Establishing a Managed Colony of Western Purple Martins

From: Stan Kostka Seattle
Date: 10/15/03

Establishing a Managed Colony of Western Purple Martins   

Western Purple Martins (Progne subis arboricola) are the largest North American swallows. They are Neotropical migrants (winter in South America, breed in North America), obligate aerial insectivores (feed almost exclusively on flying insects caught in flight), and obligate secondary cavity nesters (they do not excavate places to nest like woodpeckers, they nest in whatever they can find, including old woodpecker holes, but will use a wide variety of natural and manmade nestsites, those specifically intended for their use and otherwise).  Martins will accept many different types of artificial cavities.  It is important that they are large enough to accommodate nestlings and parents, as well as offer protection from predators .

 Western martins are known to use sites lacking elements usually associated with their nesting habitat.  The only vital element is a cluster of suitable cavities.  You are certainly free to try your own ideas, and selectively use the information given here that appeals to you.  Good Luck.

 Exactly how and where you would do this is up to you.   Across North America martins use a wide variety of natural and artificial cavities,  nestboxes of various designs made of wood, metal, and plastic, and hollow gourds both natural and plastic. Martins also nest on or in various human structures that were not intended for their use, bridges, buildings, streetlights, fire sirens, metal utility poles, etc.   They use boxes that are low to the ground, those intended for wood ducks and bluebirds, but martins also nest very high in bridges, utility poles, and in snags.  They nest below grade (underground) in crevices in the ceilings of collapsed lava tubes in Northern California.  Martins nest on human structures on small ledges  that really are not what you would consider a "cavity".   They nest directly over saltwater and freshwater, and they (Progne subis hesperia) nest in the desert in cacti  far from open water.  Martins nest in very close proximity to humans in busy marinas at sea level and  also at high elevations in remote mountain regions.  They nest in large groups and as solitary pairs. This species has shown itself to be extremely adaptable when it comes to choosing nestsites, and this fact is part of the mystery as to exactly why they declined in  the West.   Based on the behavior these birds display, it seems reasonable to assume they will  use nestboxes throughout their breeding range, and their breeding range seems to be determined primarily by the availability of nesting cavities.

 Human supplied nestboxes (and gourds) have resulted in a significant recovery of some western martin populations.  In recent decades in the US Pacific Northwest and western British Columbia, Canada, Purple Martins have shown a preference for nestboxes on or near inland marine waters (at or near sea level), although coastal, freshwater, and water free upland sites exist .

 Martins have used nestboxes in California on three occasions to date that I am aware of:  "…a specialized nest box placed upon a snag on Palomar Mtn in 1985, in a Wood Duck nest box at Essex Pond northeast of Arcata in 1985-86, in a nestbox at Loyalton… “ (Williams 1998).    The thing to remember is that martins seem to nest in all "habitats" across a wide range of elevations.  

 In New Mexico Purple Martins have been reported nesting in bird houses in two locations, a nestbox near Cloudcroft in the Sacramento Mountains, and near Luna, Catron County, in west-central NM near the AZ line, where there are (were?)some old swallow houses on poles in a wet meadow/marshy area in 1975, 1992, and 1995.  In 1995, 5 martins were counted  there (SO Williams  pers comm.)

 The limiting factor in their distribution seems to be the availability  of suitable cavities.

 Since the early days of providing nestboxes for Purple Martins much has been learned.   A 6 by 6 by 6 martin nestbox should be a thing of the past everywhere.  Martins lay more eggs and fledge more young in larger cavities.  Nest cams have shown that nestlings spend much time flapping their wings inside the nesting cavity before they fledge and therefore benefit from more space. Horizontally deep cavities offer protection from both aerial and climbing predators.  So, large horizontally deep compartments seem to be in order, and when necessary,  starling resistant entrance holes.

 Here is a design for a Purple Martin nestbox, which has been used successfully in the West, constructed of 3/4 inch thick  western red cedar , exterior plywood, or any other suitable wood.  Because of its insulating properties, wood is the best material for artificial  nesting  cavities (in  my opinion).

 Minimum inside dimension 6 inches wide, 10 to12 inches inside from front to back, 6 to7 inches high at entrance, 6 to 8 inches high at back wall.  Floor extends 3 inches out beyond front wall for a porch.  Roof extends 3 inches beyond front wall to cover porch.  Roof extends 1 inch beyond sides (and back depending on how the box is mounted) for added protection from the weather.  A sheet metal cap covering a wooden roof will greatly extend the life of the nestbox, especially when nestboxes remain outdoors year round. Some of my wood roof only boxes that were put up as recently as 1999 are beginning to look fairly ragged.     Some of Tom Lund’s nestboxes are still producing martins in Oregon after 25 years without maintenance because he put a sheet metal roof over them.  Drill a one half inch drain hole in each corner of the floor and three or more half inch ventilation holes at the top of each side wall.    Cut a rectangular starling resistant entrance hole (SREH) no less than 1 and 3/16 inches high   and no more than 1 and 1/4   inches high, 2 3/4 inches wide, flush to porch (the opening should look like a low and wide door, not a low and wide window) centered in front wall, or offset to either side (offsetting the door to either the left or right side of the front wall    makes it easier to cut out, and makes better use of available porch and floor space).   Top and sides of entrance hole must be sanded smooth.  For added starling resistance, take the small piece cutout from entry, or any small piece of 3/4 inch thick wood, and attach  it to the exterior of the front wall flush to the top of the entrance, thus thickening the entrance making it tunnel like and more starling resistant (David Fouts, pers comm).  The bottom edge of the attached piece must also be sanded smooth.  Double check the height of the entrance to make sure it is at least 1 3/16 inch high but no higher than 1 1/4 inch.  SREH will deter most starlings from nesting, and these openings will protect nesting martins from starling aggression.  Starlings may enter otherwise unprotected martin nesting cavities and kill adult martins.    Roughen the floor both inside and out to give martins a better grip when negotiating the entrance, as well as to help prevent leg splay among developing nestlings. This type of low entrance, not only makes the nestbox starling resistant, the extra interior space front to back makes the boxes more protective against gull, crow, heron, owl, and raccoon predation.   Build the box so that the sides extend down beyond the bottom of the floor a little to keep the floor dry.  The front wall will set on the floor / porch.  Also be sure to build the box and mount it so the interior is accessible for monitoring and cleaning.  Northwest martins for the most part do not have to deal with overheating in their nestboxes.  However, if you are planning for an area with high daytime summer temps (California, etc.), accommodations must be made to prevent the nesting cavity from overheating.  An overhanging double wood roof with an open air space between the two roof layers that shades the sunny sides should work.  A layer of 1 1/2 inch foam building insulation has been known to work well in hot Eastern climates.  Paint the roof white to reflect heat.  Painting the entire nestbox white is not necessary. Nest access from the front  is the most practical for managing nestlings during nest checks and banding.   Mount the front wall with screws, a latch, or nails into pre-drilled holes,  so it can be removed and reinstalled easily .   Access from either side or rear is not a good idea since martins almost always build their nests as far from the entrance hole as they can,  against the back wall, or in the back corner,  and opening the back or side can cause eggs or young to fall out.   An inspection mirror and flashlight may be needed to see down into the nestcup when accessing from the front.  When checking a nestbox that has older  nestlings,  plug the entrance when finished to prevent premature fledging,  and leave it plugged for a few minutes after the check.   Never reach into a nestbox or touch young that are 20 days or older.

 Supply a thin layer of nesting material in the rear of the box before martins arrive in the spring, more than a trace but not too much.  Often martins don't build much of a nest compared to some other cavity nesting birds, and martins seem to prefer a cavity with an existing nest.   The material you place in the box will reduce the amount of time martins spend on the ground gathering nesting material, a time when they are extremely vulnerable to predation.   Thick dry grass, straw, or hay cut into lengths that will not entangle nestlings or parents, or dry pine needles work great, or dry wood shavings.  NO sawdust. NO grass or needles that have been sprayed. NO wood shavings from treated lumber.  An elevated cache of additional nesting material in a wire basket or some other elevated container within the colony site will benefit nest-building martins.  Nesting material that does not compact, such as straw or dry pine needles, promotes airflow through the nest, and may prevent proliferation of ectoparasites.

 Another type of SREH is known as the crescent.   The crescent SREH is the top

1 3/16 to 1 1/4 inches of a 3-inch circle.   This opening is also placed flush to the floor. The top edges of the crescent opening must also be sanded smooth.

 Starling resistant entrance holes (SREH)  have the benefit of dissuading starlings from using martin nestboxes, and protecting martins from starling aggression, but SREH can be a tight squeeze for some martins.  If you have no starling problem at your site, or if you are able to intensively manage your nestboxes, and control (trap and or shoot) starlings regularly,  then entrance holes for Purple Martin nestboxes can be 2 1/4 inch round holes placed above the floor, and no porch is necessary.

 Purple Martins  generally require an open flyway around their nesting sites.  Ideally five to ten cavities  should be installed in a loose cluster  in an open area, several meters apart each,  on or near water if available, at least ten feet above ground, or well above ( five foot min)  the highest tide that will occur during the nesting season, away from trees, with  the entrances facing an open area, away from  prevailing weather if possible , allowing for viewing of all the entrances from one location  if possible, and  accessible  for nest monitoring  and  maintenance.  Although most Western nestbox martins nest in clusters of single nestboxes and gourds , they will also nest next to each other in duplex,  triplex ,  etc. setups, so several  cavities may be attached to the same pole or piling.  However, your chances of attracting more western martins sooner may be  better if there is more space between the individual cavities.  If you already have an eastern style apartment house installed that you want to save, retrofit  to enlarge the cavities and install Starling resistant entrance holes if necessary.    Supplement the surrounding area with additional single cavities, creating a loose cluster.

 Regarding site selection:  With some exceptions , a traditional Eastern style backyard Purple Martin experience is not the norm west of the Rocky Mountains.  Most  nestbox colonies in British Columbia, Washington, and Oregon are located away from the homes of the individuals or groups  responsible for them.   Many are on public lands, in wildlife refuges, and on other federal, state, county or city properties.   Many are in private marinas and at other private waterfront locations.  Purple Martins will not be bothered by nearby human activity.  In fact, quite the opposite is  true.  Human activity tends to reduce the presence of some predators,  and many martin colonies are located within busy marinas and other urban areas. 

When you locate a good site,  inquire as to the possibility of installing a cluster of Purple Martin nestboxes.  

 Initially a cluster of Purple Martin nestboxes may attract Tree Swallows   and Violet Green Swallows .  Usually only one or two  pair of each at most  because they are not  known to nest as  colonially as Purple Martins . Tree Swallows  and Violet Green Swallows  nest earlier than Purple Martins,  and you can restrict  them to the periphery of the cluster by leaving only those cavities open early in the season before martins arrive.  The presence of Tree and Violet Green Swallows may stimulate colonization of a site by martins, as long as there is a surplus of suitable cavities about 15 meters distant from the Tree and Violet Green Swallow nesting cavities. This distance is a guideline and  these species have been known to nest much closer, and in fact directly adjacent to each other in some instances.  Tree and Violet Green Swallows will tolerate the same human activity around their nest sites and internal nestbox monitoring as Purple Martins.  The presence of these smaller swallows at your colony site will give you many opportunities for observing swallow behavior during the intervening period until Purple Martins arrive.

 Regarding the installation of artificial nesting cavities for the benefit of native species of birds, it is important to understand the need to monitor the nestboxes or gourds and to some extent, if necessary, control  non-native species.  Installation of artificial nesting cavities without some subsequent control of European Starlings (Sturnus vulgaris) and House Sparrows (Passer domesticus) can be harmful to native primary and secondary cavity nesting birds, and should be discouraged. If starlings or house sparrows nest in your nestboxes, remove their nests and eggs. Multiple nest removals may be required. If necessary, nestbox traps that look like ordinary nestboxes can be used to control House Sparrows that will be undeterred by starling resistant entrances.  These traps can be operated selectively, using active and static settings,  so that only targeted species are captured and destroyed. Place nestbox traps closer to buildings than the martin nestboxes.   Properly operated traps will not harm other birds.  In 2000 and 2001 four pairs of purple martins nested and fledged young from traps at two sites in Snohomish County, Washington State. Starlings and house sparrows can also be deterred from using nestboxes by keeping the nestboxes closed until martins begin arriving in the spring. In my experience, House sparrows are more of a threat to martins than Starlings, because sparrows fill  nestboxes with nesting material to the point where martins cannot later use the box.   Also, House sparrows are multiple brooded, and will continue to nest throughout the summer, when martins are breeding.  House sparrows will destroy martin eggs, and build nests on top of and kill martin young.   Insert traps can be used to control  House sparrows.  Do not remove House sparrow nests near active martin nests unless you have eliminated the adult House sparrows.  Otherwise they will wander into nearby martin nests and destroy eggs and young.   House sparrows can be controlled in the vicinity of a martin colony year round with the use of nestbox and bait traps.

 Ideally, no Starlings or House sparrows should nest within an active martin colony. However, your ability to monitor and control  Starlings and House Sparrows should not be the ultimate deciding factor  in your decision to put up some nestboxes for martins.  A nestbox colony that supports several pairs of martins, along with a pair of Starlings and House Sparrows is better than no nestboxes that support no martins.  Martins are known to use starling nests after starling young have fledged in the same season.  And starling activity at an otherwise unoccupied  nestbox installation  may in fact be an attractant to cavity seeking martins.  Do your best to trap and kill Starlings and House sparrows, but don’t let an inability to get every one dissuade you  from establishing and maintaining a Purple Martin colony.

 Purple Martins are colonial  nesters, and the most attractive  thing to a martin is the sight and sounds of other Purple Martins. Therefore, martins can be enticed to investigate new breeding sites through the use of techniques of social attraction,  primarily by broadcasting martin dawnsong recordings on an outdoor speaker in the predawn hours starting at about 4 am until 6 am, and daytime chatter  recordings intermittently throughout the day and into the evening.  Martin decoys can also help. At nestsites that already have martins, males fly high above colony sites in the predawn hours and sing to attract additional martins to the site.  Purple Martin dawnsong recordings  and decoys are available from Eastern retailers.

 Once martins have been attracted to your site,  consider the  installation  of  climbing predator guards on poles if installed upland, or on pilings that are out of the water for a significant duration of time during low tide, or are accessible from floating docks.  A raccoon that discovers a Purple Martin colony can cause serious reproductive failure and the entire colony could be lost. Raccoons are common and widespread.  You most likely will not see them.  They are nocturnal.   In 2001 due to low water levels,  raccoon predation at Fern Ridge Reservoir in Oregon caused reduction in nesting pairs by over 50%,  and  many adult martins were killed and eaten by climbing raccoons.   In 2001 at English Boom on Camano Island,  Washington state,  raccoon tracks were discovered around the bottom of several pilings at dawn, and  some previously  banded nestlings were missing.  That day  those pilings were wrapped with 42 inch high brown sheet metal  starting a few feet up on the piling. The next morning at dawn  the bottom of all the sheet metal had muddy scratch marks.  That raccoon probably would have returned until all or most of the nestlings and some of the roosting adults in those nestboxes were eaten. The smooth sheet metal surface prevented the raccoon from climbing the pilings.   A  five foot section of smooth pvc plastic pipe slipped over an upland pole during installation should prevent raccoons from climbing.   Similarly,  squirrels , chipmunks and rats will destroy eggs and kill cavity nesting birds.   Prevent any mammals  from gaining access to your nesting cavities.

 Purple Martins often perch near their cavity entrances.  Installation of some kind of perches  (three eighth  inch wooden dowel works well)  will make the site more attractive to martins as well as assist in the identification of banded individuals.  Overhead utility wires can make a site more attractive  to nestsite seeking martins.

 In the Pacific Northwest, Purple Martins are known to nest miles away from the saltwater in residential areas not immediately adjacent to fresh water but these sites are infrequent.  Nestboxes along  marine waters and nearby freshwater lakes, rivers, and wetlands have  experienced the greatest success in recent decades. However,  it is important to remember that access to clusters of suitable cavities  is the primary determinant as to whether martins exist and persist in any area. Martins are known to nest well away from water throughout their range.


Gulls, crows, and herons are known to perch on or near martin nestboxes and prey on martins as they fledge and return to the boxes.  Any addition to the nestbox or its installation to prevent this is encouraged. 2 inch by 4 inch wire fencing can be wrapped over the top of nestboxes in an arch extending well out over the entrance hole. The wire discourages large birds from perching on the roof and snatching nestlings and returning fledglings. 

 Owls are known to hover in front of nestboxes, or hold on with one leg and beat their wings against the sides, and grab fleeing adult martins.  Owls will also reach into nestboxes and extract adults and young.  Any addition to the nestbox or its installation to prevent this is encouraged. 2 inch by 4 inch wire fencing can be wrapped out around the front of a nestbox to discourage owls.   Most people are unaware that owls have very long legs, since our view of owls is usually when they are perched with their legs tucked up under them.    Do not  harm owls or other native avian martin predators.  Rather, adjust the hardware at your managed site to discourage and defeat these predator’s  attempts.

 Do not hesitate to monitor the insides of your martin nestboxes when multiple pairs of martins are nesting.   When done appropriately, these checks  generally will not harm or bother the martins.  On the contrary, only if you know what is in there will you know if you develop a predation problem. However, solitary nesting pairs may be more sensitive to you accessing their nesting cavity, and  accessing the nesting cavity of a solitary pair before they are feeding young could possibly cause site abandonment.  So if you have only one pair nesting,  you may want to be cautious early in the season. Early in the season, and while martins are still on eggs, try to check  nests in the middle of the day when martins are usually off feeding.

 Contrary to popular opinion, it is NOT necessary to clean  out nestboxes following each season.   In fact, martins tend to prefer cavities with  old nests.    Only if there is an unusually heavy parasite load in a nest should it be removed.  Nests that have heavy parasite loads can be replaced during the nesting season.  Nests should not get wet.  If you notice wet nests,  make some adjustments to the nestbox to correct this problem.  Extend the roof perhaps.  Do not make major changes in entrance orientation  during the breeding season.

 In much of their Western range,  Purple Martins continue to nest as they have for millennia,  in natural cavities,  mostly in trees.  In  some western states, all known Purple Martins nest in  “wild” and therefore unmanaged colonies. The unique behavioral ecology of all remaining  wild western martins,  in snags or live tree cavities,  in cacti, in  lava tube caves, in steel and concrete hollow box bridges,  and elsewhere,  needs  to  be PRESERVED, PROTECTED, AND ENHANCED.   Nestbox installations may not be appropriate at some sites where martins have already established themselves without direct human intervention, where  sufficient and sustainable  cavities exist that are not threatened.  However, martins are most likely to nest nearby other martins,  therefore, if you observe martins using snags or pilings that are in advanced stages of decay and deterioration , or are threatened with removal,  and new snags are not being created in that locality,  installation  of nestboxes to maintain  and potentially increase the local martin population is appropriate.

 The intent is NOT to manage all Western martins into nestboxes indefinitely, and expand the current  Eastern conservation model throughout all of North America.

Rather, nestboxes west of the Rocky Mountains should be considered  mitigation and conservation tools, whereby threatened populations can be protected and enhanced, and martins enabled to return to localities  where they had previously nested.

 Establishment of managed western martin colonies  ideally should take place in areas where martins have historically nested,  but have experienced population reduction,  range contraction,  or extirpation,  due to loss of natural nest sites  and or nest site competition from European Starlings and House Sparrows. 

 Virtually all published literature discussing populations of western Purple Martins has focused on nest cavity availability as the primary factor affecting distribution and abundance (Williams).   Human activity (logging, agriculture, and land development) has eliminated historic nest sites, and has altered the landscape so that it supports introduced exotics (Starlings and House sparrows) that out-compete Purple Martins for most natural nest sites.  The best long term strategy to enhance recovery of Western Purple Martin populations is not to manage all martins into nestboxes indefinitely, but rather to manage the landscape to reduce exotics, and create natural martin nest sites by preserving  large emergent snags across a forested landscape, especially in areas where starlings and house sparrows are not present (Horvath).  However, now and into the foreseeable future, until land use practices change and large snags can be replaced, martins most certainly will benefit from the  installation of suitable clusters of nestboxes and or gourds, to increase abundance and expand distribution into new colonies,  both wild and managed.  By managing a nestbox colony for martins, you can  play an important role in martin conservation that goes far beyond the boundaries of your particular  site. 

 Presently less than 2% of known purple martins in Washington State are nesting in snags.   If you observe snag nesting martins anywhere,  or see  martins nesting anywhere other than in nestboxes or gourds specifically intended for them,  or hear reports second hand,  please contact  me .  Thank you.

 These recommendations are  based on known sites.  Any information based on additional  and or contradictory observation should be documented.  Do not hesitate to contact me  with additional questions, comments, and observations.

 Again, please do not  feel overwhelmed by all this information.  If you are inclined to put up some nestboxes for Purple Martins please do so, even if the site you are considering is not ideal.  Western martins are known to use sites lacking  elements usually associated with their nesting habitat.  The only vital element  is a cluster of suitable cavities.  You are certainly free to  try your own ideas, and selectively use the information  given here that appeals to you. And please let me know if you are successful  in establishing a managed colony of Western Purple Martins. 

Good Luck.   

Stan Kostka                     

360 435 7236  home  

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28603 Kunde Road

Arlington, Washington,  98223