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Our treasured terns


Sumner W. Matteson

Tern in flight against a blue sky© NANCY NABAK

When I was 18, I joined my father on a canoeing expedition down the Northwest Territories' Coppermine River to the Arctic Ocean.

The year was 1970 and we were the eighth party ever to go down the river, according to legendary Canadian wilderness canoeist Eric Morse, with whom my father consulted before deciding which Canadian river he was going to conquer next. I remember catching Arctic char on many a cast, and I'll never forget the long lakes that were part of the trek and their crystal-clear waters where you could see down 30 feet.

But I also remember my father stopping one afternoon along a flat lakeshore, an alluvial fan of sorts, and insisting we check out this congregation of birds that were quite vociferous. I ventured over, blissfully ignorant of what to expect, and soon discovered that these birds, Arctic terns, were in the midst of raising young — and darn if they would stand for some tall, skinny and unwanted potential predator in their midst!

The adults at the edge of the colony, soon followed by many others, came over and relentlessly attacked me, swooping and striking the top of my head repeatedly. My father, sitting in the stern of our canoe, laughed. Until he also was dive-bombed — at which point we quickly made a hasty retreat.

And so began a love affair with all things terns.

Four species of terns occur regularly in Wisconsin, all on the state's endangered species list: the common tern, Forster's tern, Caspian tern and black tern. The continuity of these species in our lakes and wetlands depends on our ability to safeguard them as part of the state's natural heritage legacy.

Tern feeding its chickThe common tern is the subject of extensive management efforts to help sustain its population.

Sterna hirundo: For the common good

In 1974, four years after that fateful Arctic encounter, I was asked by my good friend, James T. Harris, to join him in conducting the first-ever (University of Wisconsin Sea Grant-funded) systematic survey of colonial waterbirds along the western shore of Lake Superior. I remember meeting with naturalist and endangered species advocate Ruth Hine at the Department of Natural Resources. She was especially interested in the status and distribution of the common tern, which she feared was not very common in the state.

At that time, Jim and I found three small colonies of common terns on Lake Superior. One consisted of 24 pairs at an island of mortared stone, a square of about 15-by-5 meters, near the Washburn boat landing. A second colony occurred across Chequamegon Bay on the Ashland breakwater, with 12 pairs depositing eggs in depressions in bare rock — which ultimately filled with rain water and proved ruinous.

A third colony occupied a former Ashland pier remnant, only about 70-by-70 feet in size and supported by dilapidated railroad ties that held the island together. Here, 20 pairs of common terns could only nest in the rotting pockets of the railroad ties because the island was overgrown with paper birch and tag alder, and grasses and forbs.

Nesting success in 1974 was dismal — about four chicks for 56 nesting pairs — among the tern colonies in Chequamegon Bay. But there was one other location to investigate far to the west, the Duluth-Superior Harbor.

No colonies in Superior, but across the way in Duluth, common terns occupied four locations: At least 10 breeding pairs occupied sandy scrapes at the Sky Harbor airport on Minnesota Point; another 10 pairs occupied piles of ash and scrap along the mainland at the Minnesota Power and Light Co. (MPL); and approximately 10 to 15 pairs occupied two small, low-lying islands south of the MPL plant in the St. Louis River estuary.

The fourth site, the largest tern colony in western Lake Superior at the time, was at the Duluth Port Authority. Here — near and below the interstate highway bridge system and scattered across a broad sandy deposit — we located 150 nests in sparsely vegetated areas, along with two pairs of nesting piping plovers, one herring gull nest and one ring-billed gull nest.

Creating current habitat

Fast-forward to today. All of the common terns in the Duluth-Superior area are concentrated with about 13,000 pairs of nesting ring-billed gulls on Interstate Island, a 6-acre dredge spoil island (mostly in Minnesota) in the St. Louis River estuary that has been managed for nesting terns since the late 1980s. Back in 1974, there were fewer than 575 ring-billed gull pairs nesting across the entire Duluth-Superior area.

At Interstate, segregating the gulls from where the terns nest — through a fenced and string-grid rectangular area — has minimized gull impacts, but occasional gull predation of tern eggs and chicks occurs. More recently, high water on Lake Superior is threatening to overtake Interstate Island, where the number of nesting tern pairs has been cut in half from about 300 to less than 150 in 2017. Efforts are underway to attempt to stabilize the island.

Meanwhile, back at Ashland, we have restored the island twice, in 1986 and 2002, with Wisconsin Coastal Management and EPA funds, respectively. A complete remodel occurred in 2002 with the aid of the Bad River Band of the Lake Superior Chippewa overseeing project reconstruction.

The island, known as Ashland Tern Island, was rebuilt using a combination of a steel frame and wooden wall crib, with riprap providing protection from storm and ice activity. Once completed, the island stood about 6 feet above water and was engineered to stand intact for at least 50 years, and to withstand a 100-year storm event.

A steel mink phalange around the island's outer perimeter top was added in 2005 to deter marauding mink. Then, in spring 2018, I secured U.S. Fish and Wildlife Service (FWS) funding to repair the island's deteriorating interior wooden walls.

For reference, about 100 pairs of terns produced 179 young at the Ashland site in 2017.

Human hands fitting a band onto the leg of a ternBanding and pioneering studies using geolocators and GPS tags help track terns on their overwinter migrations.

In-depth research

At this point, let me emphasize that most of the annual common tern habitat management and monitoring in the past 30-plus years at both Ashland Tern Island and Interstate Island has been conducted by Fred Strand. The now-former DNR wildlife manager is legendary in the field for his comprehensive work with terns, and he continues his exhaustive efforts under contract with the DNR.

I have had the privilege to assist Strand annually in his work. In recent years, University of Minnesota Ph.D. student Annie Bracey also has accompanied him weekly to both tern colony sites. In addition, since 2013 Strand, Bracey and I have been involved with a pioneering study using geolocators and GPS tags to track tern movements both locally and during their fall and spring migrations to ascertain migratory pathways, stopover sites and wintering locations.

Together within a larger Great Lakes geolocator study involving common terns from Lake Winnipeg in Manitoba, Lake Huron in Ontario and Oneida Lake in New York, we have learned that our Great Lakes' common terns overwinter in Panama, Nicaragua, Venezuela and along the Gulf of Mexico. But most (70 percent) winter in southern Peru.

That makes the region very important to the Great Lakes population. It also raises the dubious proposition that the terns are highly vulnerable to a stochastic, or statistically random and unpredictable, potential climatic event affecting the local fishery in southern Peru, which could seriously impact the Great Lakes population.

Partners in conservation

One last and by no means least word on our breeding common terns comes from the shores of lower Green Bay, Lake Puckaway and Lake Butte des Morts. The DNR has many fine partners in endangered species conservation, and for common terns, one need look no further than the Green Bay office of the FWS, particularly wildlife biologist Gary Van Vreede.

Working with DNR wildlife biologist Josh Martinez, Van Vreede has spearheaded efforts to build two small wooden rafts and place them adjacent to a cell that comprises part of the mile-long Cat Island restoration chain in lower Green Bay. These rafts, lined with fine-gravel stones, have attracted 25 pairs of nesting terns and proved to be a reliable short-term solution to providing tern nesting habitat in northeastern Wisconsin.

Similarly, on Lake Puckaway, two pontoon boats, modified by Lake District staff, provide similar substrate (mixed with sand) for about 40 pairs of breeding terns. This project is monitored by longtime DNR cooperator Daryl Christensen.

Finally, under the guidance of now-retired DNR fisheries biologist Art Techlow, a small, roundish dredge spoil island was created in 2008 on Lake Butte des Morts through the FWS Natural Resource Damage Assessment and Restoration Program. Common terns immediately took to the island, but success has been difficult. One year, partygoers wiped out the nests, and in recent years great horned owl predation has taken a toll.

Despite periodic predation and highwater issues, our managed common tern sites in 2017 had a combined total of 299 breeding pairs that fledged 422 chicks, an average of 1.41 young per pair — more than adequate to maintain a stable common tern population. But the state's population has ranged widely between nearly 300 and 500 pairs over the past 15 years, with the number of nesting pairs at Interstate a determining factor.

Management goals

Over the past three to four decades, we have learned much about how to manage common terns, whose population numbers in the Great Lakes have declined from a high of about 21,000 pairs in the 1960s to around 9,000 pairs today. Reasons for the decline — most obviously apparent in Wisconsin — have been vegetative succession/habitat loss, exploding numbers of ring-billed gulls usurping nesting sites, mammalian and avian predation, human disturbance of colony sites, changing water levels and chemical contamination.

Managers across the Great Lakes, however, agree on one important point: Without ongoing active management at extant colony sites, the species stands little chance of surviving extirpation.

Wisconsin site managers have adapted to changing conditions over time and remain optimistic that, barring circumstances beyond our control, we can achieve a stable state breeding population of common terns. Perhaps it is one that will average at least 500 nesting pairs per year over a 10-year period, with a mean annual production of at least 500 young.

This goal would be a revision of the 1988-set target of 1,000 nesting pairs per year, but one that would ensure common terns will continue to frequent our large lake ecosystems for posterity.

Tern sitting on a rock near waterHabitat help is critical for the Forster's tern which thrives in large lake marshes.

Sterna forsteri: Help for a marginalized population

Forster's terns resemble common terns in size (both about a foot long with a 2½-foot wingspan) and appearance, except for the Forster's orange-red bill and frosty primaries. Forster's terns prefer to breed colonially in large lake marsh ecosystems.

Before widespread European immigrant settlement in the 19th century and the subsequent alteration or loss of wetlands, Forster's terns likely moved from site to site as habitat conditions varied with changes in water levels and muskrat activity.

The Forster's tern became state-listed as endangered in 1979 due to poor productivity at extant colonies and the small number and instability of those colonies. Since the late 1970s, 200 to 900 pairs have occurred at eight colony sites: Lakes Poygan and Butte des Morts in Winnebago County; Horicon Marsh in Dodge County; Big Muskego Lake, Waukesha County; Lake Puckaway and Hope Marsh in Green Lake County; Lower Green Bay; and Winnebago County's Rush Lake.

Today, Forster's terns often occupy marginal sites or suboptimal habitats where the emergent stands are dwindling, thinly vegetated or exposed to the vagaries of wind, waves and seiche, temporary disturbances in water level. Nowhere is this more apparent than on Lake Poygan, where for several decades the state's largest Forster's tern colony occurred. It was finally abandoned in 2017 due to the effects of persistent high water.

The delisting goal for this species is: Over a 10-year period, establish an average annual statewide nesting population of at least 800 pairs at an average annual number of 10 or more colonies, with no more than half of all nests on artificial structures and with a mean annual production of at least 800 young. This goal was established in 1988 from a recovery plan written by now-retired DNR research scientist Michael Mossman.

Over the long-term, Forster's tern management activities will likely be focused on conservation of the remaining lake wetland sites in east-central Wisconsin that support most of the state's breeding population, and on wetland habitat restoration at Lake Poygan and Lower Green Bay. There is considerable interest in habitat restoration of the lower bay, including restoration of wetlands in the Cat Island chain shadow.

"Restoration potential is high in the lower bay," said John Huff, DNR's Peshtigo-area wildlife supervisor.

The use of artificial nesting platforms is a means to halt or slow the decline of Forster's tern populations, and in some cases, such as in Lower Green Bay, provide an alternate breeding opportunity until marsh conditions are restored. Working with FWS, staff at DNR have installed a string of nesting platforms in the lower bay, strategically placed, since May 2014.

Annual surveys will likely occur to monitor colony fates and to determine if Forster's are occupying new or former colony sites — for example, along the Mississippi River in Buffalo, Trempealeau, Vernon and Crawford counties. Annual monitoring is recommended given the species' vulnerability to drought and storm events that could affect habitat conditions and available prey during its life cycle, which includes long-distance migrations to the Gulf Coast, Caribbean, Mexico and Central America.

Tern in flight against a mottled backgroundCaspian terns flourish in open cobble-gravel of Great Lakes islands.

Hydroprogne caspia: A disastrous taking of terns

In 1905, something not to be repeated occurred on Gravel Island, off the northeast coast of the upper Door peninsula and visible from Newport State Park.

Henry Ward, ornithologist with the Milwaukee Public Museum, led an expedition to Gravel Island, where he had heard the rare Caspian tern was nesting. Here, he found about 1,500 breeding birds. But this was a collecting trip for the museum, and the taking of migratory birds was not yet illegal (the Migratory Bird Treaty Act of 1916, amended in 1918, outlawed the shooting of migratory birds).

Ward and his crew blasted away, and the terns did not return the following year, which elicited his wry comment: "The absence of the terns may, perhaps, be accounted for by our shooting a number of birds the preceding year."

Prolonged human disturbance has been and may still be the most important factor affecting colony stability for this species.

Historically, ornithologists Ludwig Kumlien and Ned Hollister, who wrote "The Birds of Wisconsin" in 1903, reported Caspian terns nesting intermittently on islands off the north shore of Lake Michigan and in Green Bay from 1879 to 1893. These are the earliest known Caspian tern breeding records for Wisconsin.

Also, wildlife artist and naturalist Owen Gromme's personal field notes from his collection days at the Milwaukee Public Museum included a June 8, 1881, reference to zoologist B.F. Goss collecting Caspian tern eggs on an island (Gravel Island, perhaps?) near the northern tip of Door County.

Noteworthy is that this tern species is a Great Lakes obligate of open, cobble-gravel or barren island habitats isolated from human disturbance and mammalian predators. Colony instability (in part influenced by human disturbance), poor production and the low number of nesting pairs were determining factors in its 1989 state-endangered species listing.

Caspian terns did eventually return to Gravel Island and have nested there intermittently during the past 115 years. In 2017, I surveyed the island with Sadie Odell of the FWS, with logistical support from wildlife biologist and boat operator Randy Jurewicz, now retired from DNR. We found 1,229 Caspian tern nests amid higher portions of limestone rock and gravel, occupying the same areas where Ward found them in 1905.

Until 2014, Caspian terns also nested intermittently on Pirate Island, off the northwestern Door County peninsula and visible from Peninsula State Park. High water has submerged most of the island since that year, especially the low-lying, flat, non-vegetated portion of the island where nearly 350 pairs of Caspians previously nested on an extensive bed of bleached-white zebra mussel shells. It is unknown where the colony went, since there was not a pronounced increase of Caspians at Gravel Island, which also had been affected and reduced in size due to high water levels.

Continued monitoring will occur at the Door County colony sites to assess population stability in northeastern Wisconsin, particularly since this species is highly vulnerable to water temperature changes affecting cold freshwater prey and Great Lakes water level rises affecting habitat requirements. Less understood are what challenges Caspians face on their Gulf Coast, Mexican, Central American and Caribbean wintering grounds.

The only other Wisconsin site that holds nesting Caspian terns is the Cat Island restoration chain, managed by FWS and surveyed regularly by biologist Tom Prestby. On a narrow portion of a sparsely vegetated mile-long dike, near its northern end and surrounded by breeding ring-billed gulls, about 70 pairs of Caspian terns attempted to nest.

Although Caspians occur as breeding birds only at these two Wisconsin sites, they are regular birds of the Lake Michigan shore during summer, less conspicuous along Lake Superior. The raucous call of this gull-sized tern in flight is unmistakable. Listen and look for our gull-sized tern with the black cap at such places as Point Beach State Forest, whenever you visit the beach.

Tern in flight against a grassy backgroundOnce considered "universally common," the continental population of the black tern has declined by two-thirds since the 1960s due to wetland loss.

Chlidonias niger: 'Waif' of the wetlands

The state's smallest (10 inches) and lightest (2 ounces) regularly occurring tern species is the black tern, perhaps the most vulnerable of our tern species because of its affinity for nesting in marshes close to the water line, making it especially susceptible to storm-wave action. And yet, the black tern has persisted in our wetlands since time immemorial.

The ornithologist Arthur Cleveland Bent described this bird nearly 100 years ago as "a restless waif of the air, flitting about hither and thither with a wayward, desultory flight, light and buoyant as a butterfly."

In the early 1900s, the black tern was "universally common" in North American marshes and wet meadows. The continental population, however, has declined by two-thirds since the 1960s due to wetland loss. It is a species highly vulnerable to drought conditions, which affect its requirement of stable water levels during the breeding season, while temperature increases on its Mexican and South American non-breeding grounds could affect the availability of food resources — insects and minnows.

Concerns about population declines in the mid-1970s led to the first statewide survey of black terns in 1979, a roadside survey adjacent to suitable or potentially suitable breeding habitat. In that year, state and federal agency personnel and volunteers conducted surveys of more than 300 wetland sites.

Based on those surveys, permanent road survey transects were established in 1980 to serve as an index of statewide distribution and abundance and provide baseline data for long-term monitoring. Systematic surveys were conducted in 1980-82, 1995-97 and 2009-11.

What did we learn from these roadside surveys? A 70 percent reduction in the number of birds observed between the first and last survey periods, and a notable reduction or disappearance of sites that formerly held breeding black terns.

For example, black terns completely disappeared as breeding birds from Lake Superior wetlands, and we know from all but one site, there was no change in the amount of available breeding habitat. The roadside survey program, accordingly, spanned more than 30 years and its results provided the primary source for the bird's endangered listing in 2014.

After the bird was listed in the state, I was part of a team of three — with professor David Shealer of Loras College and naturalist Daryl Christensen — to plan a study to investigate more thoroughly the bird's actual abundance and distribution across the state. The idea was to leave roadsides and instead canoe and kayak all wetlands that contained black terns.

The process involved several steps, including classifying wetlands and compiling a list of known black tern colony sites. We also consulted state records on the birds from Wisconsin eBird and the Wisconsin Breeding Bird Atlas II.

During the first four years, we found more than 2,300 black tern breeding adults at 115 colonies in 32 counties. Most of these colonies were small to intermediate in size, with two to 25 breeding adults. We found breeding black terns throughout much of the state occupying shallow waterfowl production areas, FWS refuges, impoundments and flowages with sedge and cattail marshes, and at lake bay marshes and lake edge marshes and bogs.

Based on our surveys to date, we believe wetlands with smaller colonies may provide a buffer against catastrophic nest failures in larger wetlands, where predation pressures may be greater. With the black tern and each of the state's treasured tern species, their continued presence at lake and wetland sites will be directly tied to ongoing conservation efforts involving this unique component of Wisconsin's natural heritage.

Sumner W. Matteson is an avian ecologist with DNR's Bureau of Endangered Resources.


  • There are four regularly occurring tern species, each state-endangered, in Wisconsin: the common tern, Forster's tern, Caspian tern and black tern.
  • There also are five tern species occurring so rarely they have their own Wisconsin Society for Ornithology designation. The Arctic tern is considered "casual" in the state, observed at least once every one to five years. The least tern, sooty tern, royal tern and white-winged tern are "accidental," with "less than one record every five years" for each species.
  • Taxonomically, our Wisconsin terns belong to the class Aves, order Charadriiformes, family Laridae, subfamily Sterninae. Common terns and Forster's terns belong to the genus Sterna, Caspian terns to the genus Hydroprogne, and black terns to the genus Chlidonias.
  • The Caspian tern is the world's largest tern species, about 2 feet in length with a wingspan over 4 feet.
  • Typically, the nest of the common tern and the Forster's tern will contain three eggs, sometimes two, and rarely more than three. The Caspian tern will normally have two eggs per nest, while the black tern usually has two or three eggs in a nest.
  • The common tern, Forster's tern and black tern incubate their eggs for approximately three to four weeks and their young fledge (take wing) at about three to four weeks of age. Caspian terns take about four weeks to incubate their eggs, and their young don't fly until five to six weeks of age.
  • Breeding terns feed primarily on small forage fish such as chubs, dace, shiners, sticklebacks and sculpin, though black terns and Forster's terns pluck insects from the air and Caspian terns take small to medium-sized fish.
  • As for breeding habitat, the common tern prefers island or artificially created island sites of the upland Great Lakes and some large inland lakes. The Caspian tern typically seeks Great Lakes barren coastal islands or sometimes remote coastal dredge spoil sites. Large shallow marshes and flowages with a mix of open water and emergent stands of vegetation are the choice of the black tern, while Forster's terns have a predilection for openings or edges in lake marsh islets of emergent vegetation.