Great Lakes Moment: Detroit River common terns under threat

March 3, 2025 John Hartig, Great Lakes Now

By John Hartig, Great Lakes Now

March 3, 2025

Great Lakes Moment is a monthly column written by Great Lakes Now Contributor John Hartig. Publishing the author’s views and assertions does not represent endorsement by Great Lakes Now or Detroit PBS.

Common terns are known for their aerial acrobatics, performing rapid turns and swoops to the delight of those watching. You frequently see them flying low over the water, hovering briefly and then diving head-first into the water to spear a small fish with their pointed beak. However, these aerial acrobats are threatened in the Detroit River by natural and human-related stressors and will need human help to reach conservation goals.

The common tern is a small colonial waterbird designated as a threatened species in Michigan. They breed in the Great Lakes basin and overwinter in Florida, the Caribbean, Central America and the southwestern coast of South America.

In the 1960s, the lower Great Lakes had the largest number of common tern nests recorded in surveys — approximately 16,000-21,000 nesting pairs. By 1980, only approximately 5,000 nesting pairs were recorded in the same region. This decrease was due to several factors, including the increase of the ring-billed gull population that has displaced common terns.

Another factor in the decline of terns is human disturbance. For example, wanton human destruction occurred in the 1960s on Detroit River’s 982-acre island park called Belle Isle. In 1961, Cranbrook Institute of Science naturalist Walter Nickell and his good Detroit Audubon (now called Detroit Bird Alliance) friend named Wilbur Bull and his son Jim (who went on to become an avid birder and conservationist and serve as president of Detroit Audubon three times) walked out on an earthen dike about a half mile into the river to band terns.

This dike was adjacent to Detroit’s Belle Isle water intake. On that day there were about 1,000 nests with hundreds of spotted eggs and young terns. The journey out onto the dike required much care to avoid stepping on any eggs or chicks. This also required each person to wear a hard hat. The reason for the hard hats was that the mere presence of intruders angered the terns so much that they were dive-bombing Nickell and the Bull families. The sound of common tern beaks hitting the hard hats was like that of a percussion ensemble.

In contrast, Nickell and a Wayne State University ornithologist came back to survey the terns three years later and discovered that power mowers had run over hundreds of tern nests, chopping up hundreds of chicks and eggs into little pieces. Of the 360 nests found, only 51 escaped the carnage. The argument, at that time, was that the common terns were fouling the water intake and deterrence was needed.

Common terns have also been threatened by contaminants like polychlorinated biphenyls (PCBs) that were used in a wide range of industrial products like electronic transformers and capacitors, sealants, caulking compounds, inks and paints. The highest recorded PCB concentrations in common tern eggs from the Detroit River were found in 1972. In 1979, PCBs were banned by the Toxic Substances Control Act (TSCA). Since then, PCB levels have markedly declined, similar to the declines reported in herring gull eggs and fish from the Detroit River.

During 1960-1980, Detroit River surveys documented more than 4,500 common tern nests on Belle Isle and Mud, Grassy, Bob-Lo and Fighting islands. By 2005, no terns were nesting on these islands where they historically nested, and less than 300 common tern nests were found on two human-made bridge protection piers off the Grosse Ile toll and free bridges over the Detroit River’s Trenton Channel. This represented a 98% decline in 25 years. No systematic nesting surveys have occurred since 2011. Michigan Department of Natural Resources projects that the next common tern survey will happen in 2030.

In 2009, the Detroit Zoo, the Great Lakes Water Authority and the Detroit River International Wildlife Refuge made improvements to the artificial peninsula at the northern end of Belle Isle near the Detroit water intake to increase tern nesting success. Improvements included removing vegetation, adding gravel substrate for nesting habitat and utilizing social attraction methods (tern calls and deployed decoys) to attract terns. By 2011, terns began nesting at this site in small numbers – less than 20 pairs nested producing a limited number of chicks. The factor most limiting reproductive success became predation by raccoons, snakes and other birds. As a result, tern management efforts at this site stopped in 2018.

In response, management efforts shifted to a decommissioned South Channel range light located in Lake St. Clair near the southeastern tip of Harsen’s Island. The nongovernmental organization called Save Our South Channel Lights owns this structure. The Detroit Zoo initially worked with Save Our South Channel Lights in 2011 and 2012 to remove overgrown vegetation and create habitat more suitable for tern nesting. In response, terns began nesting, with 130 nesting pairs recorded in 2017. Detroit Zoo actively participated in field observations up until 2023. Detroit Zoo bird supervisor Jessica Jozwiak said: “This site is promising because it is isolated from the land and surrounded by a steel seawall that will minimize predation. Further, with some vegetation management, few other factors would limit tern nesting.”

However, over time the objectives of the Detroit Zoo and Save Our South Channel Lights conflicted and cooperation on tern nesting ceased.

“The common tern is a high-priority conservation species,” said Francie Cuthbert, a colonial waterbird expert and retired professor in the Department of Fisheries, Wildlife and Conservation Biology at the University of Minnesota. “However, recovery can be challenging because of special nesting requirements and predation by black-crowned night herons, great-horned owls, mink, racoons and more. The best success has been achieved at intensively-managed, human-created nesting sites.”

Multi-million-dollar habitat restoration projects were completed for Detroit River’s Stony, Celeron and Sugar islands in 2018, 2019 and 2024 — respectively. Although these three projects focused primarily on coastal wetland restoration, they each included creating some common tern nesting habitat on barrier dikes and shoals. Jozwiak cautions: “that such habitat sites will only be successful in tern nesting if there is a deterrence to predators like mink and raccoon.”

But common terns have shown an affinity for these sites.

Detroit Riverkeeper Bob Burns said: “In 2024, I observed good numbers of common terns using the Stony and Celeron island barrier dikes and shoals for loafing. For Sugar Island, it is still too early after construction to see tern activity, although I expect similar results in the future.”

Common and Caspian terns loafing on the dikes of Celeron Island. (Photo Credit: Bob Burns)

The primary factors currently limiting tern nesting in the Detroit River are predation and human disturbance. Further, natural resource managers must have realistic goals for managing a much smaller tern population than was historically in the Detroit River.

Although the future of common tern restoration in the Detroit River will be challenging, some practical things can be done to aid in recovery:

Investigating the creation of new common tern nesting habitat on the old Sugar Island ferry pier (that is free-standing and not connected to the island) that is owned by the Detroit River International Wildlife Refuge;
Recruiting nongovernmental organizations like Friends of the Detroit River, Detroit Bird Alliance and/or International Wildlife Refuge Alliance to maintain more ideal vegetation cover at the Grosse Ile bridge tern nesting sites; and
Expanding citizen science monitoring of existing and any new tern nesting sites in the Detroit River.

John Hartig is a board member at the Detroit Riverfront Conservancy. He serves as a Visiting Scholar at the University of Windsor’s Great Lakes Institute for Environmental Research and has written numerous books and publications on the environment and the Great Lakes. Hartig also helped create the Detroit River International Wildlife Refuge, where he worked for 14 years as the refuge manager.