through with the practices listed here: Use starling-resistant entrance holes. Be relentless in controlling House Sparrows and starlings. We also recommend use of the vocalization recording.
Be prepared ahead of time to deal with native nest-site competitors, too, since Eastern Bluebirds, Tree Swallows, Great Crested Flycatchers and House Wrens may show an interest in your martin houses and gourds. Have boxes and gourds up early for these desirable birds, and if necessary, briefly close martin housing to help "steer" these birds into the appropriate nesting places in your yard.