White tern

The white tern or common white tern (Gygis alba) is a small seabird found across the tropical oceans of the world.^[2] It is sometimes known as the fairy tern, although this name is potentially confusing as it is also the common name of Sternula nereis. Other names for the species include angel tern and white noddy in English, and manu-o-Kū in Hawaiian. In the Cook Islands, it is known as the kakaia.

White tern
Nominate Gygis alba alba on Ascension Island
Conservation status
Least Concern  (IUCN 3.1)[1]
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Order:	Charadriiformes
Family:	Laridae
Genus:	Gygis
Species:	G. alba
Binomial name
Gygis alba (Sparrman, 1786)

Taxonomy

The white tern was first formally described by the Swedish naturalist Anders Sparrman in 1786 under the binomial name Sterna alba.^[3] The genus Gygis was introduced by the German zoologist Johann Georg Wagler in 1832.^[4] The name Gygis is from the Ancient Greek guges for a mythical bird and the specific alba is Latin for "white".^[5]

Molecular phylogenetic studies have shown that the white tern is more closely related to the noddies (Anous) than it is to the other terns.^[6][7] This implies that "white noddy" would be a more appropriate English name;^[8] this has been taken up by at least one major text.^[9]

The white tern has four accepted subspecies:^[8][10][11]

• G. a. alba (Sparrman, 1786) — islands of the south tropical Atlantic Ocean including Fernando de Noronha, Trindade, Martin Vas Rocks, Ascension and Saint Helena islands. Larger, 25–30 cm; bill slender, slightly downcurved, all-black; legs dark grey.
• G. a. candida (J. F. Gmelin, 1789) — Indian Ocean islands (Seychelles, Mascarene Islands, and southern Maldives) east to the central Pacific Ocean but excluding range of G. a. leucopes and G. a. microrhyncha in the more southeastern parts of the central Pacific. Larger, 25–30 cm; bill stout, black with a dark blue-grey base, and slightly uptilted at the gonydeal angle; legs dark grey.
• G. a. leucopes Holyoak & Thibault, 1976 — Henderson and Pitcairn Islands. Similar to G. a. candida but with pale grey legs.
• G. a. microrhyncha H. Saunders, 1876 (Little white tern) — Line Islands, Marquesas Islands, Phoenix Islands and Kiribati. Smaller, 23–25 cm, with a slender black bill slightly uptilted at the gonydeal angle; legs pale grey to pinkish.

The little white tern, usually considered a subspecies G. a. microrhyncha of the white tern, is treated as a separate species Gygis microrhyncha by some authors,^[12] but not by either the IOC World Bird List or the Clements Checklist of Birds of the World.^[8]

One study has gone further and suggested that there may be three species of Gygis, with Gygis alba in the Atlantic Ocean, Gygis candida in the Indian and western to central Pacific Oceans, and Gygis microrhyncha in the southeast-central Pacific.^[11] This study also proposed that G. candida is gradually extending its range southeastwards, and may in the future result in the extinction of G. microrhyncha by replacing it.^[11]

Description

The white tern is 23–30 cm (9.1–11.8 in) long with a wingspan of 76–87 cm (30–34 in).^[10] It has pure white plumage, except for a dark streak along the shafts of the outer primary feathers in G. a. candida), a black eye accentuated by a narrow ring of black feathers round the eye, and a long black to bluish-black bill. The tail is shallowly forked, but like in the noddies, with the longest feathers the second-from outermost, not the outermost as in other terns. The legs are dark grey, to paler grey in G. a. leucopes and G. a. microrhyncha.^[10][13] The juvenile is white mottled with grey or greyish-brown. Nesting on coral islands, usually on trees with small branches but also on rocky ledges and on man-made structures, the white tern feeds on small fish which it catches by plunge diving.

Distribution and habitat

The white tern ranges widely across tropical regions of South Atlantic Ocean, the Pacific Ocean from the coasts of Chile and Colombia to New Zealand and along the eastern and southern coasts of Asia from China to India, South Maldives, the islands of the Indian Ocean, and the coast of South Africa.^[1] Occasional vagrants have been found in Japan, Madagascar, Mexico, and on some islands north of the Equator in the Atlantic Ocean. It is a pelagic and epipelagic bird, living along the coast and moving into wooded areas during the breeding season.

• 
  Gygis alba egg - MHNT
• 
  G. a. candida, Tench Island, Bismarck Archipelago, Papua New Guinea
• 
  G. a. candida chick, Midway Island, Hawaii
• 
  G. a. candida parents near their chick, Cousin Island, Seychelles
• 
  G. a. candida juvenile showing the mottled plumage, Cousin Island, Seychelles
• 
  G. a. candida with a fish
• 
  G. a. microrhyncha, Kiribati
• 
  G. a. candida in flight, Rarotonga, Cook Islands

Behaviour

This species is notable for laying its egg on bare thin branches in a small fork or depression without a nest. This behaviour is unusual for terns, which generally nest on the ground, and even the related tree-nesting black noddy constructs a nest. It is thought that the reason for the absence of nests is the reduction in nest parasites, which in some colonial seabirds can cause the abandonment of an entire colony.^[14] In spite of these benefits there are costs associated with tree nesting, as the eggs and chicks are vulnerable to becoming dislodged by heavy winds. For this reason the white tern is also quick to relay should it lose the egg. The newly hatched chicks have well-developed feet with which to hang on to their precarious nesting site. It is a long-lived bird, having been recorded living for 42 years.^[15]

Predators

Giant tortoises have been observed to hunt chicks of this bird on Fregate Island in the Seychelles.^[16]

Relationship with humans

The white tern, manu-o-Kū, was named Honolulu, Hawaiʻi's official bird on April 2, 2007.

New Zealand's Department of Conservation classifies the white tern as Nationally Critical, with populations having been largely decimated by the introduction of feral cats and rats on Raoul Island, the terns' only breeding site in the country.^[17] As of 2016, the white tern population in New Zealand was reported to be increasing following the eradication of introduced predators in 2002.^[17][18] Globally, the white tern has a large range that is home to several large colonies, and both recognised species are listed as Least Concern by the IUCN Red List.