Common Tern

A regular breeding resident and migrant, the Common Tern was a rare species during the Minnesota Breeding Bird Atlas (MNBBA).

In North America the Common Tern, a Holarctic species, is broadly distributed across Canada, from Alberta east to the Maritime provinces, across the Great Lakes, and south along the Atlantic Coast to South Carolina. According to surveys conducted in the 1990s, nearly 60% of the North American population breeds along the Atlantic Coast (including the Gulf of St. Lawrence), with approximately 20% on Lake Winnipeg, 6% in the Great Lakes region, and the remainder scattered on other large lakes and rivers in the northern Great Plains and along the U.S. Gulf Coast (Figure 1) (Nisbet 2002).

Assigned a Continental Concern Score of 11/20 by Partners in Flight and designated a species of Low Concern by the North American Waterbird Conservation Plan. In Minnesota, the Common Tern is officially classified as a Threatened Species and is designated a Species of Greatest Conservation Need by the Minnesota Department of Natural Resources. Audubon Minnesota has identified it as a Target Conservation Species.

A long-distance migrant, wintering primarily along the coasts of Central and South America.

An aerial diver that feeds on small fish and some crustaceans and insects.

A shallow scrape on the ground, often lined with vegetation and debris; nests on rocky substrates are usually located in natural depressions. A colonial nesting species.

The Common Tern finds its home in several of Minnesota’s largest bodies of water. When Roberts was compiling information for his 1932 book, The Birds of Minnesota, only three nesting locations had been documented for this colonial nesting species: Lake Mille Lacs (Spirit and Hennepin Islands), Lake of the Woods (Big Oak and Fourblock Islands), and Gull Lake in Cass County (a boulder reef off of Rocky Point on the lake’s west shoreline).

In the years since, the not-so-common Common Tern has been found nesting sporadically at a number of sites in northern Minnesota, including Island Lake in St. Louis County (Hines 1993), Cotton Lake in Becker County (Green and Janssen 1975), the Shipwreck Islands of Kabetogama Lake (McKearnan 1986), and Gull Lake (Roberts 1932) and Baby Lake in Cass County (Minnesota Ornithologists’ Union 2016). But for decades, the species has consistently nested in only four locations, which comprise the species’ primary population centers: (1) the Duluth-Superior harbor in Lake Superior, (2) Spirit and Hennepin Islands on Lake Mille Lacs, (3) numerous islands in Lake of the Woods, and (4) a small archipelago on Leech Lake. A history of the Common Tern’s presence at each of these breeding locations follows.

Lake Mille Lacs: Protected as federal reserves in the early 1900s, Spirit and Hennepin Islands are now known collectively as the Mille Lacs National Wildlife Refuge, the smallest wildlife refuge in the United States (0.23 ha). Tiny rocky islands devoid of vegetation, the two sites are located over 6 miles from one another and a few miles from the lake’s southern shoreline. Both have supported Common Tern nesting colonies since at least 1886 (Roberts 1932). In 1915, approximately 100 nests were reported on Hennepin Island and 150 nests on Spirit Island; 15 years later, in 1930, approximately 500 nesting pairs were reported on Hennepin Island and 80 on Spirit Island.

Regular surveys of the islands were not conducted until the 1960s and 1970s, when staff at the Rice Lake National Wildlife Refuge began closely monitoring the nesting populations. Unfortunately, the number of nesting pairs had declined significantly by then. When the first annual survey was conducted on Hennepin Island in 1963, the population was down to 100 nesting pairs; by 1976 there were only 57 nests. Regular surveys of Spirit Island did not begin until 1976, when only 11 nests were found (J. Green, personal files).

A major factor in the Common Tern’s decline was the appearance of the Ring-billed Gull on Hennepin Island in 1963 and on Spirit Island in 1970. The last nesting attempt by the Common Tern on Spirit Island was in 1998, but the species continues to nest on Hennepin Island. There, refuge staff has been collaborating with the Mille Lacs band of Ojibwe to protect the Common Tern from the growing population of the Ring-billed Gull and the recent appearance of the Double-crested Cormorant. Erosion of what little habitat remains has presented additional challenges (McDowell 2011).

Lake of the Woods: Numerous islands are found along the southern shore of this enormous lake and, farther north, in the waters off the Northwest Angle. Many of these sites, especially the rocky islets, provide suitable breeding habitat for a variety of colonial waterbirds, including gulls, cormorants, pelicans, and terns. These species also nest at several locations in the lake’s Ontario waters (Cadman et al. 2007).

Along the southern shoreline, east of Zippel Bay State Park, the Common Tern has nested for many years on a long sand spit known as Pine and Curry Island and on an adjacent point of land known as Morris Point. Because the site supported a small population of Piping Plovers, a federal Threatened Species, intensive research, monitoring, and management efforts were launched in the early 1980s. Although the focus was on the Piping Plover, the work also enabled close monitoring of the Common Tern population.

Unfortunately, the Common Tern colony was at its peak long before the Piping Plover research began, with nearly 1,000 nesting pairs reported in 1932 (Swanson, MOU Files). Between 1979 and 1999, the number of terns nesting on the island only averaged about 200 pairs per year, ranging from a low of 25 nests in 1980 to a high of 485 in 1986 (Herwig, pers. comm.). The colony’s status declined further in the ensuing years. Between 2001 and 2016 an average of only 77 nests per year were reported; this included 5 years when the terns failed to nest at all (Herwig, pers. comm.). Overall, when the lake’s water levels are low, the Common Tern is successful, but when water levels are high, nest losses due to waves, wind, and storm events can be significant.

The Common Tern has had to contend with the vagaries of weather and natural changes in water levels on its exposed nesting beaches for eons. Today, those challenges may be compounded on lakes that are managed with water control structures to address and balance multiple priorities, including recreation, hydropower generation, and downstream flooding. Such is the case on Lake of the Woods, which is carefully managed by the Lake of the Woods Control Board to balance competing interests and demands.

Occasional high water levels, caused by naturally high precipitation or management decisions, are not the only challenge for the Common Tern on this lake. The gradual erosion of sediments along the lake’s south shore has dramatically reduced the amount of sandy beach habitat on Pine and Curry Island. In a twenty-year period beginning in 1985, nearly 1,500 m of the island’s beach was lost (Herb et al. 2004). High water levels certainly may be contributing to the loss of habitat, but other factors also may be at play. For example, researchers have documented a reduction in the concentration of suspended sediments in the Rainy and Little Fork Rivers over the past 40 years, a positive trend for water quality. However, both rivers feed into Lake of the Woods and now may be providing less organic material to help replenish the island’s shorelines (Herb et al. 2004).

The Common Tern’s future on Lake of the Woods may reside entirely on the ability of the remote islets near the Northwest Angle to provide suitable breeding habitat. These sites are not surveyed every year, and even when surveys are conducted it can be difficult to reach all sites with potential habitat. Nevertheless, between 2001 and 2016, years when many of the Northwest Angle islands were surveyed, the average number of Common Tern nests per year in this region of the lake was 213 (Herwig, pers. comm.). Combined with the yearly average during the same time period on Pine and Curry Island, this yields an annual average of 290 Common Tern nests for the Minnesota portion of Lake of the Woods. Efforts by the Minnesota Department of Natural Resources to closely monitor the Common Tern on Lake of the Woods have added immeasurably to understanding the tern population on this important lake and have highlighted the significance of the Northwest Angle islands, which support more than 70% of the lake’s Common Tern population.

Leech Lake: An archipelago of three islands in the southern half of Leech Lake has been used since the early 1900s by the Common Tern and an assortment of other waterbirds, including gulls, cormorants, pelicans, and Caspian Terns. Owned and managed by the Leech Lake band of Ojibwe, the islands include Gull Island, Little Pelican Island, and Pelican Island. Until 1997, a small number of terns also nested on Little Pipe Island, located close to the lake’s southern shoreline.

At least two studies provide a comprehensive history of colonial waterbird populations on the lake (Reed et al. 1991; Mortensen and Ringle 2007). Although the Common Tern was not officially documented nesting on Leech Lake until 1933, its presence likely dates back nearly two hundred years to explorer Henry Schoolcraft, who noted the presence of “small white gulls” near Leech Lake in 1820. Based on the description of the birds, Mortensen and Ringle (2007) attribute Schoolcraft’s observations to the Common Tern.

Within the three-island archipelago, Gull Island supported the largest Common Tern colony from the 1920s through 1988 (Mortensen and Ringle 2007; Miller 1987; Reed et al. 1991). In 1933 approximately 1,000 to 1,500 breeding pairs were reported (MOU files; Miller 1987). Numbers remained high through the early 1970s but then began a gradual decline. By 1988 only 142 nests were documented. Competition with a growing nesting population of the Ring-billed Gull was considered the primary culprit. The Ring-billed Gull first arrived in 1960; at the population’s peak in 1991, the tiny island (0.16 ha) supported over 1,000 breeding pairs of gulls (Dickerman and Lefebvre 1961; Mortensen and Ringle 2007), leaving little suitable habitat for the Common Tern, which arrives later in the spring (Miller 1987; Reed et al. 1991). In 1989 the Common Tern abandoned Gull Island and moved north to Little Pelican Island.

Larger in size than Gull Island, Little Pelican Island presented its own set of challenges, including the presence of both mammalian and avian predators, encroaching vegetation, human disturbance, and the burgeoning population of the Ring-billed Gull, which also began nesting on the island in 1996. The Leech Lake Ojibwe have worked hard to address each of these challenges (Mortensen and Ringle 2007) and have succeeded in maintaining a breeding population of 100–200 pairs since 2004 (Hamilton and Cuthbert 2016). Recently, some Common Terns and Ring-billed Gulls began nesting on the largest island in the archipelago, Pelican Island.

Duluth-Superior Harbor: The history of the Common Tern in the Duluth-Superior harbor demonstrates how quickly the species can adapt to changing environmental conditions. Since the first nesting pair was discovered on Minnesota Point in 1937 (MOU files), the Common Tern has nested in several locations throughout the harbor. Biologists have characterized the harbor’s population as comprising one large, main colony that periodically moves, usually in response to encroaching vegetation or disturbance (Harris and Matteson 1975). In some years, the breeding population at the main colony is augmented by several small groups nesting at satellite sites (Penning and Cuthbert 1993). With few exceptions, most colonies in the harbor have been located on artificial islands or spits of land augmented with dredge spoils.

Several years after the Common Tern arrived in Duluth, the first major colony appeared on Hearding Island, located within the harbor. The island was created in 1934, and the Common Tern nested there for approximately 10 years (1946–1955). At its peak in 1953, the site supported 87 nesting pairs. But as vegetation succeeded to shrubs and young saplings, the site became less suitable. Although a few terns continued to nest there for several years, a new dredge-spoil island, Barkers Island, was established in 1955 just offshore of Superior, Wisconsin. Within one year of its creation, 57 nests were found on the island, and 108 were found the following year (Finseth 1957; Cohen 1958).

Data from Barkers Island beyond 1956 and 1957 are lacking, and little is clear about the species’ presence in the harbor during the 1960s. Cohen (1961) briefly mentioned a large nesting colony on Minnesota Point but provided no details regarding its location or size. Several papers repeatedly cite Cohen as the source of information for a large colony on Minnesota Point in 1960 and 1961 (Penning and Cuthbert 1993) and throughout the 1960s (Harris and Matteson 1975; Davis and Niemi 1980). Yet the only available nest counts for the entire harbor for 1961–1966 reference only three nests, found near the Sky Harbor Airport in 1961 and 1964, as well as a group of approximately 60 adults incubating in 1963 (MOU files summarized by J. Green). The actual size of the colony may have been difficult to ascertain. One observer who frequented the site in the early 1960s reported seeing a lot of terns in early June, many of which were considered lingering migrants. Some birds may have been local breeders, but nests also may have been abandoned early due to predation and human disturbance (J. Green, pers. comm.).

The history of the Sky Harbor colony during the 1970s and early 1980s is clearer. Ranging in size from 3 to 79 nesting pairs, the colony persisted, although the site was increasingly unsuitable due to encroaching vegetation and high rates of predation (Penning and Cuthbert 1993). The hazards posed by birds loafing on the runway finally prompted airport personnel to actively discourage nesting from 1986 through 1989.

Since the early 1970s, the Sky Harbor site was really only a small satellite colony. Beginning in 1971, the majority of birds began nesting at another highly disturbed dredge-spoil site, the Port Terminal. This became the main colony location until many birds moved to a small spit of land across from Wisconsin Point in 1987 and 1988.

The Common Tern’s precarious situation in the harbor, caused by constant exposure to human disturbance, competition with the Ring-billed Gull, encroaching vegetation, and high predation rates, prompted the Minnesota and Wisconsin Departments of Natural Resources to acquire and actively manage a dredge-spoil island created in the 1940s, Interstate Island, solely for the harbor’s Common Tern population. Inaccessible to the public and intensively managed to reduce vegetation and Ring-billed Gull predation, this site has consistently supported an average of approximately 190 breeding pairs since 1989 (Penning and Cuthbert 1993; A. Bracey and F. Strand, pers. comm.).

The complicated history of the Common Tern in Minnesota is not unlike that in other states throughout the Great Lakes region. Loss of habitat, disturbance, avian and mammalian predation, and competition with the Ring-billed Gull are challenging biologists throughout the species’ breeding range.

MNBBA Results: Because the Common Tern population is concentrated in a handful of sites that are largely inaccessible to the birding public, the MNBBA depended on data contributed by Dr. Francesca Cuthbert and her colleagues at the University of Minnesota. During the five-year span of the atlas, these researchers conducted a statewide census of Double-crested Cormorant and American White Pelican colonies throughout the state. Because many of these sites also support the Common Tern, the survey provided up-to-date data on three of the primary breeding locations (Hamilton and Cuthbert 2016). Data on the Duluth colony was provided by staff at the university’s Natural Resources Research Institute.

Combined, the data from the MNBBA volunteers and the university resulted in the Common Tern being reported from 0.9% (43/4,733) of the surveyed blocks and from 0.6% (13/2,337) of the priority blocks. Breeding evidence was gathered from 6 of the surveyed blocks (Figures 2 and 3; Table 1). In addition to documenting nesting at the four primary sites, a small new colony was found in 2010 nesting on Bird Island in Pelican Lake in northern Crow Wing County. The island supported approximately 10–20 nesting pairs, an estimate based on the number of fledglings that were actively being fed by adults in late July. Other significant observations during the atlas period included about 20 pairs on a long sand bar in the narrows between Upper and Lower Red Lake and a pair observed foraging in Voyageurs National Park, where the species has nested in past years. Unfortunately, no evidence of nesting was found at either site.

*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.

Minnesota’s four major Common Tern breeding populations demonstrate the range of habitats the species finds suitable during the nesting season. Although all the colonies are located on islands in large bodies of water, the nesting substrates vary from sandy depressions amidst rocky boulders to sand and gravel beaches and artificially created dredge-spoil islands (Figure 4). Scattered herbaceous vegetation or sheltered nooks among rocks and boulders provide important cover for the chicks once they have hatched. Some elevation helps protect the nesting birds from wash-overs during storms and high winds and provides good visibility. An abundant supply of small fish also is critical (Cuthbert et al. 2003).

As the species’ history in the Duluth-Superior harbor clearly illustrates, encroaching vegetation is a deterrent to nesting colonies, eventually forcing their abandonment of a site unless the vegetation is actively managed to prevent succession. Mainland sites that do not provide protection from human disturbance and predators are either avoided or used intermittently.

The North American Waterbird Conservation Plan estimated the size of the Common Tern population in North America at 300,000 breeding individuals, or 150,000 pairs (Kushlan et al. 2002). The Great Lakes population, which includes Minnesota’s Duluth-Superior harbor, has been closely monitored since the 1970s. During a three-year censusing window from 2007 to 2009, the population totaled 4,201 nesting pairs at 51 sites, an increase of 31% since the previous survey in 1998–1999 (Figure 5). The increase was attributed to management and protection efforts underway at many colonies throughout the region. Some of the biggest increases were seen in colonies at Lake Erie.

The Common Tern is widespread enough that the federal Breeding Bird Survey (BBS) provides a rough index of North American population trends for the species, although it is not a robust monitoring tool for colonial waterbirds. Since the mid-1960s, the species’ population has demonstrated a nonsignificant decline, averaging 0.59% per year. However, from 2005 to 2015 it has increased at the rate of 5.01% per year.

In Minnesota numerous estimates of the statewide population have been made over the years, ranging from a low of 541 breeding pairs in 1993 (Cuthbert et al. 2003) to a high of 800–1000 pairs in 2015 (Wessel 2015). It is difficult to compare and assess each estimate without knowing exactly how the count was conducted or which sites were included in the estimate. Nevertheless, existing data for all the major Common Tern colonies in the state were compiled to determine a statewide census estimate for five time periods spanning from the 1930s to 2015 (Table 2). In the 1930s the statewide estimate ranged from 2,580 to 3,080 nesting pairs; more recently the population has ranged from 635 to 930 pairs. Numbers vary considerably from year to year, as well as within years, depending on how and when the surveys were conducted.

The major factors impacting Common Tern populations are universal and include pesticide contaminants, human disturbance, predation, and habitat loss and degradation (Cuthbert et al. 2003). Two of the major issues in Minnesota have been the availability of suitable habitat and competition with the Ring-billed Gull and, at some sites, the Double-crested Cormorant.

Historically, the species’ low site fidelity enabled it to quickly adapt to the ephemeral nature of its open beach habitat. Natural conditions provided a revolving array of suitable habitats over time. But as natural processes have been impeded by a diversity of activities, including the construction of water level control structures, riprap banks to prevent shoreline erosion, or breakwaters that impede the natural replenishment of sediment on wave-eroded shorelines, suitable habitats have been diminished.

A loss of suitable habitat has been further exacerbated in the Great Lakes region by the rapidly expanding population of the Ring-billed Gull. The gull uses the same open nesting habitat as the Common Tern but arrives on the breeding grounds two to four weeks earlier, displacing the Common Tern from the more favored habitats and often forcing it to move into areas more prone to wave action. High densities of breeding gulls can also suppress vegetation growth due to the high acidity of the species’ fecal materials. Although the Common Tern prefers sparsely vegetated islands, some low growth provides the chicks with cover during storms or hot weather (Cuthbert et al. 2003; Nisbet 2002).

As challenging as it has been for biologists to address these threats in Minnesota, a recent analysis of the genetics of Common Tern breeding populations across North America points to the conservation importance of these efforts at a national scale (Szczys et al. 2017). Szczys et al. provided evidence supporting a proposal by other researchers that the Common Tern population in the lower Great Lakes region is the oldest population in North America and that the region has, historically, provided ideal breeding habitat. The genetic analysis, however, demonstrate a tenfold increase in the rate at which inland terns have dispersed to coastal breeding habitats since the 1960s. The authors suggest that this movement is symptomatic of the decline in habitat suitability in the Great Lakes region due to the range of threats mentioned above. Unless these issues are addressed, they predict the future of the Common Tern in the Great Lakes region is in jeopardy.

The Common Tern is considered a high priority for most states in the Great Lakes region and is a focal species for the Upper Mississippi River & Great Lakes Region Joint Venture (Soulliere et al. 2007). The Common Tern is officially classified as either Endangered, Threatened, or Extirpated in every state bordering the Great Lakes; however, in Ontario it is not considered at risk. Although it was designated a species of Low Concern by the North American Waterbird Conservation Plan (Kushlan et al. 2002), more recently it has been assigned a moderate Continental Concern Score of 11/20 which reflects both its declining population and threats to its breeding habitat (Partners in Flight 2017).

In Minnesota the Common Tern was officially listed as a Special Concern Species in 1984. Although statewide census data were limited, the species’ future seemed precarious in light of habitat threats and exploding Ring-billed Gull populations. By the early 1990s, the species had been the target of more intensive field studies that clearly demonstrated declining and fluctuating population levels, even as intensive management efforts were underway. As a result, it was officially listed as a Threatened Species in 1995. Like all state-listed species, it is also designated a Species in Greatest Conservation Need (Minnesota Department of Natural Resources 2015). The Common Tern was identified as a Target Species by Audubon Minnesota, resulting in the development of a statewide conservation plan (Pfannmuller 2014).

The Common Tern has survived in Minnesota in large part due to the remarkably creative and intensive management efforts of federal, state, and tribal resource personnel. Three of the state’s four breeding populations (Lake Mille Lacs, Leech Lake, and Duluth-Superior harbor) are actively managed to address numerous challenges, including vegetation encroachment, mammalian predation, beach erosion, and Ring-billed Gull competition (Penning and Cuthbert 1993; Mortensen and Ringle 2007; Pfannmuller 2014). Staff continue to pursue innovative options to ensure that suitable habitats are available and attractive to the Common Tern, including acquiring ownership of a dredge-spoil island in the Duluth-Superior Harbor dedicated to tern management (Interstate Island) and examining the potential of creating habitats on two modified pontoon boats in Lake Mille Lacs (Wessel 2015).

Many of these management efforts also have been implemented in Lake of the Woods. The challenges are considerable in this large, remote lake. Large-scale loss of habitat on Pine and Curry Island is one of the greatest challenges, while islands in the Northwest Angle are so remote that fieldwork is costly and difficult. Loss of nests due to high water and wave action is another major threat. Close cooperation with the Lake of the Woods Control Board, which is responsible for managing lake levels, may be essential for ensuring the species’ future in this important region.

Because the Common Tern is a fish consumer in the Great Lakes region, contaminant levels have also been a concern. Tissue analyses conducted in the Duluth-Superior harbor in the late 1970s concluded that the concentration of organochlorine compounds were high enough that potential impacts to reproductive success could not be excluded (Niemi et al. 1986). Nisbet and his colleagues noted that although the levels of organochlorine compounds persisted into the 1980s at some Great Lakes, levels have been decreasing since then (Nisbet 2002).

In the long term, climate change also poses a significant threat to the Common Tern. Significant storm events are predicted to become more common and can cause considerable damage to ground-nesting colonial waterbirds. Indeed, a recent analysis by the National Audubon Society predicted that the Common Tern could lose 80% of its current summer breeding range by the year 2080 and classified the species as “climate threatened” (Langham et al. 2015; National Audubon Society 2016).

In light of all these challenges, efforts to periodically monitor the statewide breeding population are critical to assessing the status of the species and its response to management efforts. Although the Common Tern is an uncommon species in Minnesota, many dedicated field biologists have worked extremely hard to ensure that the species remains an integral component of Minnesota’s natural heritage.