Establishing a Managed Colony of Western Purple Martins
From: Stan Kostka, Seattle, WA
Establishing a Managed Colony of Western Purple Martins updated January 2002
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 cavities, those specifically intended for their use and otherwise). In the Pacific Northwest purple martins show a preference for nesting on or near inland marine waters (at or near sea level) , although coastal, freshwater, and water-free upland sites exist. Human supplied nestboxes and gourds have resulted in a significant recovery of their populations in recent decades. Many different types of artificial human supplied cavities will be accepted by martins. It is important that they are large enough to easily accommodate all nestlings and parents, as well as offer protection from predators.
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 your site is not ideal. Western Martins are known to use sites lacking all secondary elements usually associated with their nesting habitat. The 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.
Before undertaking the installation of artificial nesting cavities for the benefit of native species of birds, it is important to understand the need to properly monitor the nestboxes and control exotic avian reproduction. Installation of artificial nesting cavities without subsequent control of Starlings (Sturnus vulgaris) and House Sparrows (Passer domesticus) is harmful to native primary and secondary cavity nesting birds, and should be discouraged. If necessary, nestbox traps that look like ordinary nestboxes to the uneducated observer, can be used to control House Sparrows that will be undeterred by starling resistant entrances. These traps can be operated selectively so that only targeted species are captured. Properly operated traps will not harm other birds. In 2000 and 2001 four pairs of martins nested and fledged young from static traps at two sites in Snohomish County, Washington State.
Since the early days of providing nestboxes for purple martins much has been learned. A 6 by 6 by 6 nestbox with a 2 and 1/4 inch round hole should be a thing of the past everywhere. Research has shown that martins lay more eggs and fledge more young in larger cavities. Nest cams have shown that nestlings spend many hours flapping their wings inside the nesting cavity before they fledge and therefore benefit from more space. Horizontally deep cavities offer better protection from both aerial and climbing predators. So, large compartments are in order, as are starling resistant entrance holes.
Here is a design for a Purple Martin nestbox which has been used successfully in the Pacific Northwest constructed of 3/4 inch exterior plywood.
minimum inside dimension 6 inches wide, 11 inches inside from front to back, 6 inches high minimum inside at entrance, 7 inches high minimum inside at back wall. floor extends 3 inches out beyond front wall for a porch. roof extends 3 1/2 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 are left outdoors year round, as almost all seem to be in the Northwest. Some of my wood roof only boxes that were put up as recently as 1999 are beginning to look fairly ragged. Some of Tom Lundís 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 two to three 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 not only makes it easier to cut out, it 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 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 and young. 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. (fitting the nestbox with an optional internal tray that slides out, while maintaining all interior and entry dimensions, is not required but will greatly aid in nestchecks and cleaning, use a material for the tray that is not slick and will insulate the birds from cold weather) 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 such as 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 seems to be the easiest, and is the most practical for managing nestlings during nestchecks, nest replacements, banding, etc. mount the front wall with screws, a latch, or in some fashion so it can be removed and reinstalled easily while maintaining entrance hole dimensions. access from either side or rear may be necessary depending on how the nestbox will be installed. for example, a nestbox installed on a piling at a dock, where the entrance is oriented out away from the dock, may need to be accessed from the side or rear. Always plug the entrance of a nestbox with nestlings before opening the side or rear to prevent premature fledging. supply a thin layer of nesting material in the rear of the box just before martins arrive in the spring. more than a trace but not too much. martins dont build much of a nest compared to some other cavity nesting birds, and research has shown that martins prefer a cavity with an existing old 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. dry grass, straw, or hay cut into lengths that will not entangle nestlings or parents, 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.
Eastern research has recently developed a purple martin nestbox entrance that is said to be "starling proof" instead of starling resistant. These entrance plates are commercially available from Eastern retailers . I am planning to test them in the Northwest in 2002. To date I have no information as to their use on the West Coast. Northwest martins (Progne subis arboricola) are generally slightly larger than Eastern martins (Progne subis subis), so the new "Excluder" entrances may have the potential to be western martin resistant (pure speculation ). I would appreciate hearing from Western martin people who try them.
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 on or near water, 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 toward the open area, away from prevailing weather if possible , allowing for viewing of the entrances from one location , and accessible for nest monitoring and maintenance. Although most Northwest Martins nest in clusters of single nestboxes separated by at least a few meters, 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 space between the individual cavities. If you already have an eastern style apartment house installed that you want to save , retrofit it to enlarge the cavities and install SREH , and supplement the surrounding area with additional cavities, creating a loose cluster.
Initially a cluster of Purple Martin cavities may attract Tree Swallows and Violet Green Swallows . Usually only one pair of each because they are not generally known to nest as densely 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. 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 . Tree and Violet Green Swallows will tolerate the same human activity around their nestsites and internal nest 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.
Once martins have been attracted to your site, seriously consider 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. A raccoon that discovers a martin nestbox colony can cause serious reproductive failure and the entire colony could be lost. Raccoons occur all across North America. 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 of over 50%, and many adult martins were killed and eaten by climbing raccoons. In 2001 at English Boom on Camano Island, Washington, raccoon tracks were discovered around the bottom of several pilings at dawn, and some previously banded nestlings were missing. That day those pilings were covered with 42 inch high brown sheet metal wrap starting a few feet up on the piling. The next morning at dawn the bottom of all the sheet metal had muddy scratch marks on them. That raccoon probably would have returned until all the nestlings and some of the 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.
Martins prefer to perch above 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.
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.
Gulls, crows, and herons are known to perch on or near martin nestboxes and prey on young 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 4inch 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.
Do not hesitate to monitor the insides of your martin nestboxes when multiple pairs of martins are nesting. When done appropriately, these checks 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 problem. However, Western 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 careful early in the season.
In much of their Western range, purple martins continue to nest as they have for millennia, in natural cavities, mostly in trees. In California, all purple martins nest in "wild" and therefore unmanaged colonies. The unique behavioral ecology of all remaining wild martin colony sites, in snag cavities or elsewhere, needs to be preserved. Nestbox installations may not be appropriate at some sites where martins have already established themselves without direct human intervention, where sufficient and sustainable natural or other cavities exist . This may be a controversial idea and discussion is welcome. Establishment of managed western martin colonies 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 nest site competition from European Starlings.
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.
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. And please let me know if you are successful in establishing a managed colony of Western Purple Martins. Good Luck.