Greater Crested Tern
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| Greater Crested Tern | ||||||||||||||
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| Sterna bergii Lichtenstein, 1823 |
The Greater Crested Tern, or Swift Tern, Sterna bergii, is a seabird of the tern family Sternidae.
It breeds in tropical and subtropical coastal parts of the Old World from South Africa around the Indian Ocean to the western Pacific and Australia. All populations disperse widely when not breeding.
The Greater Crested Tern breeds in dense colonies on coasts and islands. It nests in a ground scrape and lays two eggs. Nesting behaviour is very similar to that of Sandwich Terns, with predator avoidance by mutual protection in large colonies.
Like all Sterna terns, the Greater Crested Tern feeds by plunge diving for fish, usually from saline environments. It normally dives directly, and not from the "stepped-hover" favoured by the Arctic Tern. The offering of fish by the male to the female is part of the courtship display.
This is a large tern, 45-48 cm long with a 100 cm wingspan. Sexes are similar; the summer adult has a black cap with a long crest, a narrow white forehead band, black legs and a long sharp cold yellow bill. The back and upperwings, are medium grey with a paler rump and the underparts are white. The primary flight feathers darken during the summer.
In winter, the head becomes more extensively white and the crest is peppered with white. Juvenile birds have heavily marked upperparts and wings, with patterning of brown, white and some grey. The closed wings in particular appear to have dark bars. The head and underparts are like the winter adult. The call is a loud grating keraak noise like a gruff Sandwich Tern.
The Greater Crested Tern has four geographical races, differing mainly in the shade of the upperparts and bill colour.
- S. b. velox: breeding on the northern Indian Ocean coasts and wintering in East Africa north of the equator. Largest and darkest subspecies.
- S. b.thalassina: breeding western Indian Ocean. Smallest and palest form.
- S. b. bergii: breeding South Africa. Dark grey above and slightly larger than thalassina.
- S. b. cristata: breeding Eastern Indian Ocean, Australia and Pacific. Like bergii, with tail, rump and back concolorous.
This species has to be distinguished from the closely related Lesser Crested Tern, which is smaller, orange-billed and usually has paler upperparts.
Reference
- Seabirds by Harrison, ISBN 0-7470-8028-8
- Birds of India by Grimmett, Inskipp and Inskipp, ISBN 0-691-04910-6
- Terns of Europe and North America by Olsen and Larsson, ISBN 0-7136-4056-1nl:Geelsnavelstern