Bank Swallow

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

Breeding populations of Bank Swallows are found across the globe, including in North America, Eurasia, and northern Africa. In North America their breeding range extends from Alaska east to the Maritime Provinces, and then south to include most of the northern and central United States. Breeding densities are highest in the prairie parklands of central Manitoba south through the northern Great Plain states (Figure 1).

Assigned a Continental Concern score of 11/20 and designated a Common Bird in Steep Decline by Partners in Flight.

Long-distance migrant that winters primarily in South America.

Aerial insectivore.

Excavates its own nesting burrow in exposed cliff faces, bluffs, and banks along rivers, lakes, and streams; readily adapts to man-made environments such as sand and gravel pits. A colonial nesting species; colonies may include hundreds of nesting pairs.

Roberts (1932) considered the Bank Swallow to be a common summer resident wherever suitable nesting sites were available. He wrote,

Wherever in the state there are exposed sandbanks, its nesting holes are most certainly to be seen. It shows a preference for high, washed banks and deep cuts near water, but on the prairies and in the forests it may be found nesting by the roadside in banks no more than eighteen to twenty inches high.

Breeding evidence (including both confirmed and inferred nesting reports) was available from 12 counties covering nearly all regions of the state, including Goodhue and Lincoln Counties in the south, 7 counties across central Minnesota, and Cass, Itasca, and Polk Counties in the northern region.  Many birds were also reported nesting at Leech Lake.

Forty years later, Green and Janssen (1975) clarified that the species was least abundant in the extensive forested landscape of northern Minnesota, particularly in the far northeastern counties, where stream banks were “scarce or absent.” Janssen (1987) added that the Bank Swallow was locally common in this region around sand and gravel pits. He included a statewide distribution map that identified 32 counties where nesting had been confirmed since 1970. In 1998, Hertzel and Janssen added 3 more counties to the list.

Fieldwork conducted by the Minnesota Biological Survey (MBS) beginning in the late 1980s gave further clarity to the species’ relative abundance across the state. To date the MBS has reported 277 breeding season locations. The species was widely dispersed across the state but especially common along the Minnesota River valley, in southwestern Minnesota, and along the glacial Lake Agassiz beach ridges of northwestern Minnesota, where gravel mining is common (Minnesota Department of Natural Resources 2016).

During the MNBBA, observers reported 809 Bank Swallow records from 13.2% (628/4,764) of the surveyed atlas blocks and from 15.0% (351/2,337) of the priority blocks. Breeding was confirmed in 305 blocks (Figures 2 and 3; Table 1). The birds were reported from all 87 Minnesota counties and were found breeding in all but 6 counties (Dodge, Faribault, Freeborn, Kittson, Lake of the Woods, and Red Lake). Three of the counties with confirmed breeding were added because of blocks that straddled the boundaries of two counties (Big Stone, Koochiching, and Renville).  Forty-seven of the counties were additions to the list published by Hertzel and Janssen (1998). Records were relatively common south of a line from Crookston in northwestern Minnesota, east to Duluth.

The overall distribution of Bank Swallows in Minnesota appears to have changed little over the past 100 years. An adaptable species, it has found new opportunities in several man-made environments, including sand and gravel mines and road cuts. Two notable examples are the cluster of MNBBA records in northern Clay County, an area along the Glacial Lake Agassiz beach ridges, where gravel mining is common, and a small cluster of records in central St. Louis County, an area of taconite mining. In his comprehensive review of the species, Garrison (1999) noted no significant changes to the species’ distribution other than local shifts resulting from changes in habitat. In Maryland, for example, Bank Swallows were formerly common in coastal habitats, but shoreline development has resulted in a shift to inland locations. Overall, because it is a colonial species dependent on the availability of suitable nest sites, its distribution is quite localized.

*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.

Historically the Bank Swallow was restricted to sites where the erosive force of water cut away at stream banks, lakeshores, and ocean coasts. Its Latin name, Riparia riparia, and its common name, Bank Swallow, pay homage to its life along riparian areas. Today, however, the species has broadened its habitat to include many man-made habitats, including sand and gravel mines and vertical road cuts (Figure 4). Although many accounts suggest the species is always found near water, that association more likely characterizes its natural nest sites and not the man-made habitats it has readily adopted. Foraging habitats include a variety of open landscapes, including grasslands, pastures, agricultural fields, wetlands, open water, and occasionally woodlands (Garrison 1999).

Dense, extensively forested landscapes are generally avoided. In a long-term study of forest birds on the Chippewa and Superior National Forests in northern Minnesota, Bank Swallows were not observed on any of the monitoring sites within forest stands (Bednar et al. 2016). The species’ presence in this region of the state is largely restricted to residential and industrial developments that provide earthen banks for nesting. During the MNBBA, the primary habitats found within 200 m of point counts where Bank Swallows were detected were upland grasslands and shrub wetlands (Figure 5).

Natural or man-made, colony sites are usually composed of alluvial, sand-silt soils that are easily excavated. Substrates with very fine grains allow the birds to excavate deeper burrows, which can result in higher reproductive success. Using their bill, feet, and wings, the birds excavate a tunnel perpendicular to the bank face and approximately 60 cm deep. Their small, conical bill and their strong leg muscles are unique adaptations specifically designed for the task (Garrison 1999).

Bent (1942) noted the species was “most numerous in the glaciated sections of the country where glacial deposits of sand and gravel abound.” These deposits meet the birds’ requirement for soft, friable soils. More recently, an assessment of the species’ status in Canada noted the abundance and distribution of Bank Swallows was strongly correlated to “exposed, unconsolidated deposits of glacial lacustrine origin,” which are common in the Great Lakes region (Committee on the Status of Endangered Wildlife in Canada 2013).

When the birds return each spring, they spend several days exploring the landscape, assessing potential colony sites in the general vicinity of where they nested or where they were raised the previous season. Because of the ephemeral nature of their nesting habitat, their site tenacity is fairly low; sites successfully used in the prior year may no longer be suitable. If a site remains in good condition, however, recolonization is likely (Garrison 1999).

The Bank Swallow’s clumped distribution during the nesting season at locations that may change from year to year challenges efforts to accurately assess its population status and abundance, a problem common to other colonial nesting species. Nevertheless, the federal Breeding Bird Survey (BBS) remains the only long-term data set available for the species. Using data collected by the survey since the mid-1960s, biologists estimated the North American population is approximately 7.7 million breeding adults (Rosenberg et al. 2016). In 2013, Partners in Flight estimated that Minnesota supported 3.3% of the continental population, resulting in a current statewide estimate of 256,000 adults. Among Minnesota’s nesting swallows, only the Purple Martin is less common, with a population of approximately 40,000 birds (Partners in Flight Science Committee 2013).

Minnesota is among a suite of states in the Upper Midwest and Great Plains that support relatively high breeding densities of Bank Swallows (Figure 1). Across the BBS survey area, the average number of Bank Swallows on BBS routes is 14 birds per route per year. In Minnesota, the annual average is 39 birds. In surrounding states and provinces, it is 40 in North Dakota, 29 in Manitoba, and 26 in Wisconsin (Sauer et al. 2017).

Since the BBS began in 1966, the Bank Swallow has experienced a dramatic population decline that recently has tempered to a more stable level. Throughout southern Canada and the United States, population numbers have declined at a rate of 5.33% per year. Since 1970, the species has experienced a cumulative decline of 89% (Rosenberg et al. 2016). The good news is that in the most recent 10-year reporting period (2005–2015), the rate of decline has slowed considerably, to 0.77% per year (Sauer et al. 2017).

The decline has been nearly as steep in Minnesota, where the annual population has declined an average of 4.99% per year since 1967. Although the rate of decline has lessened to 1.52% since 2005, it is still nearly twice the decline observed at the national level (Figure 6). In general, the species is declining significantly throughout much of the eastern portion of its range, while increases are being observed in many areas farther west (Figure 7).

Numerous factors may be responsible for the Bank Swallow’s steep decline, but two in particular rise to the top. First, disturbance and nest destruction at active sand and gravel mining operations certainly are responsible for some losses. In Wellington County, Ontario, 95% of all the aggregate mines that were operational in 2011 (20 of 21) were occupied by colonies of Bank Swallows; 63% of the active pits conducted work that destroyed 32% of all the county’s  occupied burrows (Committee on the Status of Endangered Wildlife in Canada 2013). Given the species’ increasing dependence on such habitats, this level of annual loss likely has a significant impact on overall population numbers.

Of equal concern is the observation that many aerial insectivores, particularly flycatchers and swallows, have experienced significant population declines. For many species, the declines became most notable in the 1980s (Nebel et al. 2010; Smith et al. 2015). As Smith and his colleagues point out, the shear scope of the decline and the number of species affected “suggests that there are powerful and broad-scale factors at work.” The fact that declines are most acute among species in the northeastern United States has led some to speculate that the declines are a reflection of reductions in flying insects in this heavily industrialized region (Nebel et al. 2010).

In light of its steep population decline, the Bank Swallow has been assigned a Continental Concern Score of 11/20 and was identified as one of 24 common birds in steep decline (Rosenberg et al. 2016). It was officially listed as threatened in Ontario in 2013 and was deemed to be a threatened species across Canada that same year by the Committee on the Status of Endangered Wildlife in Canada (2013). It is not recognized as a conservation priority in Minnesota.

In 2016, Ontario published a detailed recovery strategy for the species (Falconer et al. 2016). A targeted recovery goal of maintaining a stable population of at least 330,000 breeding adults was established, and specific actions to help achieve that goal were proposed. The plan is an excellent compendium of data on the Bank Swallow and a model for developing local recovery plans.

Unlike many species that have adapted to man-made environments, the Bank Swallow is subject to threats resulting from their adaptations. They tolerate human activity near nesting colonies, but active excavation in sand and gravel mines can result directly in a loss of birds. There are many proposals, including in Ontario’s recent recovery strategy, to promote working in partnership with the aggregate mining industry to develop and implement best management practices that protect nesting colonies without negatively impacting the industry.

Measures to stabilize stream banks along many of the state’s rivers and streams may reduce natural nest sites and should be closely evaluated for their potential impacts on the species.

Finally, the potential effects of climate change cannot be ignored, as warming temperatures may significantly impact the Bank Swallow’s primary food source. A recent analysis of the potential impacts of climate change on 588 North American birds by the National Audubon Society included the Bank Swallow in its assessment. Their models predict that Bank Swallows could experience a significant northward shift in their breeding range, as only 22% of their current range will be suitable habitat by the year 2080 (Langham et al. 2015; National Audubon Society 2016). As a result of their analysis, the species was classified as “climate endangered”.

Resource professionals may need to pay close attention to the status and distribution of this industrious aerial excavator to ensure its future as a member of Minnesota’s rich and diverse bird community. Many of the smaller swallows, like the Bank Swallow, are easily overlooked, but as aerial insectivores, they provide an important ecosystem service.