Purple Martin

A regular breeding resident and migrant. The Purple Martin was an uncommon species during the Minnesota Breeding Bird Atlas (MNBBA).

Broadly distributed across the eastern United States and the southern regions of central and eastern Canada. Smaller populations are found along the coast of the Pacific Northwest, from southwestern British Columbia through California, and in the southwestern states of Utah, Colorado, New Mexico, and Arizona. Highest breeding densities are found in the southeastern region of the United States (Figure 1).

Assigned a Continental Concern Score of 10/20 by Partners in Flight; officially listed as a Special Concern species in Minnesota and designated a Species in Greatest Conservation Need by the Minnesota Department of Natural Resources.

Long-distance migrant that spends the winter in South America.

An aerial insectivore.

A secondary cavity nester. Eastern populations nest almost entirely in man-made nest cavities, while western populations can still be found nesting in natural cavities.

Roberts (1932) considered the Purple Martin to be a common summer resident throughout the state, noting that even in the prairies it was found in “considerable numbers in the towns and about farmhouses.” Although today we associate the species strictly with artificial nesting structures, in Roberts’s day Purple Martins were still found nesting in natural tree cavities, in cavities excavated by woodpeckers, on cliff ledges, and in rock crevices, especially in the less settled regions of the state.

Roberts’s 1932 status account of the species included his own photograph of martins nesting among large boulders on two tiny islands in Lake Mille Lacs, Spirit Island and Hennepin Island. The birds’ unique use of these boulder-strewn islands was first reported in 1886 by Washburn, an early student of the state’s ornithology. Nearly thirty years later, in 1915, Roberts and his colleague William Kilgore visited the site, describing how the martins were nesting “in the midst of the Common Tern colonies, with which birds they were at constant war, as the Terns no doubt ate the eggs and young of the Martins when they could reach them.” Despite these challenges, the martins continued to persist. A visitor in 1930 estimated 300 pairs were nesting on Spirit Island, and 50 pairs on Hennepin Island (Roberts 1932). Together the two islands, now a national wildlife refuge, supported a natural nesting colony of Purple Martins for at least 44 years, from 1886 to 1930.

Elsewhere in the state, Roberts had compiled breeding evidence (both confirmed and inferred nesting records) from 13 counties and from Itasca State Park and Cass Lake. The counties were primarily restricted to the Eastern Broadleaf Forest, the Tallgrass Aspen Parklands, and the western and southern regions of the Laurentian Mixed Forest Provinces. Their status in the far north-central and northeastern regions was not clear from Roberts’s account.

When Green and Janssen (1975) published their updated description of the species’ status several decades later, they reported that martins were a breeding resident throughout the state but were most common along the Mississippi River and about the towns and cities of central and western Minnesota. Janssen (1987) delineated a total of 36 counties, distributed across all regions of the state, where martins had been confirmed nesting since 1970. Included were 2 northern counties, Itasca and Lake. Hertzel and Janssen (1998) later added 7 more counties to the list, including several in north-central Minnesota.

Survey work by the Minnesota Biological Survey (MBS) reported a total of 101 breeding season locations of Purple Martins, restricted primarily to central and southern Minnesota. However, the MBS’s focus on natural communities precluded an in-depth survey of the small residential communities and recreational areas where the species is most common (Minnesota Department of Natural Resources 2016).

During the MNBBA, observers reported 878 Purple Martin records in 11.2% (537/4,781) of the surveyed atlas blocks and in 11.3% (265/2,337) of the priority blocks. Breeding evidence was documented in 350 blocks (Figures 2 and 3; Table 1). The birds were observed in 75 of Minnesota’s 87 counties and were confirmed breeding in 68 counties (one block straddled both Big Stone and Lac Qui Parle Counties). Thirty-two of the confirmed nesting counties were additions to the list published by Hertzel and Janssen (1998). Overall, Purple Martins were most abundant in central Minnesota, from the Twin Cities north to Leech Lake, and west to Otter Tail and Kandiyohi Counties. Certainly the focused effort of many energetic atlas observers in this region contributed to the high number of records. But the region is also populated with many lakeshore residents who actively encourage local Purple Martin colonies by erecting and maintaining martin homes on their property. The birds were virtually absent, however, from the extensively forested regions farther north in the state.

None of the confirmed breeding records documented during the MNBBA mentioned use of a natural nesting cavity. Like nesting populations throughout eastern North America, Minnesota’s Purple Martin population may now depend entirely on artificial nesting cavities constructed and maintained by local residents.

In the past 100 years, the Purple Martin has remained a species widely dispersed across all but the densely forested counties of northern Minnesota. Elsewhere the species’ breeding range has receded southward during the 20th century, particularly in northwestern Canada (Brown and Tarof 2013). Farther east, a similar retraction has been documented in Ontario, where the northern periphery of the species’ breeding range moved 56 km south during the 20 years between their first and second atlases (Cadman et al. 2007). Factors responsible for the range contraction are unclear. Cadman and his colleagues (2007) speculated that several successive years with late frosts and cold spring rains could result in a significant loss of birds, with a subsequent impact on local populations for years. On the other hand, increasingly warm spring temperatures may result in the birds arriving before sufficient food resources are available. Farther south, in the Great Plains, the installation of martin houses has facilitated a small westward expansion of the species’ core breeding range (Brown and Tarof 2013).

*Note that the definition of confirmed nesting of a species is different for Breeding Bird Atlas projects, including the definition used by the Minnesota Breeding Bird Atlas, compared with a more restrictive definition used by the Minnesota Ornithologists’ Union. For details see the Data Methods Section.

Prior to the large-scale use of Purple Martin houses, the species was found along forest edges and in riparian areas that provided an abundance of dead snags for nesting cavities and aerial insects for food. However, even prior to European settlement, Purple Martins had established a close association with humans. Hollowed-out nesting gourds were not an uncommon site in Native American villages when the first European settlers arrived. Sprunt (1942) provided an early account by ornithologist Alexander Wilson, who described how the Choctaws and Chickasaws would cut off all the top branches of sapling trees and leave just a few short branches on which to hang a hollowed-out gourd or calabash for nesting martins. When young trees were not available, Native Americans would simply erect two poles with a cross bar for hanging the gourds, a practice replicated by African American slaves on southern rice plantations (Forbush 1929). Because the Purple Martin is a fierce defender of its territory, the establishment of nesting colonies was encouraged as a means for protecting domestic poultry from hawks and eagles.

Today, due to the widespread use of artificial nest houses, Purple Martins have become quite urbanized. Nesting occurs wherever suitable nesting structures are erected, from rural towns and lake cabins to intensively developed urban and suburban landscapes (Figure 4). Indeed, the predominant habitat with 200 m of all MNBBA point counts where Purple Martins were detected was development (Figure 5).

Guidelines for erecting martin houses abound and usually recommend that the structures be placed in open areas, 40 feet from trees and within 30 to 120 feet of housing (Purple Martin Conservation Association 2016). One of the most important factors to consider when choosing a site to erect a nest box is the availability of nearby open areas. Pastures, meadows, marshes, ponds, and lakes provide critical foraging habitat for this aerial insectivore.

Long-term monitoring data collected by the federal Breeding Bird Survey (BBS) have been used to generate a North American population estimate of 7.6 million birds (Rosenberg et al. 2016). The statewide estimate for Minnesota is approximately 40,000 birds, or 0.50% of the continental population (Partners in Flight Science Committee 2013).

Across the United States breeding densities are significantly higher in the southeastern states and relatively low in Minnesota (Figure 1). Along BBS routes in Minnesota an average of 2 to 3 birds are observed per route each year; in the southeastern states of Alabama, Mississippi, and Louisiana, the average number of birds per route ranges from 30 in Mississippi to 85 in Louisiana (Sauer et al. 2017).

During a long-term monitoring program conducted on Minnesota’s two national forests, field surveyors reported an average of only 0.06 Purple Martins on 100 unlimited radius, 10 minute point counts from 1991 to 2010 on the Chippewa National Forest; on the Superior National Forest no birds were observed. This compares to an average of 191.83 for Ovenbirds and 210.5 for Red-eyed Vireos on the Chippewa, and 224.79 for Ovenbirds and 140.27 for Red-eyed Vireos on the Superior, two of the most common birds on both forests (Niemi et al. 2016). Clearly, extensively forested landscapes provide little suitable habitat for the martin.

Since the BBS began in 1966, the Purple Martin population has experienced a slow but significant survey-wide decline of 0.91% per year (Sauer et al. 2017). In Minnesota, the species’ plight has been dire, as the population has decreased at an alarmingly high rate of 6.64% per year (Figure 6). This computes to a cumulative decline of more than 96% from 1967 to 2015! Indeed, across the northern portion of the species’ breeding range, populations have been plummeting. In the Boreal Hardwood Transition region populations have declined an average of 8.35% per year since 1966; in the Atlantic Northern Forest region they have declined an average of 8.16% per year. Overall, significant population decreases are largely confined to the Great Lakes states and southeastern Canada in the northern portion of the species’ breeding range and to the southern coastal states from eastern Texas to Florida (Figure 7).

Although not studied in detail, several factors are cited as potentially responsible for the declines. As an aerial insectivore, Purple Martins are vulnerable to cold rains, sleet, and snow during migration. Early arrivals often succumb in the thousands if aerial insects are not available, and it may take years before the populations regain former levels of abundance (Sprunt 1942; Brown and Tarof 2013). Dependent on natural cavities or constructed nest boxes, Purple Martins compete with the abundant European Starlings and House Sparrows for nest sites. Indeed, suitable nest sites are considered a major factor limiting populations in some areas (Brown and Tarof 2013). A decrease in the number of people erecting and actively maintaining Purple Martin nest houses is also cited as contributing to the decline. Kelly Applegate, with the Mille Lacs Band of Ojibwe, commented that “the martin house used to be an American tradition. Back in the 50s, you had your American Flag and your martin house in the front yard. But you just have a lack of interest from generations that are coming up now” (Mohr 2011).

Finally, the Purple Martin is within a suite of aerial insectivores that have all experienced population declines, particularly in northeastern North America. Nebel and his colleagues (2010) have pointed out that the declines are steepest where human population density and industrialization are highest. They postulate that the resulting increase in atmospheric pollutants and their negative impact on aerial insects may be a strong factor in causing the declines of aerial insectivores.

At the national level the Purple Martin’s widespread distribution, adaptation to artificial nesting structures, and long-term population decline has led to the assignment of a moderate Continental Concern Score of 10/20 (Rosenberg et al. 2016). In Minnesota, however, the steeply declining population trend led to its official listing as a Species of Special Concern in 2013 (Minnesota Rules, Chapter 6134). It is also designated a Species in Greatest Conservation Need by the Minnesota Department of Natural Resources (2015).

The future of the Purple Martin, a bird beloved by many, is truly in the hands of dedicated bird enthusiasts and conservationists. Although the species has likely benefited by its adaptability to artificial nest cavities, it has become entirely dependent on people to properly manage these structures. Websites abound with recommendations on how to be a successful Purple Martin landlord, including installing predator guards and actively discouraging nest competitors, especially House Sparrows and European Starlings. In Minnesota, Perry and Bloch (1987) conducted an extensive survey of martin houses in the central region of the state and provided 12 key management recommendations. Although many factors may be responsible for the species’ demise, inattentive management by homeowners may be an important factor in some regions.

Today we think of the Purple Martin as a social and highly colonial species. Its colonial habits may be a consequence, however, of its attraction to artificial nesting structures that aggregate the birds into large colonies. Brown and Tarof (2013) note that eastern populations are nesting in high-density situations largely because they are provided nesting structures that congregate the birds. Some artificial structures include more than 100 individual nest compartments. Although nowadays it is a rare occurrence, when martins in eastern North America nest in natural cavities, they often nest as solitary pairs. As for any population, high nesting densities have their own set of problems, including vulnerability to nest predators and infestation by nest parasites, which can significantly reduce reproductive success, particularly if sites are poorly managed (Brown and Tarof 2013).

The impacts of warming temperatures on Purple Martins are predicted to be minimal to moderate (North American Bird Conservation Initiative 2010; Matthews et al. 2004). Yet, even if climate change is not a threat to this species, its future is entirely dependent on conservation efforts by the public, especially in Minnesota.