Progne subis (Linnaeus, 1758)
Basis for Listing
The Purple Martin (Progne subis) is a neo-tropical migratory swallow, with a breeding range distributed throughout much of the central and eastern United States, the west coast, and parts of Canada and Mexico. This aerial insectivore is widely distributed throughout Minnesota. While it was historically known to inhabit woodpecker holes in dead snags and other natural cavities, the decline in availability of these natural nest sites and the ease with which Purple Martins are attracted to artificial nests has resulted in their almost exclusive use of nest boxes in Minnesota today. Although not naturally colonial, the Purple Martin’s reliance on birdhouses has resulted in their aggregation in large colonies, which exposes them to several threats. They must compete with House Sparrows (Passer domesticus) and European Starlings (Sturnus vulgaris), two invasive non-native cavity-nesters that are also widely distributed in the state. These non-native species displace Purple Martins from nest boxes and are known to destroy Purple Martin eggs and kill their nestlings. Predators, such as Cooper’s Hawks (Accipiter cooperii) and Great Horned Owls (Bubo virginianus), are known to prey on Purple Martin colonies. In addition, Purple Martin colonies are vulnerable to infestation by nest parasites, such as blow flies (Calliphoridae), bird fleas (Siphonaptera), and mites (Macronyssidae). Purple Martins aggregate in the thousands at pre-migratory roost sites each fall, where they are vulnerable to any localized impact to the site.
Purple Martins are readily observed by participants in the U.S. Geological Survey’s North American Breeding Bird Survey. Survey data show a population decline of 5.7% per year in Minnesota during the period 2005-2015 and a long term decline of 6.64% per year (1966-2015; Sauer et al. 2017). This decline is one of the largest declines of any bird for which the state’s Breeding Bird Survey data are statistically significant. Purple Martins were listed as a species of special concern in 2013 due to documented declines over the past three decades and continuing threats to the state’s population.
Purple Martins are the largest swallow species in North America. Adults are approximately 20 cm (8 in.) in length, with a 40 cm (16 in.) wingspan. Long pointed wings and a forked tail are evident when they are seen making quick darting flights to catch insects. The male is a dark iridescent purple-black or blue-black, and the female has a dark back, pale gray throat and breast, and a brownish or grayish collar at the nape of the neck. Subadult males resemble the female but have variable amounts of blue-black feathering on the head and underparts. Subadult females also look similar to the adult female, but they tend to be lighter and browner than adults. Purple Martins are similar in shape to other swallows. Adult males are easily distinguished from other swallows by having a dark belly, whereas all other swallows have a light belly. Females are distinguished by the pale collar and more muddy appearance of the belly. Again, most other swallows have whiter bellies.
Both males and females vocalize, and songs have been described as “boisterous throaty chirps and creaky rattles” (Cornell University 2015). During courtship, females have a "chortle" song, and males have a "croak" song. Males also have a dawn song performed before daylight, possibly to attract other martins to the colonial nest site (Brown and Tarof 2013).
Purple Martins are found foraging for insects over cities, towns, parks, open fields, streams and rivers, and open water habitats including wetlands, marshes, and lakes (Brown and Tarof 2013). Historically, this species nested in woodpecker-created cavities in dead trees (snags) in small colonies along woodland edges or riparian areas (Brown and Tarof 2013). Today, nearly all nesting occurs in man-made nesting structures around human settlements, including highly developed cities as well as shorelines of large lakes with healthy insect populations. Purple Martins appear to not be very selective in terms of habitat, but more successful birdhouses are often located in open areas away from dense trees (Purple Martin Conservation Association 2017). There are data to suggest that populations may be limited by the number of martin houses, as colony numbers often increase as more nesting structures are provided (Brown and Tarof 2013).
Biology / Life History
This aerial insectivore is widely distributed throughout Minnesota, with fewer observations in the heavily forested northeastern part of the state (Minnesota Breeding Bird Atlas Project 2017). Purple Martins return from wintering grounds in South America via Mexico or across the Gulf of Mexico in early April through early May. Older male martins return first to areas where they nested previously. First-year nesters return to nesting grounds several weeks later. Males perform displays to attract females to their territory or compartment in man-made bird houses. This species historically relied on natural cavities and likely nested colonially or solitarily but likely in close proximity to other nesting martins (Brown and Tarof 2013). Natural cavity use in Minnesota has not been documented in over 80 years. Nesting today is colonial and in man-made bird houses. Purple Martins are seasonally monogamous (form pair bonds for one season) with a few exceptions, such as where males may have more than one mate or when birds seek extra-pair copulations (mating outside of the pair bond).
Nest building can take 10 days to 4 weeks (Brown and Tarof 2013). Nests are made from twigs, leaves, coarse grass, and mud. Martins typically lay one clutch of 3-6 pure white eggs that hatch after 15-18 days of incubation. Young are naked when hatched, yet are capable of flight after 27-36 days. Both members of the pair feed young. Martins will stay near the nest site until they begin staging, prior to migration.
After young are fledged, martins gather at traditional pre-migratory roosts at night. Purple Martins are highly social during the non-breeding season. These roosts can be made up of thousands to hundreds of thousands of individuals; they are so large that researchers often monitor weather surveillance radar to find them. Birds gather in late summer, assembling a few hours before sunset. Roosts tend to be in areas of tall and dense vegetation, including cattails (Typha spp.), bulrushes (Scirpus spp.), and even cornfields in Minnesota (North et al. 2012). A coarse habitat assessment of roosts east of the Rocky Mountains found martins to roost in a variety of land-cover types, including forest, cropland, water, and urban areas, with fewer roosts located in wetlands, scrub-shrub, barren, and grass (Bridge et al. 2015). Birds disperse for daily foraging and to move south. Roost sites are used for a number of days or weeks as birds move through an area. Minnesota roosts are also used by migrants from Canada, including Alberta, Saskatchewan, and Manitoba (M. North, unpublished data). Fall migrants generally follow a similar path as spring migrants to return to South American (North et al. 2012).
Purple Martins are primarily insectivorous, eating a large variety of flying insects, including beetles (Coleoptera), true bugs (Hemiptera), flies (Diptera), dragonflies and damselflies (Odonata), leafhoppers (Homoptera), grasshoppers and crickets (Orthoptera), butterflies and moths (Lepidoptera), wasps and bees (Hymenoptera), caddisflies (Trichoptera), and spiders (Araneida) (Brown and Tarof 2013). Diet likely reflects local availability of insects. Martins feed during the day, gathering and consuming nearly all of their food and water while on the wing; however, they have been observed gleaning insects off vegetation (Brown and Tarof 2013). They only typically land on the ground to collect nesting material or pick up grit to aid in digestion.
Conservation / Management
The primary conservation concerns for this species include the availability and management of appropriate nesting locations, exposure to unseasonably cold and rainy weather, and vulnerability of roosting locations.
Installation of nesting structures can significantly benefit local populations as long as these structures are well designed, installed properly, and maintained throughout the nesting season. Purple Martin Conservation Association is the recommended resource for nesting structure design, installation, and maintenance. Nesting structure design and placement can also affect the use and reproductive success of these structures. Houses should be placed in open areas, such as fields or lakeshores with limited tree cover in the vicinity. Houses with larger and deeper compartments (15 x 30 cm [6 to 12 in.] or larger) are preferred by martins, result in increased safety from avian predators such as owls, and can result in higher nesting success (Brown and Tarof 2013). Maintenance requires that houses that are installed are built in such a way that they can be raised and lowered safely throughout the nesting season. White pine needles are recommended for nesting material and a base of pine needles can be provided before the nesting season begins.
Maintenance includes the removal of non-native birds and taking steps to reduce parasite populations. House Sparrows and European Starlings aggressively outcompete Purple Martins, have been known to kill martins, and have been observed puncturing eggs or expelling young and eggs from nesting compartments. Additionally, these non-native species may fill nesting compartments with grasses, rendering them unusable by martins (Brown and Tarof 2013). Purple Martins may also get outcompeted by other native species such as Tree Swallows (Tachycineta bicolor), House Wrens (Troglodytes aedon), and Eastern Bluebirds (Sialia sialis). Non-native bird species, their nesting material, and eggs should be trapped or otherwise removed (all other bird species that might use Purple Martin houses are native and protected by state and federal laws). Without this removal, Purple Martins will be excluded from using nesting structures.
This species can have a number of body and nest parasites, including blow flies, bird fleas, and mites. Blood-feeding body parasites like mites can occur in large infestations (several thousand per nest) and can result in reduced clutch sizes, mortality among nestlings, weight reductions in surviving birds, and colony abandonment (Brown and Tarof 2013). Maintenance of the nesting structure includes removing nest materials at least once at the end of each nesting season, though some resources advocate for the replacement of nesting materials during the nesting season if parasites are high in number (Brown Tarof 2013). The use of insecticides on these parasites is more controversial and should be researched before use.
With changing climatic conditions, cold weather events may occur unexpectedly. Exposure to cold and rainy weather, particularly during spring and early summer, results in conditions where birds cannot find insects. After a few days without food, significant mortality can occur (Brown and Tarof 2013).
Roost sites may be vulnerable to localized impacts. Roost sites can attract upwards to hundreds of thousands of individuals. Ensuring that these sites are available on the landscape is important for staging prior to migration. While individual roosts may only be used for a number of years, martins require these large sites, with appropriate habitat for roosting and abundant insect populations for refueling prior to migration. Recent research indicates that this species has a migration strategy similar to shorebirds, where individuals from across a broad area gather at key sites to refuel for days or weeks before engaging in a pulse of migratory activity. This strategy requires the presence of good quality roost sites. The gathering of widely separated populations at predictable roost sites during migration increases vulnerability to localized reduction in food resources due to habitat loss or climate change (Fraser et al. 2013). Further, these roost sites, where large numbers of birds congregate, are vulnerable to catastrophic events.
Best Time to Search
The best time to survey for Purple Martins in Minnesota is May through late July. Roost sites are typically used from late July through early September (North et al. 2012).
Conservation Efforts in Minnesota
Due to the documented decline in Purple Martins over the past three decades, as well as the continuing threats to the state’s population, Purple Martins were listed as a species of special concern in 2013. As such, this species is now tracked in the Natural Heritage Information System. The Minnesota Department of Natural Resources (MN DNR) has also been working with partners to document roost sites and staging areas.
Minnesota has an active Purple Martin Working Group that is part of the Purple Martin Conservation Association. This group of citizen-scientists and professional biologists has worked toward improving nesting success by creating and providing educational materials for the installation and maintenance of nesting structures. A number of research projects have been initiated and published by members of this group including studies to look at the migratory behavior of the species by North et al. (2012). The MN DNR’s Nongame Wildlife Program has supported the efforts of the Purple Martin Working Group in Minnesota.
Christine Herwig (MNDNR), 2018
(Note: all content ©MNDNR)