Least Terns of the Boston Harbor

Come May, the beaches of Massachusetts shed the hush of winter. Nor'easter waves subside, verdant vegetation emerges, and the squeaky calls of birds excited to nest decorate the soundscape. These voices belong to Least Terns (Sternula antillarum), iconic seabirds of the New England coast. Small, yet mighty, these hardy birds use wings spanning a mere 50cm to carry them from their wintering grounds in the Carribean and South America thousands of miles up the east coast of the United States to breed.

A Least Tern incubating on a gravelly beach in the Boston Harbor Islands National and State Park. Notice the well camouflaged eggs protected by their parent.

NPS Photo

When Least Terns arrive in Massachusetts, adults are ready to breed and bear their finest plumage—crisp white and gray feathers cover the body and wings, a striking black cap and eyestripe crown their head, and the bill is bright yellow, finished with a black tip.

Breeding together in large groups known as colonies, mates will make small nests called "scrapes" along sandy or gravelly beaches sparse with vegetation. Scrapes dapple these life-giving shorelines during the Least Tern breeding season of May through August, but the birds aren’t the only visitors in search of a refuge here. There is another animal, that enjoys the very same habitat—beachgoing humans.

While these birds face a diversity of threats, human disturbance is among the most common. When people are too close, Least Terns get spooked and leave their nests. This makes eggs and chicks vulnerable to predation, sometimes caused by larger birds or mammals, and sometimes by unleashed dogs accompanying their owners on the beach.

Closing important nesting sections of beaches and keeping dogs away during sensitive breeding times are critical to this species’ reproductive success. Least Terns are a Species of Special Concern in Massachusetts, meaning their conservation and management is a priority for the state and should be a priority for all people who hope to see them in the years to come.

This is a tern nest, or “scrape,” after a rainstorm.  Can you see the outline of the adult tern that protected the eggs and kept them dry?

Carol Lynn Trocki

While written records of this bird’s historical distribution are lacking, documentation of the Boston Harbor breeding population began in the 1970s. Strewn across multiple islands, the Least Tern population naturally fluctuated in size and location, but a particularly drastic change in was observed in 2007.

In June of that year, volunteers monitoring a bustling Least Tern colony on Lovells Island were faced with an unfortunate surprise—the tiny, inconspicuous tern eggs that once littered the beach had vanished, leaving behind barren scrapes and aimless parents.

This European Rabbit was caught on camera investigating bait in a dummy nest.

Lauren Nolfo-Clements

Biologists were eager to steward the habitat and aid the recently decimated colony. But first, they needed to determine where things went wrong. The empty nests were a sign of predation, so researchers deployed baited game cameras to survey for egg-hungry predators. The researchers found that both mammalian and avian predators, including American Crows, brown rats, and European rabbits, could have eaten the tern eggs.

Least Terns are smart birds and good parents—the predation event in 2007 motivated the colony to find more suitable nesting locations, and Lovells was void of terns the following two years. In 2010, a small colony of 35 pairs took another shot at Lovells Island, nesting on it once again and successfully raising 10 fledglings – chicks old enough to fly. Predation is a common problem for nesting Least Terns, but efforts by Mass Audubon and the Massachusetts Department of Recreation work to control predators on Lovells Island to reduce this threat.

The lightly vegetated beaches that make up the perimeter of Lovells Island provide excellent nesting habitat for the Least Terns of Boston.

Boston Harbor Now

Tern hardship on Lovells seemed to be a thing of the past until the summer of 2012, when disaster struck again and a storm washed the terns’ scrapes into the sea. For years, invasive plants had increasingly encroached upon the birds’ shoreline habitat, pushing their nests further from safety and closer to the crashing tide.

The past two decades have brought only flighty success for Least Terns on Lovells Island. Like many coastal breeding birds, these terns must cope with a challenging combination of human disturbance, predation, and overwash of their nests.

The birds, however, do not face these challenges on their own. The Massachusetts Department of Conservation and Recreation, the National Park Service, the Mass Audubon, and dedicated volunteers work together to improve nesting conditions on Lovells Island.

From controlled burns to trash cleanups, these efforts have a real impact; in 2019, one hundred pairs of Least Terns nested on Lovells Island. Today, these organizations continue to monitor the birds and steward the land to better suit the needs of both.

Come late August, the beaches of Massachusetts begin to settle into the calm of fall. The ocean waters cool, trees prepare for dormancy, and the especially squeaky calls of a bird again decorate the soundscape. This voice belongs to a Least Tern fledgling, restlessly awaiting its first migration.

A pair of recently hatched Least Tern chicks huddle together in their nest.

NPS Photo

Contributed by: Olivia Smith, National Parks of Boston Climate Conservation Corps Crew Member

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Works Cited:

Hatch, J. J. Terns Nesting in Boston Harbor: The Importance of Artificial Sites. Bird Observer, 2001.

Mostello, C. Least Tern Fact Sheet. Massachusetts Division of Fisheries and Wildlife, 2015

Mostello, C., Burnham, J., Longsdorf, J., and Veinotte, A. Inventory of Terns, Laughing Gulls, and Black Skimmers Nesting in Massachusetts. Massachusetts Division of Fisheries & Wildlife, 2021.

Trocki, C.L., and P.W.C. Paton. Boston Harbor Islands breeding bird monitoring 2007 field season summary. National Park Service, Boston, MA, 2007.