The Greater Crested Tern (Thalasseus bergii), also called Crested Tern or Swift Tern, is a seabird

Seabird

Seabirds are birds that have 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 environmental problems and feeding niches have resulted in similar adaptations...

 in the tern

Tern

Terns are seabirds in the family Sternidae, previously considered a subfamily of the gull family Laridae . They form a lineage with the gulls and skimmers which in turn is related to skuas and auks...

 family which nests in dense colonies on coastlines and islands in the tropical and subtropical Old World

Old World

The Old World consists of those parts of the world known to classical antiquity and the European Middle Ages. It is used in the context of, and contrast with, the "New World" ....

. Its five subspecies

Subspecies

Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...

 breed in the area from South Africa

South Africa

The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...

 around the Indian Ocean

Indian Ocean

The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...

 to the central Pacific

Pacific Ocean

The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...

 and Australia

Australia

Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

, all populations dispersing

Bird migration

Bird migration is the regular seasonal journey undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat or weather. Sometimes, journeys are not termed "true migration" because they are irregular or in only one direction...

 widely from the breeding range after nesting. This large tern is closely related to the Royal

Royal Tern

The Royal Tern is a seabird in the tern family Sternidae. This bird has two distinctive subspecies. T. m. maximus breeds on the Atlantic and Pacific coasts of the southern USA and Mexico into the Caribbean. The slightly smaller T. m. albididorsalis breeds in coastal west Africa...

 and Lesser Crested Tern

Lesser Crested Tern

The Lesser Crested Tern is a seabird of the tern family Sternidae...

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 which 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

Fledge

Fledge is the stage in a young bird's life when the feathers and wing muscles are sufficiently developed for flight. It also describes the act of a chick's parents raising it to a fully grown state...

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 which has learned to follow fishing boats for jettisoned bycatch

Bycatch

The term “bycatch” is usually used for fish caught unintentionally in a fishery while intending to catch other fish. It may however also indicate untargeted catch in other forms of animal harvesting or collecting...

, and to utilise unusual nest sites such as the roofs of buildings and artificial islands in salt pans

Salt evaporation pond

Salt evaporation ponds, also called salterns or salt pans, are shallow artificial ponds designed to produce salts from sea water or other brines. The seawater or brine is fed into large ponds and water is drawn out through natural evaporation which allows the salt to be subsequently harvested...

 and sewage works

Sewage treatment

Sewage treatment, or domestic wastewater treatment, is the process of removing contaminants from wastewater and household sewage, both runoff and domestic. It includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants...

. Its eggs and young are taken by gull

Gull

Gulls are birds in the family Laridae. They are most closely related to the terns and only distantly related to auks, skimmers, and more distantly to the waders...

s and ibis

Ibis

The ibises are a group of long-legged wading birds in the family Threskiornithidae....

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

Gull

Gulls are birds in the family Laridae. They are most closely related to the terns and only distantly related to auks, skimmers, and more distantly to the waders...

s, skimmer

Skimmer

The Skimmers, Rynchopidae, are a small family of tern-like birds in the order Charadriiformes, which also includes the waders, gulls and auks. The family comprises three species found in South Asia, Africa, and the Americas....

s and skua

Skua

The skuas are a group of seabirds with about seven species forming the family Stercorariidae and the genus Stercorarius. The three smaller skuas are called jaegers in North America....

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 long bulgy legs. Most species are grey above and white below, and have a black cap which is reduced or flecked with white in the winter.

Relationships in the genus Thalasseus
The Greater Crested Tern was originally described as Sterna bergii by German naturalist Martin Lichtenstein

Martin Lichtenstein

Martin Hinrich Carl Lichtenstein was a German physician, explorer, zoologist, and herpetologist.-Biography:...

 in 1823, but was moved to its current genus, Thalasseus

Thalasseus

Thalasseus, the crested terns, is a genus of six species of seabirds in the tern family. Thalasseus signifies a "creature of the sea". It has a worldwide distribution, and many of its species are abundant and well-known birds in their ranges...

, after mitochondrial DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 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

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

s.

The Greater Crested Tern's closest relatives within its genus appear to be the Lesser Crested Tern (T. bengalensis), and the Royal Tern, (T. maximus). The DNA study did not include the critically endangered Chinese Crested Tern

Chinese Crested Tern

The Chinese Crested Tern is a seabird of the tern family Sternidae, closely related to Sandwich Tern T. sandvicensis and Lesser Crested Tern T. bengalensis...

 (T. bernsteini), but as that bird was formerly considered to be conspecific

Conspecificity

Conspecificity is a concept in biology. Two or more individual organisms, populations, or taxa are conspecific if they belong to the same species....

 with the Greater Crested Tern as a synonym

Synonym

Synonyms are different words with almost identical or similar meanings. Words that are synonyms are said to be synonymous, and the state of being a synonym is called synonymy. The word comes from Ancient Greek syn and onoma . The words car and automobile are synonyms...

 of the subspecies T. b. cristatus, it is presumably also very closely related.

The genus name of the Greater Crested Tern is derived from Greek

Greek language

Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 Thalassa, "sea", and the species epithet

Epithet

An epithet or byname is a descriptive term 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, divinities, objects, and binomial nomenclature. It is also a descriptive title...

 bergii commemorates Carl Heinrich Bergius

Karl Heinrich Bergius

Karl Heinrich Bergius , also known as Carl Heinrich Bergius, was a Prussian botanist, naturalist, cavalryman and pharmacist from Küstrin. He is notable for his natural history collecting in southern Africa....

, a Prussia

Prussia

Prussia was a German kingdom and historic state originating out of the Duchy of Prussia and the Margraviate of Brandenburg. For centuries, the House of Hohenzollern ruled Prussia, successfully expanding its size by way of an unusually well-organized and effective army. Prussia shaped the history...

n pharmacist and botanist who collected the first specimens of this tern near Cape Town

Cape Town

Cape Town is the second-most populous city in South Africa, and the provincial capital and primate city of the Western Cape. As the seat of the National Parliament, it is also the legislative capital of the country. It forms part of the City of Cape Town metropolitan municipality...

.

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

Taxonomic sequence is a sequence followed in listing of taxa which aids ease of use and roughly reflects the evolutionary relationships among the taxa...

. A similar number of other potential subspecies have been proposed, but are not considered valid.

Subspecies	Breeding range	Distinctive features	Population estimates
T. b. bergii (Lichtenstein, 1823)	Coasts of South Africa South Africa The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...  and Namibia Namibia Namibia, officially the Republic of Namibia , is a country in southern Africa whose western border is the Atlantic Ocean. It shares land borders with Angola and Zambia to the north, Botswana to the east and South Africa to the south and east. It gained independence from South Africa on 21 March...	Dark grey above, slightly larger than thalassina, least white on head	20,000 individuals (inc 6,336 breeding pairs in South Africa and up to 1,682 pairs in Namibia)
T. b. enigma (Clancey Phillip Clancey Dr Phillip Alexander Clancey DSc was a leading authority on the ornithology of South Africa.- Background and education :Phillip Clancey was born, brought up and educated in Glasgow, Scotland... , 1979)	Zambezi Zambezi The Zambezi is the fourth-longest river in Africa, and the largest flowing into the Indian Ocean from Africa. The area of its basin is , slightly less than half that of the Nile...  delta, Mozambique Mozambique Mozambique, officially the Republic of Mozambique , is a country in southeastern Africa bordered by the Indian Ocean to the east, Tanzania to the north, Malawi and Zambia to the northwest, Zimbabwe to the west and Swaziland and South Africa to the southwest... , south to Durban Durban Durban is the largest city in the South African province of KwaZulu-Natal and the third largest city in South Africa. It forms part of the eThekwini metropolitan municipality. Durban is famous for being the busiest port in South Africa. It is also seen as one of the major centres of tourism... , South Africa	Palest subspecies	8,000–10,000 individuals in Madagascar Madagascar The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...  and Mozambique
T. b. cristata (Stephens James Francis Stephens James Francis Stephens was an English entomologist.-Biography:Stephens was born in Shoreham-by-Sea and studied at Christ's Hospital.He was employed in the Admiralty office, Somerset House, from 1807 to 1845... , 1826)	Eastern Indian Ocean Indian Ocean The Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and... , Australia Australia Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...  and western Pacific Ocean Pacific Ocean The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...	Like bergii, with tail, rump and back concolorous. paler in Australia	500,000+ individuals in Australia
T. b. thalassina (Stresemann Erwin Stresemann Erwin Stresemann was a German naturalist and ornithologist.Stresemann was one of the outstanding ornithologists of the 20th century... , 1914)	Western Indian Ocean	Small and pale, larger and less pale in south of range	2,550–4,500 individuals in Eastern Africa and Seychelles Seychelles Seychelles , officially the Republic of Seychelles , is an island country spanning an archipelago of 115 islands in the Indian Ocean, some east of mainland Africa, northeast of the island of Madagascar....
T. b. velox (Cretzschmar Philipp Jakob Cretzschmar Philipp Jakob Cretzschmar was a German physician.Cretzschmar was born at Sulzbach and studied medicine at the University of Würzburg. He taught anatomy and zoology at the Senckenberg Medical Institute of Frankfurt.... , 1827)	Red Sea Red Sea The Red Sea is a seawater inlet of the Indian Ocean, lying between Africa and Asia. The connection to the ocean is in the south through the Bab el Mandeb strait and the Gulf of Aden. In the north, there is the Sinai Peninsula, the Gulf of Aqaba, and the Gulf of Suez... , Persian Gulf Persian Gulf The Persian Gulf, in Southwest Asia, is an extension of the Indian Ocean located between Iran and the Arabian Peninsula.The Persian Gulf was the focus of the 1980–1988 Iran-Iraq War, in which each side attacked the other's oil tankers... , northern Indian Ocean	Largest, heaviest, darkest and longest-billed subspecies	33,000 in Middle East Middle East The Middle East is a region that encompasses Western Asia and Northern Africa. It is often used as a synonym for Near East, in opposition to Far East...  (inc 4,000 pairs Oman Oman Oman , officially called the Sultanate of Oman , is an Arab state in southwest Asia on the southeast coast of the Arabian Peninsula. It is bordered by the United Arab Emirates to the northwest, Saudi Arabia to the west, and Yemen to the southwest. The coast is formed by the Arabian Sea on the...  and 3,500 pairs on islands off Saudi Arabia Saudi Arabia The Kingdom of Saudi Arabia , commonly known in British English as Saudi Arabia and in Arabic as as-Sa‘ūdiyyah , is the largest state in Western Asia by land area, constituting the bulk of the Arabian Peninsula, and the second-largest in the Arab World... )

Description

The Greater Crested Tern is a large tern with a long (5.4–6.5 cm, 2.1–2.6 in) yellow bill, black legs, and a glossy black crest which is noticeably shaggy at its rear. The breeding adult of the nominate subspecies T. b. bergii is 46–49 cm (18–19 in) long, with a 125–130 cm (49–51 in) wing-span; this subspecies weighs 325–397 g

Gram

The gram is a metric system unit of mass....

 (11.4–14.0 oz

Ounce

The ounce is a unit of mass with several definitions, the most commonly used of which are equal to approximately 28 grams. The ounce is used in a number of different systems, including various systems of mass that form part of the imperial and United States customary systems...

). 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

Moult

In biology, moulting or molting , also known as sloughing, shedding, or for some species, ecdysis, is the manner in which an animal routinely casts off a part of its body , either at specific times of year, or at specific points in its life cycle.Moulting can involve the epidermis , pelage...

ing, the young terns resemble the adult, but still have a variegated wing pattern with a dark bar on the inner flight feather

Flight feather

Flight feathers are the long, stiff, asymmetrically shaped, but symmetrically paired feathers on the wings or tail of a bird; those on the wings are called remiges while those on the tail are called rectrices . Their primary function is to aid in the generation of both thrust and lift, thereby...

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

Oceania is a region centered on the islands of the tropical Pacific Ocean. Conceptions of what constitutes Oceania range from the coral atolls and volcanic islands of the South Pacific to the entire insular region between Asia and the Americas, including Australasia and the Malay Archipelago...

 are in breeding plumage from September to about April, but those in Thailand

Thailand

Thailand , officially the Kingdom of Thailand , formerly known as Siam , is a country located at the centre of the Indochina peninsula and Southeast Asia. It is bordered to the north by Burma and Laos, to the east by Laos and Cambodia, to the south by the Gulf of Thailand and Malaysia, and to the...

, China and Sulawesi

Sulawesi

Sulawesi is one of the four larger Sunda Islands of Indonesia and is situated between Borneo and the Maluku Islands. In Indonesia, only Sumatra, Borneo, and Papua are larger in territory, and only Java and Sumatra have larger Indonesian populations.- Etymology :The Portuguese were the first to...

 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, especially at its breeding grounds. The territorial advertising call is a loud, raucous, crow-like

Crow

Crows form the genus Corvus in the family Corvidae. Ranging in size from the relatively small pigeon-size jackdaws to the Common Raven of the Holarctic region and Thick-billed Raven of the highlands of Ethiopia, the 40 or so members of this genus occur on all temperate continents and several...

 kerrak. Other calls include a korrkorrkorr given at the nest by anxious or excited birds, and a hard wep wep 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

The United Republic of Tanzania is a country in East Africa bordered by Kenya and Uganda to the north, Rwanda, Burundi, and the Democratic Republic of the Congo to the west, and Zambia, Malawi, and Mozambique to the south. The country's eastern borders lie on the Indian Ocean.Tanzania is a state...

, and possibly on islands around Madagascar. There is then a break in the breeding distribution of this species until Somalia

Somalia

Somalia , officially the Somali Republic and formerly known as the Somali Democratic Republic under Socialist rule, is a country located in the Horn of Africa. Since the outbreak of the Somali Civil War in 1991 there has been no central government control over most of the country's territory...

 and the Red Sea, and another discontinuity further east in southern India

India

India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

.

The Greater Crested Tern breeds on many islands in the Indian Ocean including Aldabra

Aldabra

Aldabra, the world's second largest coral atoll, is in the Aldabra Group of islands in the Indian Ocean that form part of the Seychelles. Uninhabited and extremely isolated, Aldabra is virtually untouched by humans, has distinctive island fauna including the Aldabra Giant Tortoise, and is...

 and Etoile in the Seychelles, the Chagos Archipelago

Chagos Archipelago

The Chagos Archipelago , is a group of seven atolls comprising more than 60 individual tropical islands in the Indian Ocean; situated some due south of the Maldives archipelago. This chain of islands are the southernmost archipelago of the Chagos-Laccadive Ridge a long submarine mountain range...

, and Rodrigues.

Rodrigues (island)

Rodrigues , sometimes spelled Rodriguez but named after the Portuguese explorer Diogo Rodrigues, is the smallest of the Mascarene Islands and a dependency of Mauritius...

 There are colonies on numerous Pacific islands, including Kiribati

Kiribati

Kiribati , officially the Republic of Kiribati, is an island nation located in the central tropical Pacific Ocean. The permanent population exceeds just over 100,000 , and is composed of 32 atolls and one raised coral island, dispersed over 3.5 million square kilometres, straddling the...

, Fiji

Fiji

Fiji , officially the Republic of Fiji , is an island nation in Melanesia in the South Pacific Ocean about northeast of New Zealand's North Island...

, Tonga

Tonga

Tonga, officially the Kingdom of Tonga , is a state and an archipelago in the South Pacific Ocean, comprising 176 islands scattered over of ocean in the South Pacific...

, the Society Islands

Society Islands

The Society Islands are a group of islands in the South Pacific Ocean. They are politically part of French Polynesia. The archipelago is generally believed to have been named by Captain James Cook in honor of the Royal Society, the sponsor of the first British scientific survey of the islands;...

 and the Tuamotus

Tuamotus

The Tuamotus or the Tuamotu Archipelago are a chain of islands and atolls in French Polynesia. They form the largest chain of atolls in the world, spanning an area of the Pacific Ocean roughly the size of Western Europe...

.

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, most adults move east to the Indian Ocean coastline of South Africa. Many young birds also travel east, sometimes more than 2,000 km (1,200 mi), but others move northwards along the western coast. T. b. thalassina winters on the east African coast north to Kenya

Kenya

Kenya , officially known as the Republic of Kenya, is a country in East Africa that lies on the equator, with the Indian Ocean to its south-east...

 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

Bird migration

Bird migration is the regular seasonal journey undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat or weather. Sometimes, journeys are not termed "true migration" because they are irregular or in only one direction...

, but those breeding in the Red Sea winter south along the east African coast to Kenya. T. b. cristata mostly stays within 400 km (260 mi) of its colonies, but some birds wander up to around 1,000 km (600 mi). This species has occurred as a vagrant to Hawaii

Hawaii

Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...

, New Zealand

New Zealand

New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...

, North Korea

North Korea

The Democratic People’s Republic of Korea , , is a country in East Asia, occupying the northern half of the Korean Peninsula. Its capital and largest city is Pyongyang. The Korean Demilitarized Zone serves as the buffer zone between North Korea and South Korea...

, Jordan

Jordan

Jordan , officially the Hashemite Kingdom of Jordan , Al-Mamlaka al-Urduniyya al-Hashemiyya) is a kingdom on the East Bank of the River Jordan. The country borders Saudi Arabia to the east and south-east, Iraq to the north-east, Syria to the north and the West Bank and Israel to the west, sharing...

, and Israel

Israel

The State of Israel is a parliamentary republic located in the Middle East, along the eastern shore of the Mediterranean Sea...

.

Breeding

The Greater Crested Tern breeds in colonies, often in association with other seabirds. It is monogamous

Monogamy

Monogamy /Gr. μονός+γάμος - one+marriage/ a form of marriage in which an individual has only one spouse at any one time. In current usage monogamy often refers to having one sexual partner irrespective of marriage or reproduction...

 and the pair bond is maintained through the year and sometimes in consecutive breeding seasons. The colony size is related to the abundance of pelagic fish

Pelagic fish

Pelagic fish live near the surface or in the water column of coastal, ocean and lake waters, but not on the bottom of the sea or the lake. They can be contrasted with demersal fish, which do live on or near the bottom, and reef fish which are associated with coral reefs.The marine pelagic...

 prey, and the largest documented colony, with 13,000 to 15,000 pairs, is in the Gulf of Carpentaria

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 Arafura Sea...

 in northern Australia, a region which also supports major colonies of other seabirds. Since nesting in this area follows the summer monsoon

Monsoon

Monsoon is traditionally defined as 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 the asymmetric heating of land and sea...

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

Broadly, philopatry is the behaviour of remaining in, or returning to, an individual's birthplace. More specifically, in ecology philopatry is the behaviour of elder offspring sharing the parental burden in the upbringing of their siblings, a classic example of kin selection...

, frequently changing its nest site from year to year, sometimes by more than 200 km (130 mi).
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.