Greater Crested Tern
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The greater crested tern Retrieved 28 February 2012 (''Thalasseus bergii''), also called crested tern or swift tern, is a
tern Terns are seabirds in the family Laridae that have a worldwide distribution and are normally found near the sea, rivers, or wetlands. Terns are treated as a subgroup of the family Laridae which includes gulls and skimmers and consists of e ...
in the family
Laridae Laridae is a family of seabirds in the order Charadriiformes that includes the gulls, terns, skimmers and kittiwakes. It includes around 100 species arranged into 22 genera. They are an adaptable group of mostly aerial birds found worldwide. ...
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 that originated in Europe , after Europeans became aware of the existence of the Americas. It is used to contrast the continents of Africa, Europe, and Asia, which were previously thought of by the ...
. Its five
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 species ...
breed in the area from
South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...
around the
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 th ...
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 definition, to Antarctica) in the south, and is bounded by the continen ...
and
Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
, all populations dispersing widely from the breeding range after nesting. This large tern is closely related to the
royal Royal may refer to: People * Royal (name), a list of people with either the surname or given name * A member of a royal family Places United States * Royal, Arkansas, an unincorporated community * Royal, Illinois, a village * Royal, Iowa, a cit ...
and
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 Ph ...
s, but can be distinguished by its size and bill colour. The greater crested tern has 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 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 vulnerable c ...
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 (or domestic wastewater treatment, municipal wastewater treatment) is a type of wastewater treatment which aims to remove contaminants from sewage to produce an effluent that is suitable for discharge to the surrounding envir ...
. Its eggs and young are taken by
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, m ...
s and
ibis The ibises () (collective plural ibis; 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, family Sternidae, are small to medium-sized seabirds closely related to 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, m ...
s, skimmers 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 long-tailed skua, the Arctic skua, and the pomarine skua are called jae ...
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 Hinrich Carl Lichtenstein (10 January 1780 – 2 September 1857) was a German physician, explorer, botanist and zoologist. Biography Born in Hamburg, Lichtenstein was the son of Anton August Heinrich Lichtenstein. He studied medicine a ...
in 1823, but was moved to its current genus, ''
Thalasseus ''Thalasseus'', the crested terns, is a genus of eight species of terns in the family Laridae. It has a worldwide distribution, and many of its species are abundant and well-known birds in their ranges. This genus had originally been created by ...
'',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 new s ...
in 1822, but had been abandoned until the Bridge (2005) study confirmed the need for a separate genus for the crested terns.
after
mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
l DNA 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 A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
s. The greater crested tern's closest relatives within its genus appear to be the
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 Ph ...
(''T. bengalensis''), and the
royal tern The royal tern (''Thalasseus maximus'') is a tern in the family Laridae. The species is endemic to the Americas, though strays have been identified in Europe.Buckley, P. A. and F. G. Buckley (2020). Royal Tern (Thalasseus maximus), version 1.0. ...
(''T. maximus''). The DNA study did not include the critically endangered
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 tern, ...
(''T. bernsteini'') but, as that bird was formerly considered to be
conspecific Biological specificity is the tendency of a characteristic such as a behavior or a biochemical variation to occur in a particular species. Biochemist Linus Pauling stated that "Biological specificity is the set of characteristics of living organ ...
with the greater crested tern as a
synonym A synonym is a word, morpheme, or phrase that means exactly or nearly the same as another word, morpheme, or phrase in a given language. For example, in the English language, the words ''begin'', ''start'', ''commence'', and ''initiate'' are all ...
of the subspecies ''T. b. cristatus'', it is presumably also very closely related. The generic name of the greater crested tern is derived from
Greek Greek may refer to: Greece 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 ...
''Thalassa'', "sea", and the species
epithet An epithet (, ), also byname, is a descriptive term (word or phrase) known for accompanying or occurring in place of a name and having entered common usage. It has various shades of meaning when applied to seemingly real or fictitious people, di ...
''bergii'' commemorates Carl Heinrich Bergius, a
Prussia Prussia, , Old Prussian: ''Prūsa'' or ''Prūsija'' was a German state on the southeast coast of the Baltic Sea. It formed the German Empire under Prussian rule when it united the German states in 1871. It was ''de facto'' dissolved by an em ...
n pharmacist and botanist who collected the first specimens of this tern near
Cape Town Cape Town ( af, Kaapstad; , xh, iKapa) is one of South Africa's three capital cities, serving as the seat of the Parliament of South Africa. It is the legislative capital of the country, the oldest city in the country, and the second largest ...
.Higgins & Davies (1996) 605–609 The greater crested tern has about five geographical races, differing mainly in the colour of the upperparts and bill. 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 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 species ...
''T. b. bergii'' is long, with a wing-span; this subspecies weighs 325–397  g (11.4–14.0  oz). 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 the manner in which an animal routinely casts off a part of its body (often, but not always, an outer ...
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 tail ...
s. The northern subspecies ''T. b. velox'' and ''T. b. thalassina'' are in breeding plumage from May to September or October, whereas the relevant period for the two southern African races is from December to April. For ''T. b. cristata'', the moult timing depends on location; birds from Australia and
Oceania Oceania (, , ) is a region, geographical region that includes Australasia, Melanesia, Micronesia, and Polynesia. Spanning the Eastern Hemisphere, Eastern and Western Hemisphere, Western hemispheres, Oceania is estimated to have a land area of ...
are in breeding plumage from September to about April, but those in
Thailand Thailand ( ), historically known as Siam () and officially the Kingdom of Thailand, is a country in Southeast Asia, located at the centre of the Indochinese Peninsula, spanning , with a population of almost 70 million. The country is bo ...
, China and
Sulawesi Sulawesi (), also known as Celebes (), is an island in Indonesia. One of the four Greater Sunda Islands, and the world's eleventh-largest island, it is situated east of Borneo, west of the Maluku Islands, and south of Mindanao and the Sulu Ar ...
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 production i ...
, 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'' and ''T. b. enigma'' breed in Southern Africa from Namibia to
Tanzania Tanzania (; ), officially the United Republic of Tanzania ( sw, Jamhuri ya Muungano wa Tanzania), is a country in East Africa within the African Great Lakes region. It borders Uganda to the north; Kenya to the northeast; Comoro Islands and ...
, and possibly on islands around Madagascar. There is then a break in the breeding distribution of this species until
Somalia Somalia, , Osmanya script: 𐒈𐒝𐒑𐒛𐒐𐒘𐒕𐒖; ar, الصومال, aṣ-Ṣūmāl officially the Federal Republic of SomaliaThe ''Federal Republic of Somalia'' is the country's name per Article 1 of thProvisional Constituti ...
and the Red Sea, and another discontinuity further east in southern
India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...
. The greater crested tern breeds on many islands in the Indian Ocean including
Aldabra Aldabra is the world's second-largest coral atoll, lying south-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 of the Seychelles, with a distance of 1,120 k ...
and Etoile in the Seychelles, the
Chagos Archipelago The Chagos Archipelago () or Chagos Islands (formerly the Bassas de Chagas, and later the Oil Islands) is a group of seven atolls comprising more than 60 islands in the Indian Ocean about 500 kilometres (310 mi) south of the Maldives archi ...
, and Rodrigues. There are colonies on numerous Pacific islands, including
Kiribati Kiribati (), officially the Republic of Kiribati ( gil, ibaberikiKiribati),Kiribati
''The Wor ...
,
Fiji Fiji ( , ,; fj, Viti, ; Fiji Hindi: फ़िजी, ''Fijī''), 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 ...
,
Tonga Tonga (, ; ), officially the Kingdom of Tonga ( to, Puleʻanga Fakatuʻi ʻo Tonga), is a Polynesian country and archipelago. The country has 171 islands – of which 45 are inhabited. Its total surface area is about , scattered over in ...
, the
Society Islands The Society Islands (french: Îles de la Société, officially ''Archipel de la Société;'' ty, Tōtaiete mā) are an archipelago located in the South Pacific Ocean. Politically, they are part of French Polynesia, an overseas country of the F ...
and the
Tuamotus The Tuamotu Archipelago or the Tuamotu Islands (french: Îles Tuamotu, 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, extendin ...
. 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 inhabitants in 2020 ...
, 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. thalassina'' winters on the east African coast north to
Kenya ) , national_anthem = "Ee Mungu Nguvu Yetu"() , image_map = , map_caption = , image_map2 = , capital = Nairobi , coordinates = , largest_city = Nairobi , ...
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. cristata'' mostly stays within of its colonies, but some birds wander up to around . This species has occurred as a vagrant to
Hawaii Hawaii ( ; haw, Hawaii or ) is a state in the Western United States, located in the Pacific Ocean about from the U.S. mainland. It is the only U.S. state outside North America, the only state that is an archipelago, and the only stat ...
,
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 count ...
,
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 shares borders with China and Russia to the north, at the Yalu River, Y ...
,
Jordan Jordan ( ar, الأردن; tr. ' ), officially the Hashemite Kingdom of Jordan,; tr. ' is a country in Western Asia. It is situated at the crossroads of Asia, Africa, and Europe, within the Levant region, on the East Bank of the Jordan Rive ...
, and
Israel Israel (; he, יִשְׂרָאֵל, ; ar, إِسْرَائِيل, ), officially the State of Israel ( he, מְדִינַת יִשְׂרָאֵל, label=none, translit=Medīnat Yīsrāʾēl; ), is a country in Western Asia. It is situated ...
.Snow & Perrin (1998) 770–771 In India, the Greater crested tern is protected in the
PM Sayeed Marine Birds Conservation Reserve PM Sayeed Marine Birds Conservation Reserve is the first protected area for marine birds in India. It is located in the Indian Union Territory of Lakshadweep. It was formed in 2020. It covers an area of 62 km2. The PM Sayeed Marine Birds Conservati ...
.


Behaviour


Breeding

The greater crested tern breeds 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'' ...
, often in association with other seabirds. It is
monogamous Monogamy ( ) is a form of Dyad (sociology), dyadic Intimate relationship, relationship in which an individual has only one Significant other, partner during their lifetime. Alternately, only one partner at any one time (Monogamy#Serial monogamy, ...
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 do live on or near the bottom, and reef fish that are associated with coral reef ...
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 large, shallow sea enclosed on three sides by northern Australia and bounded on the north by the eastern Arafura Sea (the body of water that lies between Australia and New Guinea). The northern boundary is ...
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 atmospheric circulation and precipitation associated with annual latitudinal oscil ...
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, 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 molluscs of the order Sepiida. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control of ...
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 In biology, altricial species are those in which the young are underdeveloped at the time of birth, but with the aid of their parents mature after birth. Precocial species are those in which the young are relatively mature and mobile from the mome ...
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''), also known as the king gull, it is a small gull. It was formerly sometimes considered to be a subspecies of the silver gull (''C. novaehollandiae''), and, as is the case with many gulls, it has t ...
, which also shares the more typical nesting sites of the nominate race. 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, part ...
s, and that species, along with Hartlaub's gull,
silver gull The silver gull (''Chroicocephalus novaehollandiae'') 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 (''Larus pacificus''), which ...
and sacred ibis, 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 Seine fishing (or seine-haul fishing; ) is a method of fishing that employs a surrounding net, called a seine, that hangs vertically in the water with its bottom edge held down by weights and its top edge buoyed by floats. Seine nets can be dep ...
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 Pacifi ...
es and
petrel Petrels are tube-nosed seabirds in the bird order Procellariiformes. Description The common name does not indicate relationship beyond that point, as "petrels" occur in three of the four families within that group (all except the albatross f ...
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 ( af, Robbeneiland) 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 word for seals (''robben''), hence the Dutch/Afrik ...
, South Africa by marine foam, generated by a combination of wave action,
kelp Kelps are large brown algae seaweeds that make up the order Laminariales. There are about 30 different genera. Despite its appearance, kelp is not a plant - it is a heterokont, a completely unrelated group of organisms. Kelp grows in "underwat ...
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. The direction of their movement is always opposite to that of the secretion of m ...
and
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'. Ph ...
. 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 class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...
s,
crustacea Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean gro ...
ns and
insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pairs ...
s. 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, ...
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 (biology), family Cheloniidae. It is the only species In biology, a spec ...
hatchlings. The great 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 (as illustrated on the right). The word ''pelagic'' is derived . The pelagic zone can be thought of as an imaginary cylinder or wa ...
fish such as
anchovy An anchovy is a small, common forage fish of the 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 species are placed in 1 ...
and
sardine "Sardine" and "pilchard" are common names for various species of small, oily forage fish in the herring family Clupeidae. The term "sardine" was first used in English during the early 15th century, a folk etymology says it comes from the Ital ...
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 more than 2,000 species in more than 200 genera. Most of gobiid fish are relatively small, typically less than in length, and the ...
. 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 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 Linnaeu ...
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 retinas of vertebrate eyes including the human eye. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cone ...
s of the
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 then ...
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 un ...
, 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 (), also called tetraterpenoids, are yellow, orange, and red organic compound, organic pigments that are produced by plants and algae, as well as several bacteria, and Fungus, fungi. Carotenoids give the characteristic color to pumpki ...
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 compo ...
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 ecosystems. The name comes from the Greek words (), meaning 'plant', and (), meaning 'wanderer' or 'drifter'. Ph ...
on which the fish feed, or observing other terns diving for food. Tern's eyes are not particularly
ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...
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 the world's most comprehensive inventory of the global conservation status of biol ...
. 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, also known as the Gulf of Siam, is a shallow inlet in the southwestern South China Sea, bounded between the southwestern shores of the Indochinese Peninsula and the northern half of the Malay Peninsula. It is around in l ...
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 and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Guine ...
where egg harvesting has caused declines. All subspecies except ''T. b. cristata'' 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 Conventio ...
). 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 or swift tern, is a tern in the family Laridae that nests in dense colonies on coastlines and islands in the tropical and subtropical Old World ...
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 or swift tern, is a tern in the family Laridae that nests in dense colonies on coastlines and islands in the tropical and subtropical Old World ...
greater crested tern The greater crested tern Retrieved 28 February 2012 (''Thalasseus bergii''), also called crested tern or swift tern, is a tern in the family Laridae that nests in dense colonies on coastlines and islands in the tropical and subtropical Old World ...