Great crested tern  (Sterna bergii)

Classified as Least Concern (LC) on the IUCN Red List (1).

The great crested tern is a large tern with an extensive range. During the breeding season, the adult has a distinctive black cap with a long crest, white neck and underparts, and a grey back and upperwing. In contrast, the non-breeding adult has a white crown with limited dark spotting, but its hind-crown remains black (2). The large bill is greenish-yellow to yellow, and the legs are black (2) (3). The juveniles primarily differ from the adults in having heavily mottled or barred brown upperparts. There is some taxonomic confusion surrounding the number of subspecies of the great crested tern, but six are currently recognised, varying mainly in the colouration of the upperparts and the amount of white on the forehead (2).

Found on coastlines in the south-east Atlantic Ocean, the Indian Ocean and the western Pacific Ocean (4).

The great crested tern forages in the shallow waters of lagoons, coral reefs, estuaries, along all types of shoreline, and also far out to sea in open water. Nesting sites are usually located on offshore islands, low-lying coral reefs, coastal islets, spits, and lagoons (2) (5).

The diet of the great crested tern consists mainly of pelagic fish from 10 - 15 centimetres in length, but it also opportunistically takes squid, crabs, insects, baby turtles, and other aquatic prey. It typically forages in groups, flying several metres above the ocean, every now and again plunging into the water or dipping its bill just under the surface to catch unsuspecting prey. Most foraging occurs within three kilometres of the colony (2) (5).

The great crested tern tends to breed in large, dense colonies or in small groups within larger mixed species colonies. Each breeding individual nests only once in any given year, with the nests being a shallow scrape in sand, gravel or coral, often packed tightly together (2) (5). The clutch size is usually a singe egg, or sometimes two, which are incubated for 25 to 30 days before hatching. The chick fledges after around 38 to 40 days but remains dependent on it its parents until it is at least 4 months of age (2).

Although most great crested terns appear to remain more or less within the vicinity of the breeding colonies throughout the year, the movement patterns of this species are poorly known (2) (5). Certainly, populations in Australia commonly disperse several hundred kilometres after breeding, while those in the Middle East typically over winter in Egypt and east Africa (2).

Whilst the great crested tern is not globally threatened, some populations are vulnerable to egg collecting, human disturbance, and injury and mortality associated with entanglement with fishing nets, lines and hooks, and human refuse (2) (5).

The great crested tern is listed under Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals (CMS), which aims to conserve migratory species throughout their range (6). It is also listed under the associated Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA), which calls upon parties to engage in a range of conservation actions to help protect and conserve bird species that are dependent on wetlands for at least part of their annual cycle (7).

Environment Agency - Abu Dhabi is a principal sponsor of ARKive. EAD is working to protect and conserve the environment as well as promoting sustainable development in the Emirate of Abu Dhabi.

View information on this species at the UNEP World Conservation Monitoring Centre.

For more information on the conservation of migratory birds:

• Convention on Migratory Species (CMS)
  http://www.cms.int
• African-Eurasian Waterbird Agreement:
  http://www.unep-aewa.org/about/introduction.htm

For more information on this and other bird species please see:

• BirdLife International:
  http://www.birdlife.org