Most of what is said here has either been proven or is the results of years of raising martins. If most of these rules are followed, then there is a good chance that you'll be able to attract a colony of purple martins.
Remember, some of the things said here might be a little different for your local so you'll have to use some judgment in some cases.
If you're just starting out and are putting up your housing, think about this. Purple Martins are not like most other birds that frequents your back yard. They actually like to see human activity around their site. With that in mind, start by looking for a place to put the housing within about 30 - 60 feet of your house, if possible. They actually prefer to be close to humans so you don't want to put them too far away. You want them close enough so you can not only see them, but enjoy them as well. It's been proven that martins will settle in a site faster if it's within close proximity of human housing and activity. It has to do with the belief that human activity will keep predators away from the site.
The correct placement of martin housing is a real art. Besides preferring their housing to be placed within a certain distance of human housing, martins also have very specific space requirements. The air space immediately surrounding their housing at the height of the housing should be unobstructed in at least a couple of directions so they can fly to and from their housing in nearly level flight. The standard rule is, there should be no trees taller than the martin house within 40 feet. In fact, the farther the martin house is placed from trees the better. If your yard violates this 40 foot rule, try mounting the housing higher relative to the trees, moving the martin house to a more open area.
The 40 foot number is commonly given by most other sites and experts. However; I have my own formula for house placement and this has served me well over the years in helping others attract purple martins.
The house should be no closer to a tree than the tree is tall. So if it's a 60' tall tree, then 60' is the magic number to place the housing from that tree.
Remember this one thing.
Trees and purple martins do not go together.
The single biggest reason people fail to attract martins is that they place their martin housing incorrectly within their yards, or their yards are inappropriate martin habitat to begin with.
NO, I'm sorry, I may love purple martins and want people to get them, but I DO NOT advocate cutting down large trees just to get them to come.
Martins will settle in housing that is placed between 12' and 20' high, however; the higher it is, the more susceptible it is to wind. They have been known to nest higher, but for the average site, anything between the two numbers mentioned will work just fine.
Definition of a predator: Any animal, bird or reptile that will take either the eggs, young or mature birds from a nest/house.
Any site that is not predator proof will have difficulty in attracting martins. If they do not feel safe, they won't stay. Regardless of the type of housing you have and the type of pole it is mounted on, make sure it has a good working predator guard. One raid by a predator and the site will be abandoned and the birds will not return. If the birds have not settled in yet, they will look the place over very intently, and if they do not feel safe, then they will simply leave. Predator guards are cheap, easy to make and are well worth the money and effort to install.
There are a number of other species of birds that will readily take over the site and the nesting cavities if given the chance. If ANY other species is allowed to settle into a martin house before martins have established themselves, that site will rarely attract nesting martins. This is because birds set up territories around their nest sites and defend then against other birds. These nest site competitors, (usually starlings and sparrows), will aggressively repel any martins that might come searching for nesting sites.
Martins are easily repelled from entire houses at un-established sites by aggressive actions of nest site competitors. Why? Because if a martin has never nested at a particular site before, it hasn't developed a site tenacity there. Without site tenacity , a martin is easily repelled. In contrast, once a martin has nested successfully at a site it rarely will be intimidated from reclaiming that site the following year.
To Be Painted White
Although martins have been known to nest in houses and gourds painted other colors, white seems to attract them best.
First of all, housing painted white reflects the heat of the sun best, so martins choosing white housing lose fewer nestlings to heat stress.
Secondly, white highlights the darkness of the entrance holes best, making the cavities more conspicuous to searching martins.
Thirdly, over the years, martins have become accustomed to seeking out white houses to nest in.
And finally, white is believed to best enhance the male martin's courtship display.
Because of all these advantages, natural selection (and/or behavioral imprinting) seems to have favored the choice of white housing by martins.
Timing is Everything
Don't open your house too early.
"wanna-be" martin landlords rush to get their martin housing opened up so as not to miss the arrival of martin
scouts in their particular area. However; this is much too early for
un-established breeding sites! Contrary to popular folklore, "scouts " are not looking for new breeding sites to lead their flocks back to. "Scouts" are nothing more than the very first martins to arrive or pass through a given area on their way back to their previous nesting sites.
The arrival of migrating martins at a location is a continual process spanning 10 - 12 weeks in the northern half of their breeding range and even longer in the southern half, with new arrivals coming daily the oldest martins arriving first and the youngest last. Although it does occasionally happen, older martins rarely can be attracted to breed at new locations. This is because martins have tremendous fidelity to the exact site where they bred the previous year. It is usually only sub adult martins (i.e. last years fledglings) that can be attracted to breed at un-established sites, because they've never bred anywhere before and have no site fidelity. Sub adult martins begin returning to any given area about 4 -5 weeks after "scouts".
that's not to say that an adult ASY martin won't start a new colony, because it
happens all the time. For whatever reason, martins are displaced from
their original site and they set out looking for another place to nest and it
just might be yours'. But the majority of the time, it isn't the ASY's,
but the SY's that settle a new site.
Generally, opening a martin house too early just results in instant occupancy by nest site competitors, a situation that often prevents martin colonization at un-established sites. Generally speaking, prospective martin landlords should not open their housing until about four weeks after the first martins are scheduled to return to their area! The only exception to this rule is if a landlord has neighbors within about a mile that have established colonies. In this case open the housing about the same time as your neighbor's birds begin arriving in good numbers. There is a slight chance you might lure some of their adult birds away if your site in far superior to theirs.
However; be prepared to defend the empty (and very inviting) house from nesting competitors, because it will happen.
Vines and Shrubs Under the Pole
Unoccupied martin housing that has tall bushes and shrubs around the bases of the pole, or vines growing up the pole, will rarely if ever, attract breeding martins. Martins tend to avoid such housing as it is much more accessible to ground based predators, such as cats, raccoons, snakes and squirrels. The solution is simple, remove the offending vines, bushes, or shrubs. And while you're at it, get and use a ground based predator guard.
Housing Really Not Built to Specifications
Please note: The following is because of recent findings/studies on nesting purple martins. Within the last 30 years, many studies have been done and many new finding have resulted from those studies, thus; the reasons given below..
Many of the published plans for martin housing and a few of the commercially manufactured houses are made to improper dimensions. Even some plans published in major encyclopedias, popular bird books, or by state and federal wildlife agencies are incorrect. (They're old and outdated). If you consult ten different sources you'll often find ten different recommendations. Part of the problem is that no one has ever scientifically tested the martins exact nesting requirements and preferences...until now.
The Purple Martin Conservation Association (Edinborough, PA) conducted some studies (as well as other martin agencies) and it was found that: A martin house having a compartment size of 6" X 6" is just too small for an 8" long bird, let alone all the young that will be in it. It was found that compartments measuring 7" X 12" are far superior.
Different entrance hole styles were also tested. Round entrance holes should be placed about 1" above the floor and have a diameter in the range of 2", although martins are known to use holes as small as 1 3/4". If your martin house does not have the dimensions stated, then modify it.
Also, a number of various SREH (Starling Resistant Entrance Holes) were also studied and the results showed that martins will readily use these SREH entrances, the crescent version being the most popular. If European Starlings are an issue where you live, definitely give these some considerations. If you plan to use these, the bottoms of these style entrances should be no higher than 1/2" above the inside floor of the compartment. Also it was discovered that inside and outside porches should be used. It makes it much easier for the short legged martins to enter while at the same time, making it harder on the long legged starlings to enter. (Just for info, ALL my housing has these Crescent styled entrances and they work).
Housing Attached To Wires
Martins love to perch on wires, but they tend to avoid houses that are attached to wires or are placed within leaping distance of them. They instinctively know that squirrels can crawl along these and gain access to the house. Never attach wires to a martin house, especially if they lead to trees, buildings or to the ground.
Most people rush into the hobby not realizing that to properly manage martins, they'll need housing that allows easy raising and lowering, and nest compartment access. Landlords need to vertically lower their housing often, (sometimes daily), to evict nest-site competitors and, (about once a week) to check on martin nestlings. Housing mounted on stationary poles or poles that tilt down are no longer practical due to the introduction and proliferation to the House Sparrow and European Starling. These type of poles should be phased out by those who currently use them. Martin housing should be mounted on poles that telescope up and down (light houses) or raise and lower with a pulley system, (heavier housing and gourd systems). If you have such a system don't be afraid to lower your houses often to check on you martins. You'll actually raise more martins if you know exactly what's going on. Such disturbances will not cause martins to abandon their nests or their colony site. Number the compartments and keep written records, it really does help.
Play the Dawnsong
The 'dawnsong' is a tape of the song sung by male martins in the early morning attempting to attract other martins to the site he has found. It can be purchased from a number of different places. Contact the Purple Martin Conservation Association and ask for their catalog. Their Email is PMCA@edinboro.edu. Once you get the catalog, you can order the tape. It can be played for 1/2 hour per side. I suggest copying a portion of it on to an endless tape so when you play it, you don't have to go and flip the tape over all the time. You can let it run for a couple of hours during the morning hours. It has full instructions as to when and how to play it.
Here's a tip. Birds need nutrients to help harden their eggshells. You can help by presenting them with crushed eggshells. These can be acquired in a number of different ways. One, you can save the shells from the eggs you have for breakfast each morning. Now this can take some time, so here's another couple of ideas of how to acquire some faster.
One, ask your friends to save you their eggshells and add them to your 'stash'.. You'll be surprised as how fast your 'stash' grows.
Two, go down to your local diner and ask them to save you the shells from their eggs for one morning. Just bring them a clean 5 gallon bucket and ask them to throw the eggshells in it for you. Then, you pick it up at a pre-determined time. Don't leave them with a bucket full of eggshells on their hands (the food rangers will get them). Now all you have to do is get them home, put them in an old pillow case, tie the top securely, throw it in the washer on the rinse cycle, (do not use soap), then throw it into the dryer until dry. When done, you have a bag full of crushed eggshells for your birds. Then you can stick a post in the ground with a flat surfaced tray attached to the top of it and put some of the eggshells on it. Just beware, once found, they will be mobbed, and that includes every bird in the neighborhood. If you spend some time standing near the post, some of the other wild birds won't come around. Give it a shot. You'll be amazed at what happens once the martins find this offering.
IF...you're outside and see or hear a purple martin in your area, BE around the housing. You don't have to be right under it, just close enough so that they can see you and come and investigate. They instinctively know the site is inhabited by humans and believe it or not, if they're looking for a place to set up housing, they'll investigate it much closer. I don't know how to tell folks this enough...purple martins like human activity. YES, at first when you approach the site, they will fly away, after all, they are wild birds. But once they realize that you are part of the site and don't mean them any harm, they will soon begin to completely ignore you. PLEASE trust me on this one. Your being around the site WILL NOT scare your martins away. I try to drive this point home at every site I try to get established.
So there you have it. A few tips that, if followed, will enhance your chances of getting martins. I'm not going to say that they will work every time, but for the most part, following these tips really does help.