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Terns are
seabird Seabirds (also known as marine birds) are birds that are adapted to life within the marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same envir ...
s in the
family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...
Laridae that have a worldwide distribution and are normally found near the
sea The sea, connected as the world ocean or simply the ocean, is the body of salty water that covers approximately 71% of the Earth's surface. The word sea is also used to denote second-order sections of the sea, such as the Mediterranean Sea, ...
,
river A river is a natural flowing watercourse, usually freshwater, flowing towards an ocean, sea, lake or another river. In some cases, a river flows into the ground and becomes dry at the end of its course without reaching another body of ...
s, or
wetland A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free (Anoxic waters, anoxic) processes prevailing, especially in t ...
s. Terns are treated as a subgroup of the family Laridae which includes
gull Gulls, or colloquially seagulls, are seabirds of the family Laridae in the suborder Lari. They are most closely related to the terns and skimmers and only distantly related to auks, and even more distantly to waders. Until the 21st century ...
s and
skimmer Skimmer may refer to: Animals *Skimmer (bird), a common name for birds in the genus ''Rynchops'' *Skimmer (dragonfly), a common name for dragonflies in the family Libellulidae *Water strider or skimmer, a common name for insects in the family Ge ...
s and consists of eleven
genera Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclat ...
. They are slender, lightly built birds with long, forked tails, narrow wings, long bills, and relatively short legs. Most species are pale grey above and white below, with a contrasting black cap to the head, but the
marsh tern The name marsh tern refers to terns of the genus ''Chlidonias'', which are typically found in freshwater marshes, rather than coastal locations. The genus name ''Chlidonias'' is from Ancient Greek , "swallow-like", from , " swallow". Species T ...
s, the Inca tern, and some noddies have dark
plumage Plumage ( "feather") is a layer of feathers that covers a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage differ between species and subspecies and may vary with age classes. Within species, ...
for at least part of the year. 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 long-lived birds and are relatively free from natural predators and
parasites Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...
; most species are declining in numbers due directly or indirectly to human activities, including habitat loss, pollution, disturbance, and predation by
introduced mammal An introduced species, alien species, exotic species, adventive species, immigrant species, foreign species, non-indigenous species, or non-native species is a species living outside its native distributional range, but which has arrived there ...
s. The
Chinese crested tern The Chinese crested tern (''Thalasseus bernsteini'') is a tern in the family Laridae. It is the county bird of Lienchiang County, Fuchien. Description It is closely related to the Sandwich tern, ''T. sandvicensis'', and the lesser crested te ...
is in a
critical Critical or Critically may refer to: *Critical, or critical but stable, medical states **Critical, or intensive care medicine * Critical juncture, a discontinuous change studied in the social sciences. *Critical Software, a company specializing ...
situation and three other species are classed as
endangered An endangered species is a species that is very likely to become extinct in the near future, either worldwide or in a particular political jurisdiction. Endangered species may be at risk due to factors such as habitat loss, poaching and in ...
. 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 The West Indies is a subregion of North America, surrounded by the North Atlantic Ocean and the Caribbean Sea that includes 13 independent island countries and 18 dependencies and other territories in three major archipelagos: the Greate ...
because they are believed to have
aphrodisiac An aphrodisiac is a substance that increases sexual desire, sexual attraction, sexual pleasure, or sexual behavior. Substances range from a variety of plants, spices, foods, and synthetic chemicals. Natural aphrodisiacs like cannabis or cocai ...
properties.


Description

Terns range in size from the least tern, at in length and weighing ,Maehr & Kale (2005) p. 111.Olsen & Larsson (1995) p. 136. to the Caspian tern at , .Harrison (1988) p. 368. They 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 male can be 2–5% larger than the female 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. The noddies (genera ''Anous'', ''Procelsterna'' and ''Gygis'') have unusual notched-wedge shaped tails, the longest tail feathers being the middle-outer, rather than the central or outermost.Harrison (1988) pp. 387–390.Hutton & Drummond (2011) p. 226. Although their legs are short, terns can run well. They rarely swim, despite having webbed feet, usually landing on water only to bathe. The majority of sea terns have light grey or white body
plumage Plumage ( "feather") is a layer of feathers that covers a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage differ between species and subspecies and may vary with age classes. Within species, ...
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. The pale plumage is conspicuous from a distance at sea, and may attract other birds to a good feeding area for these fish-eating species. When seen against the sky, the white underparts also help to hide the hunting bird from its intended prey. The Inca tern has mainly dark plumage, and three species that mainly eat insects, the black tern, white-winged tern, and black-bellied tern, have black underparts in the breeding season. The ''Anous'' noddies have dark plumage with a pale head cap. The reason for their dark plumage is unknown, but it has been suggested that in tropical areas, where food resources are scarce, the less conspicuous colouration makes it harder for other noddies to detect a feeding bird. Plumage type, especially the head pattern, is linked to the
phylogeny A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological s ...
of the terns, and the pale-capped, dark-bodied noddies are believed to have diverged earlier than the other genera from an ancestral white-headed gull, followed by the partially black-headed ''
Onychoprion ''Onychoprion'', the "brown-backed terns", is a genus of seabirds in the family Laridae. The genus name is from Ancient Greek , "claw" or "nail", and , "saw". Species Although the genus was first described in 1832 by Johann Georg Wagler the fou ...
'' and '' Sternula'' groupings. Juvenile terns typically have brown- or yellow-tinged upperparts, and the feathers have dark edges that give the plumage a scaly appearance. They have dark bands on the wings and short tails. In most species, the subsequent
moult In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is the manner in which an animal routinely casts off a part of its body (often, but not always, an outer ...
does not start until after migration, the plumage then becoming more like the adult, but with some retained juvenile feathers and a white forehead with only a partial dark cap. By the second summer, the appearance is very like the adult, and full mature plumage is usually attained by the third year. After breeding, terns moult into a winter plumage, typically showing a white forehead. Heavily worn or aberrant plumages such as melanism and
albinism Albinism is the congenital absence of melanin in an animal or plant resulting in white hair, feathers, scales and skin and pink or blue eyes. Individuals with the condition are referred to as albino. Varied use and interpretation of the term ...
are much rarer in terns than in gulls.


Voice

Terns have a wide repertoire of vocalisations. For example, the
common tern The common tern (''Sterna hirundo'') is a seabird in the family Laridae. This bird has a circumpolar distribution, its four subspecies breeding in temperate and subarctic regions of Europe, Asia and North America. It is strongly migrator ...
has a distinctive
alarm An alarm device is a mechanism that gives an audible, visual or other kind of alarm signal to alert someone to a problem or condition that requires urgent attention. Alphabetical musical instruments Etymology The word ''alarm'' comes from t ...
, ''kee-yah'', also used as a warning to intruders, and a shorter ''kyar'', given as an individual takes flight in response to a more serious threat; this quietens 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.Hume (1993) pp. 68–75. Parents and chicks can locate one another by call,Burton (1985) p. 123. and
sibling A sibling is a relative that shares at least one parent with the subject. A male sibling is a brother and a female sibling is a sister. A person with no siblings is an only child. While some circumstances can cause siblings to be raised separa ...
s 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 tern The Arctic tern (''Sterna paradisaea'') is a tern in the family Laridae. This bird has a circumpolar breeding distribution covering the Arctic and sub-Arctic regions of Europe (as far south as Brittany), Asia, and North America (as far south ...
s, since flight calls are unique to each species.Constantine (2006) pp. 73–77.


Taxonomy

The bird
order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...
Charadriiformes Charadriiformes (, from ''Charadrius'', the type genus of family Charadriidae) is a diverse order of small to medium-large birds. It includes about 390 species and has members in all parts of the world. Most charadriiform birds live near water a ...
contains 18 coastal seabird and
wader 245px, A flock of Dunlins and Red knots">Red_knot.html" ;"title="Dunlins and Red knot">Dunlins and Red knots Waders or shorebirds are birds of the order Charadriiformes commonly found wikt:wade#Etymology 1, wading along shorelines and mudflat ...
families. Within the order, the terns form a lineage with the
gull Gulls, or colloquially seagulls, are seabirds of the family Laridae in the suborder Lari. They are most closely related to the terns and skimmers and only distantly related to auks, and even more distantly to waders. Until the 21st century ...
s, and, less closely, with the
skimmer Skimmer may refer to: Animals *Skimmer (bird), a common name for birds in the genus ''Rynchops'' *Skimmer (dragonfly), a common name for dragonflies in the family Libellulidae *Water strider or skimmer, a common name for insects in the family Ge ...
s,
skua The skuas are a group of predatory seabirds with seven species forming the genus ''Stercorarius'', the only genus in the family Stercorariidae. The three smaller skuas, the long-tailed skua, the Arctic skua, and the pomarine skua are calle ...
s, and
auk An auk or alcid is a bird of the family Alcidae in the order Charadriiformes. The alcid family includes the murres, guillemots, auklets, puffins, and murrelets. The word "auk" is derived from Icelandic ''álka'', from Old Norse ''alka'' (a ...
s. Early authors such as
Conrad Gessner Conrad Gessner (; la, Conradus Gesnerus 26 March 1516 – 13 December 1565) was a Swiss physician, naturalist, bibliographer, and philologist. Born into a poor family in Zürich, Switzerland, his father and teachers quickly realised his tale ...
, Francis Willughby, and William Turner did not clearly separate terns from gulls, but
Linnaeus Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his ennoblement in 1761 as Carl von Linné Blunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomial nomenclature, the ...
recognised the distinction in his 1758 ''
Systema Naturae ' (originally in Latin written ' with the ligature æ) is one of the major works of the Swedish botanist, zoologist and physician Carl Linnaeus (1707–1778) and introduced the Linnaean taxonomy. Although the system, now known as binomial ...
'', placing the gulls in the genus ''
Larus ''Larus'' is a large genus of gulls with worldwide distribution (by far the greatest species diversity is in the Northern Hemisphere). Many of its species are abundant and well-known birds in their ranges. Until about 2005–2007, most gu ...
'' and the terns in ''Sterna''. He gave ''Sterna'' the description ''rostrum subulatum'', " awl-shaped bill", referring to the long, pointed bills typical of this group of birds, a feature that distinguishes them from the thicker-billed gulls.Linnaeus (1758) p. 84.Jobling (2010) p. 338. Behaviour and morphology suggest that the terns are more closely related to the gulls than to the skimmers or skuas, and although
Charles Lucien Bonaparte Charles Lucien Jules Laurent Bonaparte, 2nd Prince of Canino and Musignano (24 May 1803 – 29 July 1857), was a French naturalist and ornithologist. Lucien and his wife had twelve children, including Cardinal Lucien Bonaparte. Life and career ...
created the family Sternidae for the terns in 1838, for many years they were considered to be a subfamily, Sterninae, of the gull family, Laridae. Relationships between various tern species, and between the terns and the other Charadriiformes, were formerly difficult to resolve because of a poor
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
record and the misidentification of some finds.Gochfeld & Burger (1996) pp. 624–645. Following
gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...
tic research in the early twenty-first century, the terns were historically treated as a separate family: Sternidae. Most terns were formerly treated as belonging to one large genus, '' Sterna'', with just a few dark species placed in other genera; in 1959, only the noddies and the Inca tern were excluded from ''Sterna''. A recent analysis of DNA sequences supported the splitting of ''Sterna'' into several smaller genera. One study of part of the cytochrome b gene sequence found a close relationship between terns and a group of waders in the suborder
Thinocori The seedsnipes are a small family, Thinocoridae, of small gregarious waders which have adapted to a herbivorous diet. The family is divided into two genera, '' Attagis'' and '' Thinocorus'', each containing two species. The family has a South A ...
. These results are in disagreement with other molecular and morphological studies, and have been interpreted as showing either a large degree of molecular
convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...
between the terns and these waders, or the retention of an ancient
genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
. The word "stearn" was used for these birds in
Old English Old English (, ), or Anglo-Saxon, is the earliest recorded form of the English language, spoken in England and southern and eastern Scotland in the early Middle Ages. It was brought to Great Britain by Anglo-Saxon settlers in the mid-5th ...
as early as the eighth century, and appears in the poem '' The Seafarer'', written in the ninth century or earlier. Variants such as "tearn" occurred by the eleventh century, although the older form lingered on in Norfolk dialect for several centuries.Hume (1993) pp. 12–13. As now, the term was used for the inland black tern as well as the marine species.Jobling (2010) p. 365.Lockwood (1984) pp. 147, 153. Some authorities consider "tearn" and similar forms to be variants of "stearn", while others derive the English words from Scandinavian equivalents such as Danish and Norwegian ''terne'' or Swedish ''tärna'', and ultimately from
Old Norse Old Norse, Old Nordic, or Old Scandinavian, is a stage of development of North Germanic dialects before their final divergence into separate Nordic languages. Old Norse was spoken by inhabitants of Scandinavia and their overseas settlement ...
''þerna''.Merriam-Webster (2014) Linnaeus adopted "stearn" or "sterna" (which the naturalist William Turner had used in 1544 as a Latinisation of an English word, presumably "stern", for the black tern)Turner (1544), p. 78. or a North Germanic equivalent for his genus name ''Sterna''.Linnaeus (1758) p. 155.


Species

The
cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...
shows the relationships between the tern genera, and the currently recognised species, based on
mitochondrial DNA Mitochondrial DNA (mtDNA or mDNA) is the DNA located in mitochondria, cellular organelles within eukaryotic cells that convert chemical energy from food into a form that cells can use, such as adenosine triphosphate (ATP). Mitochondrial D ...
studies, are listed below: In addition to extant species, the fossil record includes a
Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...
palaeospecies, '' Sterna milne-edwardsii''. The genera ''Anous'', ''Procelsterna'' and ''Gygis'' are collectively known as noddies, the ''Chlidonias'' species are the marsh terns, and all other species comprise the sea terns.Barlow ''et al.'' (1987) p. 212.Perrins & Arlott (1987) p. 130.


Distribution and habitat

Terns have a worldwide distribution, breeding on all continents including Antarctica. The northernmost and southernmost breeders are the Arctic tern and Antarctic tern respectively.Harrison (1988) pp. 370–371. Many terns breeding in temperate zones are long-distance migrants, and the Arctic tern probably sees more annual daylight than any other animal as it migrates from its northern breeding grounds to Antarctic waters, a return journey of more than . A common tern that hatched in Sweden and was found dead five months later on
Stewart Island Stewart Island ( mi, Rakiura, 'Aurora, glowing skies', officially Stewart Island / Rakiura) is New Zealand's third-largest island, located south of the South Island, across the Foveaux Strait. It is a roughly triangular island with a total ...
,
New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island coun ...
, must have flown at least .Newton (2010) pp. 150–151. Actual flight distances are, of course, much greater than the shortest possible route. Arctic terns from Greenland were shown by radio geolocation to average on their annual migrations. Most terns breed on open sandy or rocky areas on coasts and islands. The yellow-billed, large-billed, and
black-fronted tern The black-fronted tern (''Chlidonias albostriatus''), also known as sea martin, ploughboy, inland tern, riverbed tern or tarapiroe,Rod Morris and Alison Ballance, ''"Rare Wildlife of New Zealand"'', Random House, 2008 is a small tern generally f ...
s breed only on rivers, and common, least and little terns also sometimes use inland locations. The
marsh tern The name marsh tern refers to terns of the genus ''Chlidonias'', which are typically found in freshwater marshes, rather than coastal locations. The genus name ''Chlidonias'' is from Ancient Greek , "swallow-like", from , " swallow". Species T ...
s,
Trudeau's tern The snowy-crowned tern (''Sterna trudeaui''), also known as Trudeau's tern, is a species of tern in the family Laridae. It is found in Argentina, south-east Brazil, Chile, Paraguay and Uruguay, and as a vagrant in the Falkland Islands. The tern's ...
and some Forster's terns nest in inland marshes. The
black noddy The black noddy or white-capped noddy (''Anous minutus'') is a seabird from the family Laridae. It is a medium-sized species of tern with black plumage and a white cap. It closely resembles the lesser noddy (''Anous tenuirostris'') with which i ...
and the white tern nest above ground level on cliffs or in trees. Migratory terns move to the coast after breeding, and most species winter near land, although some marine species, like the
Aleutian tern The Aleutian tern (''Onychoprion aleuticus'') is a migratory bird living in the subarctic region of the globe most of the year. It is frequently associated with the Arctic tern, which it closely resembles. While both species have a black cap, the ...
, may wander far from land. The
sooty tern The sooty tern (''Onychoprion fuscatus'') is a seabird in the family Laridae. It is a bird of the tropical oceans, returning to land only to breed on islands throughout the equatorial zone. Taxonomy The sooty tern was described by Carl Linnae ...
is entirely oceanic when not breeding, and healthy young birds are not seen on land for up to five years after fledging until they return to breed. They lack waterproof plumage, so they cannot rest on the sea. Where they spend the years prior to breeding is unknown.


Behaviour

The terns are birds of open habitats that typically breed in noisy
colonies In modern parlance, a colony is a territory subject to a form of foreign rule. Though dominated by the foreign colonizers, colonies remain separate from the administration of the original country of the colonizers, the '' metropolitan state'' ...
and lay their eggs on bare ground with little or no nest material. 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. The white tern, uniquely, lays its single egg on a bare tree branch. Depending on the species, one to three eggs make up the
clutch A clutch is a mechanical device that engages and disengages power transmission, especially from a drive shaft to a driven shaft. In the simplest application, clutches connect and disconnect two rotating shafts (drive shafts or line shafts). ...
. 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
vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
s. Many terns are long-distance migrants, and the
Arctic tern The Arctic tern (''Sterna paradisaea'') is a tern in the family Laridae. This bird has a circumpolar breeding distribution covering the Arctic and sub-Arctic regions of Europe (as far south as Brittany), Asia, and North America (as far south ...
may see more daylight in a year than any other animal.


Breeding

Terns are normally
monogamous Monogamy ( ) is a form of dyadic relationship in which an individual has only one partner during their lifetime. Alternately, only one partner at any one time ( serial monogamy) — as compared to the various forms of non-monogamy (e.g., pol ...
, although trios or female-female pairings have been observed in at least three species. Most terns breed annually and at the same time of year, but some tropical species may nest at intervals shorter than 12 months or asynchronously. Most terns become sexually mature when aged three, although some small species may breed in their second year. Some large sea terns, including the sooty and
bridled tern The bridled tern (''Onychoprion anaethetus'') is a seabird of the family Laridae. It is a bird of the tropical oceans. The scientific name is from Ancient Greek. The genus comes from ' meaning "claw" or "nail", and , meaning "saw". The specific ...
s, are four or older when they first breed. Terns normally breed in
colonies In modern parlance, a colony is a territory subject to a form of foreign rule. Though dominated by the foreign colonizers, colonies remain separate from the administration of the original country of the colonizers, the '' metropolitan state'' ...
, and are site-faithful if their habitat is sufficiently stable. A few species nest in small or dispersed groups, but most breed in colonies of up to a few hundred pairs, often alongside other seabirds such as gulls or skimmers. Large tern species tend to form larger colonies, which in the case of the sooty tern can contain up to two million pairs. Large species nest very close together and sit tightly, making it difficult for aerial predators to land among them. Smaller species are less closely packed and mob intruders. Peruvian and
Damara tern The Damara tern (''Sternula balaenarum'') is a species of small tern in the family Laridae which breeds in the southern summer in southern Africa and migrates to tropical African coasts to winter. Description At in length the Damara tern is a ...
s have small dispersed colonies and rely on the
cryptic Cryptic may refer to: In science: * Cryptic species complex, a group of species that are very difficult to distinguish from one another * Crypsis, the ability of animals to blend in to avoid observation * Cryptic era, earliest period of the Earth ...
plumage of the eggs and young for protection. The male selects a
territory A territory is an area of land, sea, or space, particularly belonging or connected to a country, person, or animal. In international politics, a territory is usually either the total area from which a state may extract power resources or a ...
, which he defends against conspecifics, and re-establishes a
pair bond In biology, a pair bond is the strong affinity that develops in some species between a mating pair, often leading to the production and rearing of offspring and potentially a lifelong bond. Pair-bonding is a term coined in the 1940s that is freque ...
with his mate or attracts a new female if necessary. Courtship involves ritualised flight and ground displays, and the male often presents a fish to his partner. Most species have little or no nest, laying the eggs onto bare ground, but Trudeau's tern, Forster's tern and the marsh terns construct floating nests from the vegetation in their wetland habitats. Black and lesser noddies build nests of twigs, feathers and excreta on tree branches, and
brown Brown is a color. It can be considered a composite color, but it is mainly a darker shade of orange. In the CMYK color model used in printing or painting, brown is usually made by combining the colors orange and black. In the RGB color model ...
,
blue Blue is one of the three primary colours in the RYB colour model (traditional colour theory), as well as in the RGB (additive) colour model. It lies between violet and cyan on the spectrum of visible light. The eye perceives blue when ...
, and grey noddies make rough platforms of grass and seaweed on cliff ledges, in cavities or on other rocky surfaces.Watling (2003) pp. 206–207. The Inca tern nests in crevices, caves and disused burrows, such as that of a
Humboldt penguin The Humboldt penguin (''Spheniscus humboldti'') is a medium-sized penguin. It resides in South America, its range mainly contains most of coastal Peru. Its nearest relatives are the African penguin, the Magellanic penguin and the Galápagos pen ...
. The white tern is unique in that it lays its single egg on a bare tree branch. Tropical species usually lay just one egg, but two or three is typical in cooler regions if there is an adequate food supply. The time taken to complete the
clutch A clutch is a mechanical device that engages and disengages power transmission, especially from a drive shaft to a driven shaft. In the simplest application, clutches connect and disconnect two rotating shafts (drive shafts or line shafts). ...
varies, but for temperate species incubation takes 21–28 days. The eggs of most gulls and terns are brown with dark splotches, so they are difficult for predators to spot on the beach. The precocial chicks
fledge Fledging is the stage in a flying animal's life between hatching or birth and becoming capable of flight. This term is most frequently applied to birds, but is also used for bats. For altricial birds, those that spend more time in vulnera ...
in about four weeks after hatching. Tropical species take longer because of the poorer food supply. Both parents incubate the eggs and feed the chicks, although the female does more incubating and less fishing than her partner. Young birds migrate with the adults. Terns are generally long-lived birds, with individuals typically returning for 7–10 breeding seasons. Maximum known ages include 34 for an Arctic tern and 32 for a sooty. Although several other species are known to live in captivity for up to 20 years, their greatest recorded ages are underestimates because the birds can outlive their rings. Interbreeding between tern species is rare, and involves closely related species when it occurs. Hybrids recorded include common tern with roseate, Sandwich with lesser-crested, and black with white-winged.Olsen & Larsson (1995) p. 10.


Feeding

Most terns hunt fish by diving, often hovering first, and the particular approach technique used can help to distinguish similar species at a distance.Hume & Pearson (1993) pp. 54–55. Sea terns often hunt in association with
porpoise Porpoises are a group of fully aquatic marine mammals, all of which are classified under the family Phocoenidae, parvorder Odontoceti (toothed whales). Although similar in appearance to dolphins, they are more closely related to narwhals a ...
s or predatory fish, such as
bluefish The bluefish (''Pomatomus saltatrix'') is the only extant species of the family Pomatomidae. It is a marine pelagic fish found around the world in temperate and subtropical waters, except for the northern Pacific Ocean. Bluefish are known as t ...
,
tuna A tuna is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae (mackerel) family. The Thunnini comprise 15 species across five genera, the sizes of which vary greatly, ranging from the bullet tuna (max length: ...
or
bonito Bonitos are a tribe of medium-sized, ray-finned predatory fish in the family Scombridae – a family it shares with the mackerel, tuna, and Spanish mackerel tribes, and also the butterfly kingfish. Also called the tribe Sardini, it consists ...
s, since these large marine animals drive the prey to the surface. Sooty terns feed at night as the fish rise to the surface, and are believed to sleep on the wing since they become waterlogged easily. Terns of several species will feed on
invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chorda ...
s, following the plough or hunting on foot on
mudflat Mudflats or mud flats, also known as tidal flats or, in Ireland, slob or slobs, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A global analysis published in 2019 suggested that tidal f ...
s. 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.Svensson ''et al.'' (2009) p. 206. An individual tern's foraging efficiency increases with its age.Burger J; Gochfeld M "Laridae, Sternidae and Rynchopidae" in Steele ''et al.'' (2001) p. 1469–1480. The gull-billed tern is an opportunist predator, taking a wide variety of prey from marine, freshwater and terrestrial habitats. Depending on what is available it will eat small crabs, fish,
crayfish Crayfish are freshwater crustaceans belonging to the clade Astacidea, which also contains lobsters. In some locations, they are also known as crawfish, craydids, crawdaddies, crawdads, freshwater lobsters, mountain lobsters, rock lobsters, m ...
,
grasshopper Grasshoppers are a group of insects belonging to the suborder Caelifera. They are among what is possibly the most ancient living group of chewing herbivorous insects, dating back to the early Triassic around 250 million years ago. Grasshopp ...
s and other large insects, lizards and
amphibian Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arbo ...
s.
Warm-blooded Warm-blooded is an informal term referring to animal species which can maintain a body temperature higher than their environment. In particular, homeothermic species maintain a stable body temperature by regulating metabolic processes. The on ...
prey includes
mice A mouse ( : mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus'' ...
and the eggs and chicks of other beach-breeding birds; least terns, little terns and members of its own species may be victims. The greater crested tern will also occasionally catch unusual
vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () ( chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with ...
species such as agamid lizards and
green sea turtle The green sea turtle (''Chelonia mydas''), also known as the green turtle, black (sea) turtle or Pacific green turtle, is a species of large sea turtle of the family Cheloniidae. It is the only species in the genus ''Chelonia''. Its range exten ...
hatchlings, and follows trawlers for discards. The eyes of terns cannot accommodate under water, so 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
retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which the ...
s; birds that have to look through an air/water interface have more deeply coloured
carotenoid Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, ...
pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compou ...
s in the oil drops than other species.Varela, F J; Palacios, A G; Goldsmith T M "Color vision in birds" in Ziegler & Bischof (1993) pp. 77–94. The pigment also improves visual contrast and sharpens distance vision, especially in hazy conditions,Sinclair (1985) pp. 88–100. and helps terns to locate shoals of fish, although it is uncertain whether they are sighting the
phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. ...
on which the fish feed, or other feeding birds.Lythgoe (1979) pp. 180–183. The red colouring reduces
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30  PHz) to 400 nm (750  THz), shorter than that of visible light, but longer than X-rays. UV radiation ...
sensitivity, which in any case is an adaptation more suited to terrestrial feeders like the gulls, and this protects the eye from UV damage.Coles (2007) p. 30. File:Sternula albifrons 2 - Little Swanport.jpg, Little tern in flight showing the forked tail File:Gull billed Tern I IMG 9309.jpg, The gull-billed tern will sometimes prey on the chicks and young of other terns File:Chlidonias hybrida 2 (Marek Szczepanek).jpg, The
whiskered tern The whiskered tern (''Chlidonias hybrida'') is a tern in the family Laridae. The genus name is from Ancient Greek ''khelidonios'', "swallow-like", from ''khelidon'', "swallow". The specific ''hybridus'' is Latin for ''hybrid''; Peter Simon Pall ...
is an insect-eating
marsh tern The name marsh tern refers to terns of the genus ''Chlidonias'', which are typically found in freshwater marshes, rather than coastal locations. The genus name ''Chlidonias'' is from Ancient Greek , "swallow-like", from , " swallow". Species T ...


Predators and parasites

The inaccessibility of many tern colonies gave them a measure of protection from
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur ...
ian predators, especially on islands, but
introduced species An introduced species, alien species, exotic species, adventive species, immigrant species, foreign species, non-indigenous species, or non-native species is a species living outside its native distributional range, but which has arrived there ...
brought by humans can seriously affect breeding birds. These can be predators such as foxes,
raccoon The raccoon ( or , ''Procyon lotor''), sometimes called the common raccoon to distinguish it from other species, is a mammal native to North America. It is the largest of the procyonid family, having a body length of , and a body weight of ...
s,
cat The cat (''Felis catus'') is a domestic species of small carnivorous mammal. It is the only domesticated species in the family Felidae and is commonly referred to as the domestic cat or house cat to distinguish it from the wild members of ...
s and rats, or animals that destroy the habitat, including
rabbit Rabbits, also known as bunnies or bunny rabbits, are small mammals in the family Leporidae (which also contains the hares) of the order Lagomorpha (which also contains the pikas). ''Oryctolagus cuniculus'' includes the European rabbit sp ...
s,
goat The goat or domestic goat (''Capra hircus'') is a domesticated species of goat-antelope typically kept as livestock. It was domesticated from the wild goat (''C. aegagrus'') of Southwest Asia and Eastern Europe. The goat is a member of the a ...
s and pigs. Problems arise not only on formerly mammal-free islands, as in New Zealand, but also where an alien
carnivore A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose food and energy requirements derive from animal tissues (mainly muscle, fat and other s ...
, such as the
American mink The American mink (''Neogale vison'') is a semiaquatic species of mustelid native to North America, though human intervention has expanded its range to many parts of Europe, Asia and South America. Because of range expansion, the American mink i ...
in
Scotland Scotland (, ) is a country that is part of the United Kingdom. Covering the northern third of the island of Great Britain, mainland Scotland has a border with England to the southeast and is otherwise surrounded by the Atlantic Ocean to ...
, presents an unfamiliar threat.Hume (1993) pp. 112–119. Adult terns may be hunted by owls and
raptor Raptor or RAPTOR may refer to: Animals The word "raptor" refers to several groups of bird-like dinosaurs which primarily capture and subdue/kill prey with their talons. * Raptor (bird) or bird of prey, a bird that primarily hunts and feeds on v ...
s, and their chicks and eggs may be taken by
heron The herons are long-legged, long-necked, freshwater and coastal birds in the family Ardeidae, with 72 recognised species, some of which are referred to as egrets or bitterns rather than herons. Members of the genera ''Botaurus'' and ''Ixobrychu ...
s,
crow A crow is a bird of the genus '' Corvus'', or more broadly a synonym for all of ''Corvus''. Crows are generally black in colour. The word "crow" is used as part of the common name of many species. The related term "raven" is not pinned scientifica ...
s or gulls. Less obvious nest predators include
ruddy turnstone The ruddy turnstone (''Arenaria interpres'') is a small cosmopolitan wading bird, one of two species of turnstone in the genus ''Arenaria''. It is now classified in the sandpiper family Scolopacidae but was formerly sometimes placed in the plov ...
s in the Arctic, and gull-billed terns in little tern colonies. Adults may be robbed of their catch by avian kleptoparasites such as
frigatebird Frigatebirds are a family of seabirds called Fregatidae which are found across all tropical and subtropical oceans. The five extant species are classified in a single genus, ''Fregata''. All have predominantly black plumage, long, deeply forke ...
s, skuas, other terns or large gulls.Harrison (1988) pp. 320–323. External
parasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson h ...
s include
chewing lice The Mallophaga are a possibly paraphyletic section of lice Louse ( : lice) is the common name for any member of the clade Phthiraptera, which contains nearly 5,000 species of wingless parasitic insects. Phthiraptera has variously been reco ...
of the genus '' Saemundssonia'', feather lice and fleas such as '' Ceratophyllus borealis''.Rothschild & Clay (1953) p. 82, 130. Lice are often host specific, and the closely related common and Arctic terns carry quite different species.Rothschild & Clay (1953) p. 135. Internal parasites include the
crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapoda, decapods, ostracoda, seed shrimp, branchiopoda, branchiopods, argulidae, fish lice, krill, remipedes, isopoda, isopods, barnacles, copepods, ...
'' Reighardia sternae'', and
tapeworms Cestoda is a class of parasitic worms in the flatworm phylum (Platyhelminthes). Most of the species—and the best-known—are those in the subclass Eucestoda; they are ribbon-like worms as adults, known as tapeworms. Their bodies consist of ...
such as '' Ligula intestinalis'' and members of the genera ''
Diphyllobothrium ''Diphyllobothrium'' is a genus of tapeworms which can cause diphyllobothriasis in humans through consumption of raw or undercooked fish. The principal species causing diphyllobothriasis is ''D. latum'', known as the broad or fish tapeworm, or ...
'' and '' Schistocephalus''.Rothschild & Clay (1953) p. 194–197. Terns are normally free of blood parasites, unlike gulls that often carry ''
Haemoproteus ''Haemoproteus'' is a genus of alveolates that are parasitic in birds, reptiles and amphibians. Its name is derived from Greek: ''Haima'', "blood", and ''Proteus'', a sea god who had the power of assuming different shapes. The name ''Haemopr ...
'' species. An exception is the brown noddy, which sometimes harbours
protozoa Protozoa (singular: protozoan or protozoon; alternative plural: protozoans) are a group of single-celled eukaryotes, either free-living or parasitic, that feed on organic matter such as other microorganisms or organic tissues and debris. Histo ...
of that genus. In 1961 the common tern was the first wild bird species identified as being infected with avian influenza, the H5N3 variant being found in an outbreak involving South African birds. Several species of terns have been implicated as carriers of
West Nile virus West Nile virus (WNV) is a single-stranded RNA virus that causes West Nile fever. It is a member of the family '' Flaviviridae'', from the genus '' Flavivirus'', which also contains the Zika virus, dengue virus, and yellow fever virus. The v ...
.Takken & Knols (2007) p. 137–140.


Relationships with humans

Terns and their eggs have long been eaten by humans and island colonies were raided by sailors on long voyages since the eggs or large chicks were an easily obtained source of
protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
. Eggs are still illegally harvested in southern Europe, and adults of wintering birds are taken as food in West Africa and South America. The roseate tern is significantly affected by this hunting, with adult survival 10% lower than would otherwise be expected. In the West Indies, the eggs of roseate and sooty terns are believed to be
aphrodisiac An aphrodisiac is a substance that increases sexual desire, sexual attraction, sexual pleasure, or sexual behavior. Substances range from a variety of plants, spices, foods, and synthetic chemicals. Natural aphrodisiacs like cannabis or cocai ...
s, and are disproportionately targeted by egg collectors. Tern skins and feathers have long been used for making items of clothing such as capes and hats, and this became a large-scale activity in the second half of the nineteenth century when it became fashionable to use feathers in
hatmaking Hat-making or millinery is the design, manufacture and sale of hats and other headwear. A person engaged in this trade is called a milliner or hatter. Historically, milliners, typically women shopkeepers, produced or imported an inventory of ...
. This trend started in Europe but soon spread to the Americas and Australia. White was the preferred colour, and sometimes wings or entire birds were used. Terns have sometimes benefited from human activities, following the plough or fishing boats for easy food supplies, although some birds get trapped in nets or swallow plastic. Fishermen looked for feeding tern flocks, since the birds could lead them to fish shoals.
Overfishing Overfishing is the removal of a species of fish (i.e. fishing) from a body of water at a rate greater than that the species can replenish its population naturally (i.e. the overexploitation of the fishery's existing fish stock), resulting in t ...
of small fish such as sand eels can lead to steep declines in the colonies relying on these prey items. More generally, the loss or disruption to tern colonies caused by human activities has caused declines in many species. Pollution has been a problem in some areas, and in the 1960s and 1970s DDT caused egg loss through thinning of the shells. In the 1980s, organochlorides caused severe declines in the
Great Lakes The Great Lakes, also called the Great Lakes of North America, are a series of large interconnected freshwater lakes in the mid-east region of North America that connect to the Atlantic Ocean via the Saint Lawrence River. There are five lakes ...
area of the US. Because of their sensitivity to pollutants, terns are sometimes used as indicators of contamination levels. Habitat enhancements used to increase the breeding success of terns include floating nest platforms for black, common and Caspian terns, and artificial islands created for a number of different species. More specialised interventions include providing nest boxes for roseate terns, which normally nest in the shelter of tallish vegetation, and using artificial eelgrass mats to encourage common terns to nest in areas not vulnerable to flooding.


Conservation status

A number of terns face serious threats, and the
Chinese crested tern The Chinese crested tern (''Thalasseus bernsteini'') is a tern in the family Laridae. It is the county bird of Lienchiang County, Fuchien. Description It is closely related to the Sandwich tern, ''T. sandvicensis'', and the lesser crested te ...
is classed as " critically endangered" by
BirdLife International BirdLife International is a global partnership of non-governmental organizations that strives to conserve birds and their habitats. BirdLife International's priorities include preventing extinction of bird species, identifying and safeguarding ...
. It has a population of fewer than 50 birds and a breeding range of just 9 km2 (3.5 mi2). It is declining due to egg collection, human disturbance and the loss of coastal wetlands in China. Three other species are categorised as "
endangered An endangered species is a species that is very likely to become extinct in the near future, either worldwide or in a particular political jurisdiction. Endangered species may be at risk due to factors such as habitat loss, poaching and in ...
", with declining populations of less than 10,000 birds. The South Asian black-bellied tern is threatened by habitat loss, egg collecting for food, pollution and predation. In New Zealand, the black-fronted tern is facing a rapid fall in numbers due to predation by introduced mammals and Australian magpies. Disturbance by cattle and sheep and by human activities is also a factor. The Peruvian tern was initially damaged by the collapse of anchoveta stocks in 1972, but breeding colonies have subsequently been lost due to building, disturbance and pollution in their coastal wetlands. The
Australasia Australasia is a region that comprises Australia, New Zealand and some neighbouring islands in the Pacific Ocean. The term is used in a number of different contexts, including geopolitically, physiogeographically, philologically, and ecologic ...
n fairy tern is described as " vulnerable". Disturbance by humans, dogs and vehicles, predation by introduced species and inappropriate water level management in
South Australia South Australia (commonly abbreviated as SA) is a States and territories of Australia, state in the southern central part of Australia. It covers some of the most arid parts of the country. With a total land area of , it is the fourth-largest o ...
are the main reasons for its decline. Five species are "
near threatened A near-threatened species is a species which has been categorized as "Near Threatened" (NT) by the International Union for Conservation of Nature as that may be vulnerable to endangerment in the near future, but it does not currently qualify fo ...
", indicating less severe concerns or only potential vulnerability. The elegant tern is so categorised because 95% of the population breeds on one island,
Isla Rasa Isla Rasa is an island in the Gulf of California east of the Baja California Peninsula. The island is uninhabited and is part of the Mexicali Municipality. The 0.21 sq./mi island has three small ponds and has small shed located in the center of t ...
in the
Gulf of California The Gulf of California ( es, Golfo de California), also known as the Sea of Cortés (''Mar de Cortés'') or Sea of Cortez, or less commonly as the Vermilion Sea (''Mar Bermejo''), is a marginal sea of the Pacific Ocean that separates the Baja C ...
, and the
Kerguelen tern The Kerguelen tern (''Sterna virgata'') is a tern of the southern hemisphere. This seabird mainly breeds colonially in the Kerguelen Islands, as its common name implies. However, smaller colonies are also found in the Prince Edward Islands (i. ...
has a population of less than 5,000 adults breeding on small and often stormy islands in the southern
Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or ~19.8% of the water on Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia to the east. To the south it is bounded by ...
. Three species, the Inca, Damara, and
river tern The Indian river tern or just river tern (''Sterna aurantia'') is a tern in the family Laridae. It is a resident breeder along inland rivers from Iran east into the Indian Subcontinent and further to Myanmar to Thailand, where it is uncommon ...
s, are expected to decline in the future due to habitat loss and disturbance. Some tern
subspecies In biological classification, subspecies is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics ( morphology), but that can successfully interbreed. Not all specie ...
are endangered, including the California least tern and the
Easter Island Easter Island ( rap, Rapa Nui; es, Isla de Pascua) is an island and special territory of Chile in the southeastern Pacific Ocean, at the southeasternmost point of the Polynesian Triangle in Oceania. The island is most famous for its nearl ...
race of the grey noddy. Most tern species are declining in numbers due to the loss or disturbance of breeding habitat, pollution and increased predation. Gull populations have increased over the last century because of reduced persecution and the availability of food from human activities, and terns have been forced out of many traditional nesting areas by the larger birds. A few species are defying the trend and showing local increases, including the Arctic tern in
Scandinavia Scandinavia; Sámi languages: /. ( ) is a subregion in Northern Europe, with strong historical, cultural, and linguistic ties between its constituent peoples. In English usage, ''Scandinavia'' most commonly refers to Denmark, Norway, and S ...
, Forster's tern around the Great Lakes, the
Sandwich tern The Sandwich tern (''Thalasseus sandvicensis'') is a tern in the family Laridae. It is very closely related to the lesser crested tern (''T. bengalensis''), Chinese crested tern (''T. bernsteini''), Cabot's tern (''T. acuflavidus''), and ele ...
in eastern North America and its yellow-billed subspecies, the Cayenne tern, in the
Caribbean The Caribbean (, ) ( es, El Caribe; french: la Caraïbe; ht, Karayib; nl, De Caraïben) is a region of the Americas that consists of the Caribbean Sea, its islands (some surrounded by the Caribbean Sea and some bordering both the Caribbean ...
. Terns are protected by international legislation such as the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA) and the US-Canada
Migratory Bird Treaty Act of 1918 The Migratory Bird Treaty Act of 1918 (MBTA), codified at (although §709 is omitted), is a United States federal law, first enacted in 1918 to implement the convention for the protection of migratory birds between the United States and Canada ...
. Parties to the AWEA agreement are required to engage in a wide range of conservation strategies described in a detailed action plan. The plan is intended to address key issues such as species and habitat conservation, management of human activities, research, education, and implementation. The North American legislation is similar, although there is a greater emphasis on protection.


See also

*
Seabird breeding behavior The term seabird is used for many families of birds in several orders that spend the majority of their lives at sea. Seabirds make up some, if not all, of the families in the following orders: Procellariiformes, Sphenisciformes, Pelecaniformes, and ...


References


Cited texts


External links


Tern videos
on the Internet Bird Collection {{Authority control Extant Miocene first appearances * Taxa named by Charles Lucien Bonaparte