The Goldilocks Colony

Since I have started mentoring purple martin landlords in Missouri, there are two questions I'm frequently asked; 1) "what is the minimum number of cavities I should offer?"; and 2) "how large should I grow my colony?".  Too small a colony and a single catastrophic event can wipe it out.  Too large a colony and problems can go undetected until it's too late to correct them.  With migration still underway in this 2014 season, now is a good time to start addressing those questions and determine for yourself whether you'd like to stick with a smaller colony or plan for a larger colony.

Small Colonies
I've done some research on the minimum number of martins required to sustain a colony throughout weather calamities and natural attrition and here is what I've found. First, the PMCA recommends that landlords should offer at least 24 total nesting compartments and strive to build their colonies to at least 12-15 breeding pairs.
Consider the following from the PMCA site: http://www.purplemartin.org/main/toptwelve.html

"Too many martin landlords offer just one 12-unit martin house. While this is enough to start a colony, it is not enough to ensure long-term colony site survival.  Landlords should offer multiple houses and/or at least 24 total nesting compartments. They should strive to build their colonies to at least 12-15 breeding pairs. Because conventionally-designed martin houses rarely have active nests in more than 50 - 60% of their rooms (due to male porch domination), this means a landlord should be offering at least 24-30 total compartments. Don’t put ‘all of your eggs in one basket.’ If you have only one house and a summer storm blows it to the ground, smashing the eggs and killing the nestlings, you could be out of the martin business. Landlords with single houses (and thus smaller colonies) run a much higher risk of losing all their birds from a weather calamity, or from one year to the next due to natural attrition — approximately half of all adult martins die each year, and about 75% of the fledglings. Give your martins a diversity of nesting choices: try adding some gourds to your housing set-up. "

Luther & Fonda's small, but very well-managed colony.  Luther uses Troyer Vertical gourds with the Conley II entrance.  He conducts nest checks and eliminates any house sparrows checking out his housing (note the repeating house sparrow trap attached to the pole).

**Note that the article refers to the "conventionally-designed houses" when talking about the 50-60% occupancy.  One landlord I know offered a 12-cavity house for many years and was only able to get 5-6 pair to nest. Last year after adding another 12-room house they were able to get 10 pair to nest, effectively doubling their colony.  The article also states that martins are far more productive in gourds. I would submit that besides the obvious reason of more room offered in gourds, there is also less porch domination with gourds, especially when staggered, which will increase your occupancy rate.
Large Colonies
Some landlords strive to achieve the label of "super colony" for their sites.  A de facto super colony is one that offers 100 nest cavities.  I have visited a couple of these super colonies and talked with their owners.  Most are retired and management of purple martins is their sole hobby for the summer.

Photo provided by Jonathan Spangler.  Large colonies need an abundant food supply both for the adults and for the fast-growing new fledges.  ASY female with Cicada breakfast for the kids.

Consider some of the points below by Mr. Jamie Hill-founder of the PMCA - below: http://www.purplemartin.org/forumarchives/archive/q&awith.htm

21. Q: Some martin enthusiasts become so addicted to the hobby that their main goal each year is to attract more martins than the previous year until they have built up a "super colony" of over 100 pairs of martins. Do you think martin colonies can be too large?
A:  Yes. Large colonies become giant predator magnets. I know of no martin colony over 75 pairs that doesn't have hawk and owl problems. One site we know with over 300 pairs is hit repeatedly each day by aerial predators. With such high levels of predation, the breeding success of martins in such colonies is probably depressed. The level of aggressive fighting among martins is also greater in larger colonies, as is the spread of disease and parasites. Competition for food is also increased, forcing martins to fly farther from their housing to find food for their nestlings. This causes life-shortening stress on parents and increases the time between feedings for the nestlings, slowing their growth.

While Mr. Hill makes some good general points, you would need to take into account your own circumstances when growing your colony.  For example, Jerry in Troy, MO experienced frequent owl attacks in 2012 with only 38 nesting pair in residence and had to install owl guards to protect his colony in 2013.  An awareness of the resident predators in your area will help give you an idea of the challenges you will face with increasing the size of your colony.

Jerry's colony in Troy, MO offering 44 Troyer gourds with Conley II entrances in 2012.

Just Right?
I urge anyone that desires to grow their colony to go slowly.  Going slowly allows to you observe your colony, deal with challenges along the way and make adjustments quickly.  You will know when you've reached the "sweet spot" when you still have time to enjoy your martins, rather than being constantly frustrated and having to address issues.
More from Mr. Hill's interview:

22. Q:  What do you feel is the "ideal" martin colony size that can be properly managed?

A:  The answer to this question depends on how much time the landlord has to devote to the hobby. A colony should only be as big as a landlord can properly manage. During the breeding season, a nest check of 100 nesting compartments can take about 30 minutes, if you move fast (as you should). This needs to be done every four days during the egg-laying period and every 5-7 days during the nestling period. A landlord should also walk underneath his housing once or twice each day to look for evidence of problems. Trapping and shooting House Sparrows and starlings is another factor determining how many cavities should be offered. No more than what the landlord can keep 100% starling and sparrow free. And don't forget annual maintenance. Housing needs to be taken down, nests cleaned out, housing washed off, repainted (if necessary), stored for the winter, and put back up in spring. Only build your colony to the size you can manage. For some people this is one 12-unit house. For others, it's several hundred gourds or a dozen multi-room houses.

An example of a medium-sized colony.  Mark & Teresa, Brandon, SD - photo taken 7/24/2013.  24-gourd system:  first year, 2012 - 8 nesting pair, 2013-21 nesting pair.  Mark & Teresa also have another 16 gourd rack system with 15 out of the 16 Super Gourds occupied every year.

Personal Perspective:
Based on Mr. Hill's recommendation of 30 minutes to conduct a nest check of 100 compartments, I can see that I need to work on my nest check timing!  I currently offer a total of 84 cavities - 72 Troyer gourds and 12 cavities in my Trendsetter house - and it is taking me about 45 minutes.  I have found that 84 cavities is all that I want to manage at this time, along with my full-time job.  At the end of each season, when I'm hauling all my cleaning gear out and staring at all my housing I have to clean up, anticipating the inevitable week of back pain afterwards, I just can't fathom the idea of adding more housing at this time.  Of course, once I retire, all bets are off and I reserve the right to change my mind (insert loud laughter from my husband here).  Ha!

Who wouldn't love a rack full of martins?

Full management of a large colony takes a lot of time, planning, money and effort.  So, based on the above information, here are some questions to ask yourself when trying to decide the correct size colony for you:
1. How involved do you want to be with your colony? Do you want to do weekly nest checks and record their progress? If so, you will need to consider how much time that will take and weigh it against how much available time you have to devote to those activities.
2. If you're not very involved, do you have frequent predator or nest-site competitor problems that keep you busy? As Mr. Hill states, hawks are drawn to the noise made by a large colony but on the flip side of that, I have found that a larger colony such as mine usually has more eyes in the sky that can watch out for hawk attacks. Hawk attacks can also be minimized through a few other strategies mentioned in an earlier article.
If you're using SREH, then you should have minimum problems with starlings.  By the way, we just have to face it; English House Sparrows will always be a problem, but the problem is manageable with traps and frequent shooting / elimination.  As far as other predators, with the proper guards installed, you can prevent snake and raccoon attacks and also reduce the impact an owl can have on your colony (updated since this article was written - I'll be sharing some recent owl attack experiences).
3. Is there enough food around to support a larger colony at your site?  One of the reasons I've become so interested in native wildflowers and ponds was because I was concerned about there being an ample supply of bugs to support my large colony.  Increased numbers of martins require increased numbers of insects, not only for the adults, but also for the young they would raise here.  So as not to cause them to have to fly further to find that food, I've learned to attract that food to my backyard through native wildflower plantings and a large pond.

Native wildflower plantings help draw in many insects that not only provide pollination benefits, but also provide food for your martin colony.

Of course, not everyone has the land for these things. But you can certainly increase your insect populations through native wildflower plantings which will attract thousands of beneficial bugs (yes, including bees), to your area. An interesting side-note:  Purple Martins (and other insect-eating birds) will not eat the Monarch butterfly as their host milkweed plant causes them to taste bitter to all birds.  However, they will eat the numerous other butterflies, moths, wasps, bees, cicadas, butterflies and dragonflies that are drawn to the wildflowers and any water features (ponds included) that you may add to your yard.

Common Green Darner dragonfly is found near water sources.

So, for those of you that would like to keep a small colony, the minimum number of cavities offered should be at least 24. Beyond that, it is strictly up to you and what you feel comfortable managing. My plan for my martins when I retire is still up in the air. I love martins. I am extremely passionate about them. My heart wants to setup purple martin gourd racks as far as my boundaries permit, but my head knows better. So, I will probably negotiate a compromise between the two someday. Maybe after ensuring that many of the landlords I am mentoring have well-established colonies, I might choose to setup 1 or 2 more racks and after that, do further public outreach.

Photo provided by Jonathan Spangler.  ASY male providing a Common Green Darner to the kids.

To me, there is nothing better than sitting out on your porch, enjoying the raucous vocalizations and chatter of over 150+ purple martins and laughing at their antics throughout the various stages of their breeding cycle and rearing of their young.  Even the young fledglings provide me with enormous amounts of entertainment and I delight in their shenanigans.  Regardless of the stresses they face, whether being chased by hawks, awakened by an owl knocking in the middle of the night, or fighting off house sparrows and starlings, the indomitable, determined spirit of the martin persists and he surges forth with his delightful chatter against all odds with one thing in mind. To further his bloodline, against all odds.
We can increase his odds of being more successful.  It is easy to get caught up on the romantic idea of a larger colony.  But you will do the purple martin more harm than good if you become frustrated with difficult issues and neglect, or worse, decide to remove your housing.  There is nothing sadder than seeing a flock of martins return to a site whose housing has been removed.
Whether you have a small or a large, well-managed colony, make sure that you are able to manage it in such a way that keeps both you and your purple martins happy while helping them increase their numbers, despite the odds.