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News An unlikely pair of terns going strong for 13 years

A pair of terns has been producing hybrid chicks in Buzzards Bay since 2007. Biologists were surprised to find offspring produced by a male arctic tern and a female common tern.
  • Division of Fisheries and Wildlife

Media Contact for An unlikely pair of terns going strong for 13 years

Marion Larson, MassWildlife

Hybrid tern chick (Hybrid between arctic and common tern)

In spring, small seabirds called terns abandon their winter homes to trek up to thousands of miles northward to Massachusetts for one purpose: to raise young terns on beaches. For species like terns where males and females share nesting duties, choosing a partner is serious business, especially after a big travel investment! Terns evaluate potential mates and renew bonds with a previous mate through a variety of behaviors. Performing aerial displays, presenting gifts of fish, and conducting other courtship rituals are all part of the “test.”

There are four species of tern that nest in the Commonwealth: common, least, roseate, and arctic. All have black caps, gray backs, and white bellies. While many people find it hard to tell the species apart, the terns know who’s who. The species differ in leg and bill color, pattern on the cap, body size, voice, flight characteristics, and details of courtship behavior. In fact, some of these differences may have evolved to avoid “mistakes” in mate selection. Individuals generally must look, sound, and behave a certain way to be considered suitable partners.

When pairs consisting of individuals of different species do form, they are usually less successful at nesting: many eggs fail to hatch and chicks are less likely to survive. In other words, it is generally not in a tern’s best interest to select a partner of a different species.

Terns are very sensitive to disturbance and require hands-on protection and management to be successful, so MassWildlife works with several of the most important tern colonies annually. In 2007, MassWildlife was surprised to observe a common tern and an arctic tern with two chicks in a large nesting colony in Buzzards Bay. Through genetic testing, the chicks were determined to be the biological offspring of the mixed-species pair—this was the first confirmation that common and arctic terns can hybridize (interbreed).

Given the “tests” potential mates must pass, how and why did this pair form? MassWildlife speculates that there are a couple of main factors. The number of arctic terns nesting in the state in recent years can be counted on one hand and there seem to be more males than females—bad news for our arctic tern, a male. He probably became well aware of his prospects of finding an arctic tern female and eventually considered other options. Massachusetts’ common tern population numbers in  the thousands, however, so why did the female common tern accept him as a mate? Quite possibly youthful ignorance: in 2007, she was likely a first-time nester based on her age, which could be determined by her numbered leg band. MassWildlife bands terns each year to support our monitoring and research studies.

Over the years, this capable pair has produced many apparently healthy hybrid young, at least one of which returned to the island as an adult to nest. As of this past summer, the common and arctic tern were still together—a pair bond lasting at least 13 years, just a year shy of the longest known pair bond for common terns. Stay tuned in 2020!

Media Contact for An unlikely pair of terns going strong for 13 years

Division of Fisheries and Wildlife 

MassWildlife is responsible for the conservation of freshwater fish and wildlife in the Commonwealth, including endangered plants and animals. MassWildlife restores, protects, and manages land for wildlife to thrive and for people to enjoy.