Crested Tern
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The greater crested tern Retrieved 28 February 2012 (''Thalasseus bergii''), also called crested tern, swift tern, or great crested tern, is a
tern Terns are seabirds in the family Laridae, subfamily Sterninae, that have a worldwide distribution and are normally found near the sea, rivers, or wetlands. Terns are treated in eleven genera in a subgroup of the family Laridae, which also ...
in the family
Laridae Laridae is a family of seabirds in the order Charadriiformes that includes the gulls, terns (including white terns), noddies, and skimmers. It includes around 100 species arranged into 22 genera. They are an adaptable group of mostly aerial bird ...
that nests in dense colonies on coastlines and islands in the tropical and subtropical
Old World The "Old World" () is a term for Afro-Eurasia coined by Europeans after 1493, when they became aware of the existence of the Americas. It is used to contrast the continents of Africa, Europe, and Asia in the Eastern Hemisphere, previously ...
. Its four or five
subspecies In Taxonomy (biology), biological classification, subspecies (: subspecies) is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics (Morphology (biology), morpholog ...
breed in the area from
South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...
around the
Indian Ocean The Indian Ocean is the third-largest of the world's five oceanic divisions, covering or approximately 20% of the water area of Earth#Surface, Earth's surface. It is bounded by Asia to the north, Africa to the west and Australia (continent), ...
to the central
Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean, or, depending on the definition, to Antarctica in the south, and is bounded by the cont ...
and
Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
, all populations dispersing widely from the breeding range after nesting. This large tern is closely related to other species in its genus, but can be distinguished by its darker, battleship-grey upperparts, white forehead even in full breeding plumage, and greenish-tinged yellow (not orange or black) bill colour. The greater crested tern has dark grey upperparts, white underparts, a yellow bill, and a shaggy black crest that recedes in winter. Its young have a distinctive appearance, with strongly patterned grey, brown and white plumage, and rely on their parents for food for several months after they have
fledge Fledging is the stage in a flying animal's life between egg, 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 ...
d. Like all members of the genus ''Thalasseus'', the greater crested tern feeds by plunge diving for fish, usually in marine environments; the male offers fish to the female as part of the courtship ritual. This is an adaptable species that has learned to follow fishing boats for jettisoned
bycatch Bycatch (or by-catch), in the fishing industry, is a fish or other marine species that is caught unintentionally while fishing for specific species or sizes of wildlife. Bycatch is either the wrong species, the wrong sex, or is undersized or juve ...
, and to use unusual nest sites such as the roofs of buildings and artificial islands in salt pans and
sewage works Sewage treatment is a type of wastewater treatment which aims to remove contaminants from sewage to produce an effluent that is suitable to discharge to the surrounding environment or an intended reuse application, thereby preventing water p ...
. Its eggs and young are taken by
gull Gulls, or colloquially seagulls, are seabirds of the subfamily Larinae. They are most closely related to terns and skimmers, distantly related to auks, and even more distantly related to waders. Until the 21st century, most gulls were placed ...
s and
ibis The ibis () (collective plural ibises; classical plurals ibides and ibes) are a group of long-legged wading birds in the family Threskiornithidae that inhabit wetlands, forests and plains. "Ibis" derives from the Latin and Ancient Greek word f ...
es, and human activities such as fishing, shooting and egg harvesting have caused local population declines. There are no global conservation concerns for this bird, which has a stable total population of more than 500,000 individuals.


Taxonomy

The terns, subfamily Sterninae, are small to medium-sized seabirds closely related to the
gull Gulls, or colloquially seagulls, are seabirds of the subfamily Larinae. They are most closely related to terns and skimmers, distantly related to auks, and even more distantly related to waders. Until the 21st century, most gulls were placed ...
s,
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
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 Arctic skua, the long-tailed skua, and the pomarine skua, are called ...
s. They are gull-like in appearance, but typically have a lighter build, long pointed wings (which give them a fast, buoyant flight), a deeply forked tail and short legs. Most species are grey above and white below, and have a black cap that is reduced or flecked with white in the winter.Snow & Perrin (1998) 764 The greater crested tern was originally described as ''Sterna bergii'' by German naturalist
Martin Lichtenstein Martin H nrich Carl Lichtenstein (10 January 1780 – 2 September 1857) was a German physician, List of explorers, explorer, botanist and zoologist. He explored parts of southern Africa and collected natural history specimens extensively and ...
in 1823, but was moved to its current genus ''
Thalasseus ''Thalasseus'', the crested terns, is a genus of eight species of medium-large to large terns in the family Laridae. The species have a worldwide distribution in temperate and tropical seas, mostly between about 43° N and S latitude, but to 60 ...
'',This genus had originally been created by
Heinrich Boie Heinrich Boie (4 May 1794, Meldorf, Holstein – 4 September 1827, Bogor, West Java, Indonesia) was a German zoologist. He was the brother of Friedrich Boie. In the field of herpetology they described 49 new species of reptiles and several ne ...
in 1822, but had been abandoned until the study by Bridge (2005) confirmed the need for a separate genus for the crested terns.
after
mitochondria A mitochondrion () is an organelle found in the cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...
l
DNA Deoxyribonucleic acid (; DNA) is a polymer composed of two polynucleotide chains that coil around each other to form a double helix. The polymer carries genetic instructions for the development, functioning, growth and reproduction of al ...
studies confirmed that the three main head patterns shown by terns (no black cap, black cap, black cap with a white forehead) corresponded to distinct
clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
s. The closest relative of greater crested tern is the
critically endangered An IUCN Red List critically endangered (CR or sometimes CE) species is one that has been categorized by the International Union for Conservation of Nature as facing an extremely high risk of extinction in the wild. As of December 2023, of t ...
species
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. Taxonomy It is most closely related to greater crested tern ''T. bergii'', and has hybridised with i ...
(''T. bernsteini''), with the next closest being a group of three orange-billed species,
West African crested tern The West African crested tern (''Thalasseus albididorsalis'') is a bird species in the family Laridae. Until 2020 it was considered a subspecies of the New World royal tern, ''Thalasseus maximus''.R. Terry Chesser, Shawn M. Billerman, Kevin J. Bu ...
''T. albididorsalis'',
lesser crested tern The lesser crested tern (''Thalasseus bengalensis'')Bridge, E. S.; Jones, A. W. & Baker, A. J. (2005)A phylogenetic framework for the terns (Sternini) inferred from mtDNA sequences: implications for taxonomy and plumage evolution. ''Molecular Phy ...
(''T. bengalensis''), and
royal tern The royal tern (''Thalasseus maximus'') is a tern in the family Laridae. The species is endemic to the Americas, though vagrants have been identified in Europe.Buckley, P. A. and F. G. Buckley (2020). Royal Tern (Thalasseus maximus), version 1. ...
(''T. maximus''). Earlier studies of the genus ''Thalasseus'' had not included ''T. bernsteini'', as genetic material was not then available, but otherwise had broadly similar results. The generic name of the greater crested tern is derived from
Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...
''Thalassa'', "sea", and the species
epithet An epithet (, ), also a byname, is a descriptive term (word or phrase) commonly accompanying or occurring in place of the name of a real or fictitious person, place, or thing. It is usually literally descriptive, as in Alfred the Great, Suleima ...
''bergii'' commemorates Karl Heinrich Bergius, a
Prussia Prussia (; ; Old Prussian: ''Prūsija'') was a Germans, German state centred on the North European Plain that originated from the 1525 secularization of the Prussia (region), Prussian part of the State of the Teutonic Order. For centuries, ...
n pharmacist and botanist who collected the first specimens of this tern near
Cape Town Cape Town is the legislature, legislative capital city, capital of South Africa. It is the country's oldest city and the seat of the Parliament of South Africa. Cape Town is the country's List of municipalities in South Africa, second-largest ...
.Higgins & Davies (1996) 605–609 The greater crested tern has four or five geographical subspecies (depending on authority), differing mainly in the darkness of the upperparts and the bill dimensions. These are listed below in
taxonomic sequence Taxonomic sequence (also known as systematic, phyletic or taxonomic order) is a sequence followed in listing of taxa which aids ease of use and roughly reflects the evolutionary relationships among the taxa. Taxonomic sequences can exist for taxa ...
. A similar number of other potential subspecies have been proposed, but are not considered valid.


Description

The greater crested tern is a large tern with a long () yellow bill, black legs, and a glossy black crest that is noticeably shaggy at its rear. The breeding adult of the
nominate subspecies In biological classification, subspecies (: 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. ...
''T. b. bergii'' is long, with a wing-span; weight . The forehead and the underparts are white, the back and inner wings are dusky-grey. In winter, the upperparts plumage wears to a paler grey, and the crown of the head becomes white, merging at the rear into a peppered black crest and mask. The adults of both sexes are identical in appearance, but juvenile birds are distinctive, with a head pattern like the winter adult, and upperparts strongly patterned in grey, brown, and white; the closed wings appear to have dark bars. After
moult In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is a process by which an animal casts off parts of its body to serve some beneficial purpose, either at ...
ing, the young terns resemble the adult, but still have a variegated wing pattern with a dark bar on the inner
flight feather Flight feathers (''Pennae volatus'') are the long, stiff, asymmetrically shaped, but symmetrically paired pennaceous feathers on the wings or tail of a bird; those on the wings are called remiges (), singular remex (), while those on the tai ...
s. The northern subspecies ''T. b. velox'' and ''T. b. thalassinus'' are in breeding plumage from May to September or October, whereas the relevant period for the two southern African subspecies is from December to April. For ''T. b. cristatus'', the moult timing depends on location; birds from Australia and
Oceania Oceania ( , ) is a region, geographical region including Australasia, Melanesia, Micronesia, and Polynesia. Outside of the English-speaking world, Oceania is generally considered a continent, while Mainland Australia is regarded as its co ...
are in breeding plumage from September to about April, but those in
Thailand Thailand, officially the Kingdom of Thailand and historically known as Siam (the official name until 1939), is a country in Southeast Asia on the Mainland Southeast Asia, Indochinese Peninsula. With a population of almost 66 million, it spa ...
, China and
Sulawesi Sulawesi ( ), also known as Celebes ( ), is an island in Indonesia. One of the four Greater Sunda Islands, and the List of islands by area, world's 11th-largest island, it is situated east of Borneo, west of the Maluku Islands, and south of Min ...
have this appearance from February to June or July. The royal tern is similar in size to this species, but has a heavier build, broader wings, a paler back and a blunter, more orange bill. The greater crested often associates with the lesser crested tern, but is 25% larger than the latter, with a proportionately longer bill, longer and heavier head, and bulkier body. Lesser crested tern has an orange-tinted bill, and in immature plumage it is much less variegated than greater crested. The greater crested tern is highly
vocal The human voice consists of sound made by a human being using the vocal tract, including talking, singing, laughing, crying, screaming, shouting, humming or yelling. The human voice frequency is specifically a part of human sound producti ...
, especially at its breeding grounds. The territorial advertising call is a loud, raucous, crow-like '. Other calls include a ' given at the nest by anxious or excited birds, and a hard ' in flight.


Distribution and habitat

The greater crested tern occurs in tropical and warm temperate coastal parts of the Old World from South Africa around the Indian Ocean to the Pacific and Australia. The subspecies ''T. b. bergii'' (including the doubtfully distinct ''T. b. enigma'') breeds in southern Africa from Namibia to
Tanzania Tanzania, officially the United Republic of Tanzania, is a country in East Africa within the African Great Lakes region. It is bordered by Uganda to the northwest; Kenya to the northeast; the Indian Ocean to the east; Mozambique and Malawi to t ...
, and possibly on islands around Madagascar. There is then a break in the breeding distribution of the species until ''T. b. velox'' is met in
Somalia Somalia, officially the Federal Republic of Somalia, is the easternmost country in continental Africa. The country is located in the Horn of Africa and is bordered by Ethiopia to the west, Djibouti to the northwest, Kenya to the southwest, th ...
, the Red Sea, and the Persian Gulf, and another discontinuity further east in southern
India India, officially the Republic of India, is a country in South Asia. It is the List of countries and dependencies by area, seventh-largest country by area; the List of countries by population (United Nations), most populous country since ...
. The subspecies ''T. b. thalassinus'' breeds on many islands in the Indian Ocean including
Aldabra Aldabra, the world's second-largest coral atoll (the largest is Kiritimati), is located east of the continent of Africa. It is part of the Aldabra Group of islands in the Indian Ocean that are part of the Outer Islands (Seychelles), Outer Islands ...
and Etoile in the Seychelles, the
Chagos Archipelago The Chagos Archipelago (, ) or Chagos Islands (formerly , and later the Oil Islands) is a group of seven atolls comprising more than 60 islands in the Indian Ocean about south of the Maldives archipelago. This chain of islands is the southernmo ...
, and Rodrigues. In the Pacific Ocean, there are colonies of ''T. b. cristatus'' on numerous Pacific islands, including
Kiribati Kiribati, officially the Republic of Kiribati, is an island country in the Micronesia subregion of Oceania in the central Pacific Ocean. Its permanent population is over 119,000 as of the 2020 census, and more than half live on Tarawa. The st ...
,
Fiji Fiji, officially the Republic of Fiji, is an island country in Melanesia, part of Oceania in the South Pacific Ocean. It lies about north-northeast of New Zealand. Fiji consists of an archipelago of more than 330 islands—of which about ...
,
Tonga Tonga, officially the Kingdom of Tonga, is an island country in Polynesia, part of Oceania. The country has 171 islands, of which 45 are inhabited. Its total surface area is about , scattered over in the southern Pacific Ocean. accordin ...
, the
Society Islands The Society Islands ( , officially ; ) are an archipelago in the South Pacific Ocean that includes the major islands of Tahiti, Mo'orea, Moorea, Raiatea, Bora Bora and Huahine. Politically, they are part of French Polynesia, an overseas country ...
and the
Tuamotus The Tuamotu Archipelago or the Tuamotu Islands (, officially ) are a French Polynesian chain of just under 80 islands and atolls in the southern Pacific Ocean. They constitute the largest chain of atolls in the world, extending (from northwest to ...
, as well as more widely on islets off the coasts of China, Taiwan, the Philippines, Indonesia, New Guinea, and Australia south to Tasmania. The nests are located on low‑lying sandy, rocky, or coral islands, sometimes amongst stunted shrubs, often without any shelter at all. When not breeding, the greater crested tern will roost or rest on open shores, less often on boats, pilings, harbour buildings and raised salt mounds in lagoons. It is rarely seen on tidal creeks or inland waters. All populations of greater crested tern disperse after breeding. When Southern African birds leave colonies in Namibia and
Western Cape Province The Western Cape ( ; , ) is a province of South Africa, situated on the south-western coast of the country. It is the fourth largest of the nine provinces with an area of , and the third most populous, with an estimated 7 million inhabi ...
, most adults move east to the Indian Ocean coastline of South Africa. Many young birds also travel east, sometimes more than , but others move northwards along the western coast. ''T. b. thalassinus'' winters on the east African coast north to
Kenya Kenya, officially the Republic of Kenya, is a country located in East Africa. With an estimated population of more than 52.4 million as of mid-2024, Kenya is the 27th-most-populous country in the world and the 7th most populous in Africa. ...
and Somalia and may move as far south as Durban. Populations of ''T. b. velox'' breeding from the Persian Gulf eastwards appear to be sedentary or dispersive rather than truly migratory, but those breeding in the Red Sea winter south along the east African coast to Kenya. ''T. b. cristatus'' mostly stays within of its colonies, but some birds wander up to around . This species has occurred as a vagrant to
Hawaii Hawaii ( ; ) is an island U.S. state, state of the United States, in the Pacific Ocean about southwest of the U.S. mainland. One of the two Non-contiguous United States, non-contiguous U.S. states (along with Alaska), it is the only sta ...
,
New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...
,
North Korea North Korea, officially the Democratic People's Republic of Korea (DPRK), is a country in East Asia. It constitutes the northern half of the Korea, Korean Peninsula and borders China and Russia to the north at the Yalu River, Yalu (Amnok) an ...
,
Jordan Jordan, officially the Hashemite Kingdom of Jordan, is a country in the Southern Levant region of West Asia. Jordan is bordered by Syria to the north, Iraq to the east, Saudi Arabia to the south, and Israel and the occupied Palestinian ter ...
, and
Israel Israel, officially the State of Israel, is a country in West Asia. It Borders of Israel, shares borders with Lebanon to the north, Syria to the north-east, Jordan to the east, Egypt to the south-west, and the Mediterranean Sea to the west. Isr ...
.Snow & Perrin (1998) 770–771 In India, the greater crested tern is protected in the PM Sayeed Marine Birds Conservation Reserve.


Behaviour


Breeding

The greater crested tern breeds in
colonies A colony is a territory subject to a form of foreign rule, which rules the territory and its indigenous peoples separated from the foreign rulers, the colonizer, and their '' metropole'' (or "mother country"). This separated rule was often or ...
, often in association with other seabirds. It is
monogamous Monogamy ( ) is a relationship of two individuals in which they form a mutual and exclusive intimate partnership. Having only one partner at any one time, whether for life or serial monogamy, contrasts with various forms of non-monogamy (e.g. ...
and the pair bond is maintained through the year and sometimes in consecutive breeding seasons.Higgins & Davies (1996) 610–611 The colony size is related to the abundance of
pelagic fish Pelagic fish live in the pelagic zone of ocean or lake waters—being neither close to the bottom nor near the shore—in contrast with demersal fish that live on or near the bottom, and reef fish that are associated with coral reefs. ...
prey, and the largest documented colony, with 13,000 to 15,000 pairs, is in the
Gulf of Carpentaria The Gulf of Carpentaria is a sea off the northern coast of Australia. It is enclosed on three sides by northern Australia and bounded on the north by the eastern Arafura Sea, which separates Australia and New Guinea. The northern boundary ...
in northern Australia, a region which also supports major colonies of other seabirds. Since nesting in this area follows the summer
monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in Atmosphere of Earth, atmospheric circulation and precipitation associated with annu ...
al flooding, it is presumably a response to fish stocks rising, probably due to river run-off providing extra nutrient to the Gulf. This tern does not show
site fidelity Philopatry is the tendency of an organism to stay in or habitually return to a particular area. The causes of philopatry are numerous, but natal philopatry, where animals return to their birthplace to breed, may be the most common. The term derives ...
, frequently changing its nest site from year to year, sometimes by more than . A male greater crested tern establishes a small area of the colony in preparation for nesting, and initially pecks at any other tern entering his territory. If the intruder is another male, it retaliates in kind, and is normally vigorously repelled by the incumbent. A female entering the nest area reacts passively to the male's aggression, enabling him to recognise her sex and initiate pair formation by display, including head raising and bowing; this behaviour is frequently repeated during nesting to reinforce the bond between the pair. Terns also use fish as part of the courtship ritual. One bird flies around the colony with a fish in its beak, calling loudly; its partner may also fly, but the pair eventually settle and the gift is exchanged. The nest is a shallow scrape in the sand on open, flat or occasionally sloping ground. It is often unlined, but sometimes includes stones or
cuttlefish Cuttlefish, or cuttles, are Marine (ocean), marine Mollusca, molluscs of the order (biology), suborder Sepiina. They belong to the class (biology), class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique ...
bones. One, sometimes two, eggs are laid and incubated by both parents for 25 to 30 days prior to hatching. The eggs are cream with blackish streaks. Egg laying is synchronised within a breeding colony and more tightly so within sub-colonies. Parents do not recognize their own eggs or newly hatched chicks, but are able to distinguish their chicks by the time they are two days-old, shortly before they begin to wander from the nest. The
precocial Precocial species in birds and mammals are those in which the young are relatively mature and mobile from the moment of birth or hatching. They are normally nidifugous, meaning that they leave the nest shortly after birth or hatching. Altricial ...
chicks, which are very pale with black speckling, are brooded and fed by both parents, but may gather in crèches when older. The young terns fledge after 38 to 40 days, but remain dependent on the parents after leaving the colony until they are about four months old.Cooper (2006) 760–764 In South Africa, this species has adapted to breeding on the roofs of building, sometimes with
Hartlaub's gull Hartlaub's gull (''Chroicocephalus hartlaubii'') is a small gull in the genus ''Chroicocephalus''. It was formerly (as with other related gulls) placed in the genus ''Larus'' until genetic research demonstrated that the old broad view of that gen ...
, which also shares the more typical nesting sites of the nominate subspecies. In 2000, 7.5% of the population of this subspecies bred on roofs. Artificial islands in salt pans and sewage works have also recently been colonised by this adaptable seabird. Adult terns have few predators, but in Namibia immature birds are often robbed of their food by
kelp gull The kelp gull (''Larus dominicanus''), also known as the Dominican gull, is a gull that breeds on coasts and islands through much of the Southern Hemisphere. The nominate ''L. d. dominicanus'' is the subspecies found around South America, pa ...
s, and that species, along with Hartlaub's gull,
silver gull The silver gull (''Chroicocephalus novaehollandiae'') is a gull in Oceania. It is the most common gull of Australia. It has been found throughout the continent, but particularly at or near coastal areas. It is smaller than the Pacific gull (''L ...
and
sacred ibis The African sacred ibis (''Threskiornis aethiopicus'') is a species of ibis, a wading bird of the family Threskiornithidae. It is native to much of Africa, as well as small parts of Iraq, Iran and Kuwait. It is especially known for its role in ...
, has been observed feeding on eggs or nestlings, especially when colonies are disturbed. Smaller subcolonies with a relatively larger numbers of nests located on the perimeter are subject to more predation. In Australia, predation by cats and dogs, and occasional deaths by shooting or collisions with cars, wires or light-towers have been documented. Commercial fisheries can have both positive and negative effects on the greater crested tern. Juvenile survival rates are improved where trawler discards provide extra food, and huge population increases in the southeastern Gulf of Carpentaria are thought to have been due to the development of a large prawn trawl fishery. Conversely, purse-seine fishing reduces the available food supply, and sizeable fluctuations in the numbers of great crested terns breeding in the Western Cape of South Africa are significantly related to changes in the abundance of pelagic fish, which are intensively exploited by purse-seine fishing. Terns may be killed or injured by collisions with trawl warps, trapped in trawls or discarded gear, or hooked by
longline fishing Longline fishing, or longlining, is a commercial fishing angling technique that uses a long ''main line'' with baited hooks attached at intervals via short branch lines called ''snoods'' or ''gangions''.albatross Albatrosses, of the biological family Diomedeidae, are large seabirds related to the procellariids, storm petrels, and diving petrels in the order Procellariiformes (the tubenoses). They range widely in the Southern Ocean and the North Paci ...
es and
petrel Petrels are tube-nosed seabirds in the phylogenetic order Procellariiformes. Description Petrels are a monophyletic group of marine seabirds, sharing a characteristic of a nostril arrangement that results in the name "tubenoses". Petrels enco ...
s, there is little evidence that overall numbers are significantly affected. An unusual incident was the incapacitation of 103 terns off
Robben Island Robben Island () is an island in Table Bay, 6.9 kilometres (4.3 mi) west of the coast of Bloubergstrand, north of Cape Town, South Africa. It takes its name from the Dutch language, Dutch word for seals (''robben''), hence the Dutch/Afrika ...
, South Africa by marine foam, generated by a combination of wave action,
kelp Kelps are large brown algae or seaweeds that make up the order (biology), order Laminariales. There are about 30 different genus, genera. Despite its appearance and use of photosynthesis in chloroplasts, kelp is technically not a plant but a str ...
mucilage Mucilage is a thick gluey substance produced by nearly all plants and some microorganisms. These microorganisms include protists which use it for their locomotion, with the direction of their movement always opposite to that of the secretion of ...
and
phytoplankton Phytoplankton () are the autotrophic (self-feeding) components of the plankton community and a key part of ocean and freshwater Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...
. After treatment, 90% of the birds were fit to be released.


Feeding

Fish are the main food of the greater crested tern, found to make up nearly 90% of all prey items with the remainder including
cephalopod A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...
s,
crustacea Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...
ns and
insect Insects (from Latin ') are Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...
s. Unusual
vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...
prey included agamid lizards and
green 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 exte ...
hatchlings. The greater crested tern feeds mostly at sea by plunge diving to a depth of up to , or by dipping from the surface, and food is usually swallowed in mid-air. Birds may forage up to from land in the breeding season. Prey size ranges from in length and up to in weight. Shoaling
pelagic The pelagic zone consists of the water column of the open ocean and can be further divided into regions by depth. The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or water column between the sur ...
fish such as
anchovy An anchovy is a small, common forage fish of the Family (biology), family Engraulidae. Most species are found in marine waters, but several will enter brackish water, and some in South America are restricted to fresh water. More than 140 speci ...
and
sardine Sardine and pilchard are common names for various species of small, oily forage fish in the herring suborder Clupeoidei. The term "sardine" was first used in English during the early 15th century; a somewhat dubious etymology says it com ...
are typical prey, but bottom-living species are taken as discards from commercial fishing. This tern actively follows trawlers, including at night, and during the fishing season trawl discards can constitute 70% of its diet. Prawn fishing is particularly productive in providing extra food, since prawns usually represent only 10–20% of the catch, the remaining being bycatch, mainly fish such as
cardinalfish Cardinalfishes are a family, Apogonidae, of ray-finned fishes found in the Atlantic, Indian, and Pacific Oceans; they are chiefly marine, but some species are found in brackish water and a few (notably '' Glossamia'') are found in fresh water. A ...
and
gobies Gobiidae or gobies is a family of bony fish in the order Gobiiformes, one of the largest fish families comprising over 2,000 species in more than 200 genera. Most of gobiid fish are relatively small, typically less than in length, and the fam ...
. A study of an area of the
Great Barrier Reef The Great Barrier Reef is the world's largest coral reef system, composed of over 2,900 individual reefs and 900 islands stretching for over over an area of approximately . The reef is located in the Coral Sea, off the coast of Queensland, ...
where the number of breeding great crested terns has grown ten-fold, probably due to extra food from trawl
by-catch Bycatch (or by-catch), in the fishing industry, is a fish or other marine species that is caught unintentionally while fishing for specific species or sizes of wildlife. Bycatch is either the wrong species, the wrong sex, or is undersized or juve ...
, suggested that lesser crested and
sooty tern The sooty tern (''Onychoprion fuscatus'') is a tern in the family Laridae. It is a seabird of the tropical oceans, and remarkably, has evolved the ability to fly for years at a time, skimming the sea surface for food, and returning to land only ...
s have moved away and now breed on a part of the reef where fishing is banned. It is possible that the large increase in the number of greater crested terns may have affected other species through competition for food and nesting sites. Terns have red oil droplets in the
cone cell Cone cells or cones are photoreceptor cells in the retina of the vertebrate eye. Cones are active in daylight conditions and enable photopic vision, as opposed to rod cells, which are active in dim light and enable scotopic vision. Most v ...
s of the
retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...
s of their eyes. This improves contrast and sharpens distance
vision Vision, Visions, or The Vision may refer to: Perception Optical perception * Visual perception, the sense of sight * Visual system, the physical mechanism of eyesight * Computer vision, a field dealing with how computers can be made to gain und ...
, especially in hazy conditions. Birds that have to see through an air/water interface, such as terns and gulls, have more strongly coloured
carotenoid Carotenoids () are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, archaea, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, corn, tomatoes, cana ...
pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), chemically unreactive in water or another medium; in contrast, dyes are colored sub ...
s in the cone oil drops than other avian species.Varela, F. J.; Palacios, A. G.; Goldsmith T. M. (1993) "Vision, Brain, and Behavior in Birds" in Harris, Philip; Bischof, Hans-Joachim ''Vision, Brain, and Behavior in Birds: a comparative review'' Cambridge, Massachusetts: MIT Press 77–94 The improved eyesight 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 Aquatic ecosystem, ecosystems. The name comes from the Greek language, Greek words (), meaning 'plant', and (), mea ...
on which the fish feed, or observing other terns diving for food. Tern's eyes are not particularly
ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
sensitive, an adaptation more suited to terrestrial feeders like the gulls.


Status

The greater crested tern has a widespread distribution range, estimated at 1–10 million square kilometres (0.4–3.8 million square miles). The population has not been quantified, but it is not believed to approach the thresholds for either the size criterion (fewer than 10,000 mature individuals) or the population decline criterion (declining more than 30% in ten years or three generations) of the
IUCN Red List The International Union for Conservation of Nature (IUCN) Red List of Threatened Species, also known as the IUCN Red List or Red Data Book, founded in 1964, is an inventory of the global conservation status and extinction risk of biological ...
. For these reasons, the species is evaluated as being of Least Concern at the global level. However, there are concerns for populations in some areas such as the
Gulf of Thailand The Gulf of Thailand (), historically known as the Gulf of Siam (), is a shallow inlet adjacent to the southwestern South China Sea, bounded between the southwestern shores of the Indochinese Peninsula and the northern half of the Malay Peninsula. ...
where the species no longer breeds, and in
Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania, between the Indian Ocean, Indian and Pacific Ocean, Pacific oceans. Comprising over List of islands of Indonesia, 17,000 islands, including Sumatra, ...
where egg harvesting has caused declines. All subspecies except ''T. b. cristatus'' are covered under the ''Agreement on the Conservation of African-Eurasian Migratory Waterbirds'' (
AEWA The Agreement on the Conservation of African-Eurasian Migratory Waterbirds, or African-Eurasian Waterbird Agreement (AEWA), is an independent international treaty developed under the auspices of the United Nations Environment Programme's Convent ...
). Parties to the 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.


References


Bibliography

* Cooper, John (2006)
Potential impacts of marine fisheries on migratory waterbirds of the Afrotropical Region: a study in progress
(PDF extract) in Boere, C.A.; Galbraith G.C.; Stroud D.A. (eds) ''Waterbirds around the world.'' Edinburgh: The Stationery Office, * * * * *


External links

* (Greater crested tern = ) Swift tern
Species text in The Atlas of Southern African Birds
{{Authority control
greater crested tern The greater crested tern Retrieved 28 February 2012 (''Thalasseus bergii''), also called crested tern, swift tern, or great crested tern, is a tern in the family Laridae that nests in dense colonies on coastlines and islands in the tropical and ...
Birds of East Africa Birds of Southern Africa Birds of Asia Birds of Oceania Birds of the Indian Ocean
greater crested tern The greater crested tern Retrieved 28 February 2012 (''Thalasseus bergii''), also called crested tern, swift tern, or great crested tern, is a tern in the family Laridae that nests in dense colonies on coastlines and islands in the tropical and ...
greater crested tern The greater crested tern Retrieved 28 February 2012 (''Thalasseus bergii''), also called crested tern, swift tern, or great crested tern, is a tern in the family Laridae that nests in dense colonies on coastlines and islands in the tropical and ...