Category Archives: Birds

Arctic Tern (Sterna paradisaea)

Birds, Kodiak Wildlife, , , , ,

The Arctic tern (Sterna paradisaea) is one of three species of terns found in Alaska. The other two species are the Aleutian tern (Onychoprion aleutica) and the Caspian tern (Sterna caspia). Terns belong to the family Laridae, which also includes gulls.

Arctic terns have a circumpolar range. They breed in the Arctic and subarctic regions of North America, Europe, and Asia, and they winter at the southern tips of Africa and South America, all the way to the edge of the Antarctic ice. In the United States, Arctic terns nest as far south as New England on the east coast and Washington State on the west coast. In Alaska, the Arctic tern has the largest breeding range of any Alaskan water bird. Arctic terns nest from Point Barrow through the Southeast Panhandle, and everywhere in between those two points.

Since Arctic terns breed in Arctic and subarctic areas and then migrate as far as the edge of the Antarctic ice to spend the winter, biologists believe they have the longest migration of any animal. The only animal whose migration may rival that of the Arctic tern is the sooty shearwater which migrates between New Zealand and the North Pacific. In a 2010 study, biologist Carsten Egevang and his colleagues fitted 11 Arctic terns with miniature geolocators, and they learned Arctic terns migrate even further than was previously believed. Some individual terns in the study traveled nearly 50,000 miles (more than 80,000 km) round trip. Because Arctic terns spend summer in the high latitudes of the northern hemisphere and then travel to the high latitudes of the southern hemisphere for the summer there, they see more sunlight every year than any other animal species on the planet.

Arctic terns measure 14 to 17 inches (36-43 cm) in length and have a wingspan of 29 to 33 inches (74-84 cm). Their bodies are white or gray during the breeding season, and a black patch covers the head and forehead. They have a sharply pointed red bill and short red legs. Their deeply forked tail resembles the tail of a swallow and is the reason for their nickname, “Sea swallow.” Terns are agile and quick in the air and can even hover above the water while searching for food. Because they have small, webbed feet, terns do not swim well and do not remain in the water any longer than it takes to catch their prey. A tern flies with its bill pointed down toward the water, and when it sees a fish or other prey, it dives into the water, grasps the prey, and flies away with the fish in its beak. During the non-breeding season, a tern’s legs and beak turn black, and the black patch on the head shrinks. Also during the non-breeding season, terns molt and lose most of their feathers. If they lose their feathers faster than they can be replaced, they may be flightless for a short period.

Arctic terns mate for life, and in Alaska, they arrive at their breeding areas in early to late May. During their courtship, the male performs a “fish flight.” He carries a fish in his bill and flies low over the female on the ground. If she sees him, she will join him in a high climb and flight. Terns nest in solitary pairs or colonies of a few to several hundred pairs. A tern’s nest is little more than a shallow depression in the ground, and nests usually have little or no lining material. Terns nest near fresh or salt water on beaches, spits, and small islands. The female lays one to three eggs that are brown or green and lightly speckled. Both sexes incubate the eggs, and the eggs hatch in about 23 days. The young terns immediately leave the nest and hide in nearby vegetation. The parents catch small fish to feed the chicks for the next 25 days until the chicks have fledged. Arctic terns are very aggressive during the breeding season, and they will attack intruders by crying loudly and repeatedly diving at the intruder’s head. Less than three months after they arrive at their breeding colonies, Arctic terns begin their long migration south.

Arctic terns eat small fish, insects, and invertebrates. During the non-breeding season, they are pelagic and forage at the edges of the pack ice, icebergs, and ice floes near shore.

The worldwide population of Arctic terns is between one and two million breeding pairs. Several hundred thousand pairs nest in Alaska. Because terns nest on the ground, their eggs, and chicks are susceptible to predation by foxes, rats, raccoons, gulls, and other seabirds. Arctic terns are also susceptible to pollution, human disturbance, and decreased food availability due to warming ocean temperatures. Arctic terns may live into their late twenties or early thirties.

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Arctic Terns

Birds, Kodiak Wildlife

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Artic terns are our most punctual spring visitors. Their arrival at the rookery near my house occurs somewhere between May 11th and May 13th each year. They can’t afford to be casual, because they are on a tight schedule. These beautiful, little birds that are distant cousins of gulls have one of the longest migrations of any animal. How far they travel is still not certain, but they fly at least from Alaska to Antarctica and back in a year, a distance of 25,000 miles (40,234 km). Only the sooty shearwater has a migration as long, or perhaps even longer, than an arctic tern’s, since they travel between New Zealand and the North Pacific.

Another interesting fact about Arctic terns is that since they spend summers in Alaska during our long days and then fly to Antarctica for the summer there with the corresponding long days, this species spends more hours in daylight than any other animal.

Arctic terns are slim and graceful. They have the largest breeding range of any water bird in Alaska, nesting from Point Barrow in the north to the Southeastern Panhandle. An adult Arctic tern is gray to white in color. Its pointed beak and its legs are red, and it has a black patch over the head and forehead. It has webbed feet, long, gray wings, and a forked tail. Because of their long wings and forked tails, terns are very agile and can make sudden turns and even hover in place. They look much like swallows and are sometimes called “sea swallows.”

Soon after their arrival in Alaska in May, they choose mates and begin nesting. During mating, the male performs a “fish flight,” in which he carries a small fish in his bill and flies low over a female on the ground. If she notices him, she will join him in a high climb and flight. An Arctic tern’s nest is a simple, scraped, shallow depression in the ground with very little or no lining material. The female lays two eggs that are brown or green in color and are lightly speckled. Both parents incubate the eggs, which hatch in approximately 23 days. As soon as they hatch, the chicks quickly leave the exposed nest and hide in nearby vegetation. The parents bring the chicks small fish to eat for the next 25 days, until the chicks fledge.

Less than three months after their arrival, by about mid-August here, terns leave their breeding areas and start their southward migration. It is interesting to watch the interaction between adults and their chicks when the young terns are just learning to fly. Because their webbed feet are small, terns don’t swim well. They hover over water until they spot a small fish and then dive into the water to catch it, but they don’t sit on the water, so young terns must learn to land on solid surfaces, which is tricky. Once a young tern lands, sometimes it is hesitant to take off again, and the adults will dive-bomb it until they force it to leave its safe perch. This behavior may seem harsh to an observer, but the adults must prepare the young Arctic tern for a 12,000-mile flight in the very near future.