Terns: Characteristics, Behavior, Species and Arctic Tern Migrations

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Arctic tern by James Audobon

Terns are mostly sea birds. They have webbed feet and are good swimmers. Terns are sometimes group together with gulls. Terns are smaller than gulls and have wings and feet that are designed for flying. They tend to be plunge divers capable of diving deeper in the water than gulls. Terns primarily eat small fish they catch by diving into the surface of the sea. A male courting a female, brings her one or two fish, carrying them crossways in her bill. Downy tern chicks are almost invisible crouching in the brush. When an intruder approaches, sometimes the adult flies off and leaves the chicks alone. The chicks don't move regardless of how close the predator approaches

Terns are slender, lightly built birds with long, forked tails, narrow wings, long bills, and relatively short legs. The sexes are identical in appearance, but young birds are readily distinguishable from adults. Terns have a non-breeding plumage, which usually involves a white forehead and much-reduced black cap. Terns are longer-billed, lighter-bodied, and more streamlined than gulls, and their long tails and long narrow wings give them an elegance in flight. Male and female plumages are identical, although the males can be two to five percent larger than the females and often has a relatively larger bill. Sea terns have deeply forked tails, and at least a shallow "V" is shown by all other species. [Source: Wikipedia +]

Terns are long-lived birds and are relatively free from natural predators but most species are declining in number due directly or indirectly to human activities, including habitat loss, pollution, disturbance, and predation by introduced mammals. The Chinese crested tern is critically threatened and three other species are classed as endangered. International agreements provide a measure of protection, but adults and eggs of some species are still used for food in the tropics. The eggs of two species are eaten in the West Indies because they are believed to have aphrodisiac properties. +

Humans have caused health and habitat problems for terns. Humans have damaged eggs and nests and injured and killed chicks because the coastal areas where terns nest overlap with popular beaches and places where people picnic and sunbathe. Nature photographers and bird watchers sometimes disturb nesting terns. Humans also cause problems for the terns with pollution with chemicals that weaken eggshells and cause birth defects. Some adults and chicks die when they become tangled in netting or plastic. In the 19th century, terns were removed from nearly all of their former habitats when they were exploited commercially for their eggs and their feathers. [Source: Kristina Sepe, Animal Diversity Web (ADW) /=]

Websites and Resources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Encyclopedia of Life eol.org; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems ; MarineBio marinebio.org/oceans/creatures

Tern Behavior and Feeding

Terns are birds of the sea and open habitats that typically breed in noisy colonies and lay their eggs on bare ground with little or no nest material. Depending on the species, one to three eggs make up the clutch. Most species feed on fish caught by diving from flight, but the marsh terns are insect-eaters, and some large terns will supplement their diet with small land vertebrates. Many terns are long-distance migrants. [Source: Wikipedia]

Terns have a wide repertoire of vocalisations. The common tern is known for its distinctive kee-yah alarm call, also used as a warning to intruders, and a shorter kyar, given as an individual takes flight in response to a more serious threat. The latter quiets the usually noisy colony while its residents assess the danger. Other calls include a down-slurred keeur given when an adult is approaching the nest with a fish, and a kip uttered during social contact. Parents and chicks can locate one another by call, and siblings also recognise each other's vocalisations from about the twelfth day after hatching, which helps to keep the brood together. Vocal differences reinforce species separation between closely related birds such as the least and little terns, and can help humans distinguish similar species, such as common and Arctic terns, since flight calls are unique to each species.

Arctic tern chick
Most terns hunt fish by diving, often hovering first, and the particular approach technique used can help to distinguish similar species at a distance. Sea terns often hunt in association with porpoises or predatory fish, such as bluefish, tuna or bonitos, since these large marine animals drive the prey to the surface. Terns of several species will feed on invertebrates, following the plough or hunting on foot on mudflats. The marsh terns normally catch insects in the air or pick them off the surface of fresh water. Other species will sometimes use these techniques if the opportunity arises. An individual tern's foraging efficiency increases with its age.

Terns cannot use their eyes underwater. They rely on accurate sighting from the air before they plunge-dive. Like other seabirds that feed at the surface or dive for food, terns have red oil droplets in the cones of their retinas; birds that have to look through an air/water interface have more deeply coloured carotenoid pigments in the oil drops than other species. The pigment also improves visual contrast and sharpens distance vision, especially in hazy conditions, and helps terns to locate shoals of fish.

Tern Species

There are about 40 species of tern, with some debate over species and subspecies classification. The majority of sea terns have light grey or white body plumage as adults, with a black cap to the head. The legs and bill are various combinations of red, orange, yellow, or black depending on species. Most are pale grey above and white below, with a contrasting black cap to the head, but the marsh terns, the Inca tern, and some noddies have dark plumage for at least part of the year. Terns range in size from the least tern, at 23 centimeters (9.1 inches) in length and weighing 30–45 grams (1.1–1.6 ounces) to the Caspian tern at 48–56 centimeters (19–22 inches), 500–700 grams (18–25 ounces). [Source: Wikipedia]

Arctic terns are known for their long migrations. Sooty terns take flight when the leave their nests and don’t land or settle on land until it nests three or four years later. They feed at night as the fish rise to the surface, and are believed to sleep on the wing since they become waterlogged easily. Noddy terns, or noddies, are noted for their tameness. They have unusual notched-wedge shaped tails, the longest tail feathers being the middle-outer, rather than the central or outermost. Although their legs are short, terns can run well. They rarely swim, despite having webbed feet, usually landing on water only to bathe. Gull-billed terns are an opportunist predators that feed on a wide variety of prey from marine, freshwater and land habitats, including small crabs, fish, crayfish, grasshoppers other large insects, lizards, amphibians, mice and the eggs and chicks of least terns, little terns and members of their own species may be victims. The greater crested tern will also occasionally catch unusual vertebrate species such as agamid lizards and green sea turtle hatchlings, and follows trawlers for discards.

Most terns breed on open sandy or rocky areas on coasts and islands. The yellow-billed, large-billed, and black-fronted terns breed only on rivers, and common, least and little terns also sometimes use inland locations. Marsh terns, Trudeau's terns and some Forster's terns nest in inland marshes. Marsh terns construct floating nests from the vegetation in their wetland habitats, and a few species build simple nests in trees, on cliffs or in crevices. White terns uniquely lay a single egg on a bare tree branch. The black noddy and the white tern nest above ground level on cliffs or in trees

Arctic terns are not the only long-distance migrants. Many terns breeding in temperate zones migrate long-distances. A common tern that hatched in Sweden and was found dead five months later on Stewart Island, New Zealand, is estimated to have flown at least 25,000 kilometers (16,000 miles). In addition to Arctic terns, there are Antarctic terns. They range throughout the southern oceans and are found on small islands around Antarctica as well as on the shores of the mainland. Feeding primarily of small fish and crustaceans, they are very similar in appearance to the closely related Arctic terns, but are stockier. The Antarctic tern does not migrate like the Arctic tern does, but it can still be found on a very large range. This tern species is actually more closely related to the South American tern.

Arctic Terns

Arctic terns (Scientific name: Sterna paradisaea) have the longest migrations of any bird. Every years they flies up to 40,233 kilometers (25,000 miles) between the Arctic and the Antarctic and back, with much of the flying done over water. It nests on the Arctic region. When the young are old enough to fly the whole family flies south to Antarctica. The tern’s wings are long and powerful and they can make steady progress even against strong headwinds. Their average lifespan in the wild is 34 years.

Arctic terns begin their journeys southward in August, Those that winter in Arctic Canada and Greenland fly across the Atlantic and meet those that have flown south from the Russia Arctic. Some fly to the Cape of Good Hope in South Africa via West Africa. Others fly across the Atlantic again to the east coast of South America to the Cape Horn. A third group follows the Pacific coast from Alaska to Chile. All three groups fly to Antarctica. In February they begin flying back north.

Arctic terns are native to Arctic regions and have been introduced to other places. All terns live along seacoasts and around interior lakes and marshes. Arctic terns live in tundra and taiga areas, forests and icecaps and along lakes and coastal areas in the circumpolar region. Among the places with nestin sites are Northern European islands and peninsulas from Iceland to Northern Russia-Siberia, the British Isles, Netherlands, Denmark, Sweden, the Baltic Nations, Northern Alaska, extreme of Northern Canada, Greenland, Newfoundland, and south along Atlantic Coast to Massachusetts. The birds winter in s in S. Hemisphere in subantarctic and Antarctic waters of Atlantic, Pacific, and Indian Oceans. [Source: Robin Street, Animal Diversity Web (ADW) /=]

Arctic terns were once hunted for their feathers. Now there populations are fairly healthy and stable. The International Union for Conservation of Nature (IUCN) Red List classifies them as a species of “Least Concern”. They have no special status according to the Convention on the International Trade in Endangered Species (CITES). Foxes, raccoons, weasels, rats, gulls, and other seabirds are all predators of terns and their eggs. Massive spraying of marshes with insecticides such as DDT for mosquito control killed many terns through their consumption of DDT-laden minnows. In the last decade of the 19th century and in the first decade of the 20th century, plume hunters killed tens of thousands of terns for their plumage for women's hats.

Arctic Tern Characteristics, Behavior, Reproduction and Feeding

Arctic terns are 36 to 43 centimeters (14 to 17 inches) long with a wingspread of 74 to 84 centimeters (29 to 33 inches). Their average weight is 100 grams (3.52 ounces) /=\ They are white with black caps and gray mantles, and a deeply-forked tail. In spring and summer, the entire bill is blood-red. Their legs are so short that the birds appear to be crouched when standing. [Source: Robin Street, Animal Diversity Web (ADW) /=]

Arctic terns walk with a mouse-like glide and utter a shrill "kee-kee" or "kee-kahr". Many migrate from the Arctic summer breeding grounds to Antarctica for a "second summer", making them the animal that sees the most sun. Terns are very sociable and nest in large colonies, which are not always in the same area each year. They are vigorous in defending their nesting colonies and are most successful in breeding when they nest close together.

Arctic terns feed on small fishes such as capelin, sand launae, sand eel, and small crustaceans. They sense using vision, touch, sound and chemicals usually detected with smell. When hunting they hover 30-40 feet over the water on beating wings and then dive suddenly into the water with a splash, often completely submerging to catch their prey.

Arctic terns nest in colonies defended by the males in the rocky or sandy beaches of the far north. Tern courtship is performed through a "fish flight" by the male: after much aerial chasing and screaming, the male offers a small fish to the female. They can even scream with a fish in their mouths. Most terns mate for life. The nests usually consist of a hollow in sand, gravel or moss. In June-July, two or three brown or greenish eggs with brown speckles are incubated for 21-22 days. Young fly about 21-28 days after hatching

Arctic Tern Migrations

Arctic terns are famous for their annual migrations between Arctic breeding grounds and the Antarctic and back again each year. It is estimated that these birds probably sees more annual daylight than any other animal as they experience two summers per year and do so where the daylight hours are particularly long. They terns usually migrate over open ocean sufficiently far from coastal area so the birds are rarely seen from land outside the breeding season. [Source: Wikipedia]

The shortest distance between the Arctic and Antarctica is 19,000 kilometers (12,000 miles). A return journey involves doubling that figure, with 38,000 kilometers (24,000 miles) being the result. Actual flight distances are, of course, much greater than the shortest possible route. Arctic terns from Greenland were shown by radio geolocation to average 70,000 kilometers (43,000 miles) on their annual migrations.

In the summer of 1982, an Arctic tern was ringed as an unfledged chick on the Farne Islands, Northumberland, U.K, It reached Melbourne, Australia in October, just three months after fledging — a journey of more than 22,000 kilometers (14,000 miles). In July 1928, a chick ringed in Labrador, Canada was found in South Africa four months later. A 2013 tracking study of half a dozen Arctic terns breeding in the Netherlands showed an average annual migrations of around 48,700 kilometers (30,300 miles). On their way south, these birds roughly followed the coastlines of Europe and Africa.

Arctic tern nesting areas in red, migration routes in green and wintering areas in blue

A 2010 study using tracking devices attached to the birds showed that previous research had seriously underestimated the annual distances travelled by the Arctic tern. Eleven birds that bred in Greenland or Iceland covered 70,900 kilometers (44,100 miles) on average in a year, with a maximum of 81,600 km (50,700 miles). The difference from previous estimates is due to the birds taking meandering courses rather than following a straight route as was previously assumed. The birds follow a somewhat convoluted course in order to take advantage of prevailing winds. The average Arctic tern lives about 30 years and thus, based on the above research, travels some 2.4 million kilometers (1.5 million miles) during their lifetime — the equivalent of three roundtrip journeys between the Earth to the Moon.

Common Terns

Common terns (Scientific name: Sterna hirundo) are found from northern Canada south to the Caribbean Sea, as well as throughout Europe, Northern Asia, North Africa, and the Middle East. Some winter as far south as Peru and Argentina. The average lifespan is about nine to 10 years. The oldest common tern ever recorded was 25 years old.[Source: Kristina Sepe, Animal Diversity Web (ADW) /=]

Common terns live in temperate, land, saltwater, marine and freshwater environments in lakes and coastal areas. They almost always reside in colonies. The colonies tend to be along ocean coasts, although they are also found on the shores of large lakes. The two things necessary for a colony of terns, or a "ternery," are isolation from predators and a reliable source of food nearby. The birds also must be able to communicate visually and vocally with the rest of the colony from their nests. They nest among rocks and cliffs.

In the 19th century, terns were exploited commercially for their eggs and feathers but their populations are considered stable and healthy today. The International Union for Conservation of Nature (IUCN) Red List classifies them as a species of “Least Concern”. They have no special status according to the Convention on the International Trade in Endangered Species (CITES).

Common Tern Characteristics, Behavior and Communication

Common terns have an average weight of 120 grams (4.23 ounces), an average length of 37 centimeters (14.57 inches) and an average wingspan of 27 centimeters (10.63 inches). Both sexes are roughly equal in size and look similar. Male are a little larger. [Source: Kristina Sepe, Animal Diversity Web (ADW) /=]

common tern with fish

According to Animal Diversity Web: Because the coloring of common terns changes significantly as seasons change, they are often difficult to identify from plumage alone. They are most easily identified by their black head and red bill. The tail is forked and the tail feathers are more elongated than those of most terns. The wings are pointed and the inner and outer parts of each wing are the same width. The body of common terns is whitish-gray and the underparts are much paler than the upperparts, particularly in adult terns. The female is usually smaller than the male, although only slightly. The bill is usually pointed downward when the tern flies. Other notable characteristics include an exceptionally powerful head and neck and unusually long legs, which distinguish them from other terns such as Arctic terns. /=\

Common terns are motile (move around as opposed to being stationary), migratory (make seasonal movements between regions, such as between breeding and wintering grounds), territorial (defend an area within the home range), social (associates with others of its species; forms social groups) and colonial (living together in groups or in close proximity to each other).

In common tern colonies there is no clearly organized hierarchy among the birds; all appear to be equal. Although all the terns migrate and live together, each family unit is responsible for its own feeding and care of eggs and chicks. They often defend feeding territories. Terns nest during the breeding season and they migrate at the end of the season. Common terns and sense using vision, touch, sound and chemicals usually detected with smell. They communicate with vision, touch and sound, mostly with their unusual, hoarse voices, and they have three different, distinct calls. During mating, communication is mainly visual and tactile.

Common Terns Feeding and Predators

The diet of common terns is often limited to fishes such as whiting, herring, and haddock. They also eat sand launces, insects, crabs, shrimp, other crustaceans, annelids, mollusks, fish eggs, and in certain cases, echinoderms. Terns that nest near bodies of fresh water often consume minnows in place of fish like herring and whiting.

Often, if food is particularly abundant, the terns will catch more fish than necessary. Common terns sometimes catch fish that are too big for them to swallow. This, combined with their tendency to catch as much as is available even if it is not needed, explains why it is not unusual to see fish scattered around terns' nesting grounds. At the beginning of the breeding season, terns may eat insects, annelids, and echinoderms in addition to fish. However, throughout the later parts of the breeding season, the tern's diet is much more limited. [Source: Kristina Sepe, Animal Diversity Web (ADW) /=]

Common terns are very good at catching insects. Many have been reported to fly near the surface of the water and pick insects off the surface while in flight. It is rare for them to eat dead food. They usually fly at great heights before diving for their prey, a behavior uncommon in other terns. The terns dive into the water after a fish, come to the surface, shake the water from themselves, and fly off with the fish. When a solitary tern catches fish in the same spot repeatedly, other terns from its colony join it. /=\

Known predators include: red foxes, raccoons, striped skunks, minks, long-tailed weasels, squirrels, dogs, cats, rats, gulls, herons, hawks, falcons, owls, blue jays, grackles, reptiles and ants. When a predator comes too close to a tern colony, any terns that spot the predator begin to call loudly to the rest of the colony. Adult terns come over to mob the predator while the chicks take cover in the high grass or in their nests.

Also, the sheer number of terns in a colony aids in the strategy of "passive avoidance". In other words, the probability of any one tern being harmed by a predator is much less because of the number of other terns that the predator could choose instead. A common tactic among members of colonies, and in fact among all members of tern colonies and gull colonies, is called a "panic." This means that an entire colony of terns flies up making noise, falls silent suddenly, and then swoops back down toward the ground. This can be very threatening to potential predators and often assures that the colony will be left alone, particularly by smaller predators such as blue jays or grackles. /=\

Common Terns Reproduction, Nesting and Offspring

Arctic tern
Common terns engage in seasonal breeding. It is rare for a pair to produce more than one clutch per summer. The breeding season is from early to mid-summer. The average number of eggs per season is three. The time to hatching ranges from three to 4 weeks, with the fledging age ranging from 27 to 30 days. On average males and females reach sexual maturity at age three years. On average males reach sexual maturity at three years. [Source: Kristina Sepe, Animal Diversity Web (ADW) /=]

Common terns are monogamous (having one mate at a time). They migrating to their breeding grounds shortly after the beginning of spring. In April, male terns establish their territories at the colony and engage in what is called "courtship feeding," in which they bring fish to females as a way of courting them. Premating displays are accompanied by the male tern posturing followed by the two terns circling each other. The males mount the females for one- to two-minute intervals before copulation actually takes place. Common terns are known for wildly flapping their wings during and directly after copulating. /=\

It is rare to see a pair of common terns produce more than one clutch per summer. They nest among rocks and cliffs. The nests are made up of shells and debris or of dead vegetation. During the pre-weaning stage provisioning and protecting are done by females and males. One of the parents attends the nest at all times after the eggs hatch; the female is often, but not always, the one standing guard. Common terns become very aggressive after their chicks learn to move on their own because of the likelihood that the chicks will be harmed or killed by predators. Both males and females bring food back to the nest, but males are usually more involved in feeding than females are. Chicks are semi- precocial (hatched with eyes open, covered with down, and capable of leaving the nest soon after hatching). Young terns usually learn to fly when they are 27 to 30 days old. but do not reach sexual maturity for three years. /=\

Image Sources: Wikimedia Commons, NOAA, Tern species from "The foraging ecology of Caspian Terns (Hydroprogne caspia) on Peel-Harvey Estuary, south-western Australia" by S. Stockwell, Environmental Science, 2019

Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated May 2023

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