Gull-billed Tern (Gelochelidon nilotica)

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The Gull-billed Tern is a medium-sized, heavy-billed tern with a broad distribution, and notably varied diet. Watch for this uncommon tern flying into the wind over marshlands, catching flying insects in the air, plucking prey from the ground or water surface, or nesting among other terns and seabirds.


Contents

Taxonomy

The Gull-billed Tern, Gelochelidon nilotica, formerly known as Sterna nilotica is a seabird of the tern family Sternidae belongs to genus Gelochelidon. There are 6 subspecies of gull billed tern differing mainly in size and minor plumage details.

Distribution

Gull-billed tern breeds in warmer parts of the world in southern Europe, temperate and eastern Asia, both coasts of North America, eastern South America and Australia. This bird has a number of geographical races, differing mainly in size and minor plumage details. All forms show a post-breeding dispersal, but the northern breeders are most migratory, wintering south to Africa, the Caribbean and northern South America, southern Asia and New Zealand.


Physical Characteristics

Gull-billed tern is about 33-38 cm in length and has an average mass of 233 grams. Body mass appears to be similar between sexes. During the breeding season, this bird has a white back and wings, black cap and stout black bill. During the non-breeding season, gull-billed terns lack the black cap. The Gull-billed Tern can be identified in all plumages by its thick, relatively short, black bill and shallowly forked tail. The Gull-billed Tern gets its name from the fact that its bill is thicker and more gull-like than in other terns but it might have been more helpful to call it the 'gull-winged tern' as it is the broader wings and more leisurely gull-like flight which make it stand out from other terns. Its relatively large size and black bill separate it from all except Sandwich Tern but notice that the upper wing looks much cleaner, uniformly pearl-grey with just a vague dark border to the primaries. Also, the neat black crown and the stout black bill give the head a much tidier look. From below, the black trailing edge to the primaries is more distinct than on any other tern. In winter and juvenile plumages the head looks strikingly white apart from a finger of black leading backwards from the eye.


Habitat

Gull-billed terns found in colonies close to the coastal lagoons and inside the grounds, close to the salt-water marshes. It likes the sandy beaches and the coastal marshes. It is migrating. It breed in colonies, and often reproduces in company of other species of terns.


Diet

Gull-billed terns feed by "aerial-dipping" in which the bird surveys the feeding area by passing back and forth, then dives down to pluck the prey item from land or water. This species is capable of exploiting locally abundant prey including many kinds of terrestrial and aquatic species like insects, small mammals, batrachians, shellfish and sometimes of eggs, chicks and fish.. Specific preferences include invertebrates and worms in plowed fields, fish and crustaceans.


Behavior

Gull-billed terns emit a kind of squealing similar to its product by a grasshopper. The adults emit deep and guttural gek-gek-gek or gu-saw. The youthful ones have soft “pi-iip” or a rapid “pi-pi-iip”. The cry of alarm is a nasillard kvay-kvay-kvay. Gull-billed terns are rather quiet in winter. Contrary to the other terns, Gull-billed terns seeks its food in the grounds, posed on a perch and flying away to catch insects with the flight. It can also continue them on the ground. Gull-billed terns have a nimble flight which enables him to catch its preys in flight and on the surface of water. It often makes on the spot, with the manner of the falcons. Its flight is gracious but rather stiff, resembling more that of a seagull, with the wings maintained right, less folded backwards

Breeding

This species breeds colonially, often with other terns and skimmers, in coastal marshes and sandy beaches, occasionally in the presence of other species of terns. Two to three spotted buff eggs are laid in a shell-lined shallow depression or occasionally a well-made cup of dead marsh grass. Young are semiprecocial. The maximum age of a gull-billed tern recorded in nature 15 years.


References

  • The Clements Checklist of Birds of the World: James F. Clements, Jared Diamond, John W. Fitzpatrick.
  • Wetlands International Waterbird Population Estimates - Fourth Edition.
  • Ali, S. & Ripley, D. (1964-74 ) Handbook of the Birds of India & Pakistan (Vols. 1-10). Bombay: OUP
  • Grimmet, R Inskipp, T., & Inskipp, C. (1998) Birds of the Indian Subcontinent. UK: A&C Black.
  • Inskipp, T. et al. (1996) An Annotated Checklist of the Birds of the Oriental Region. Sandy, UK: OBC.
  • Kazmierczak, K. & van Perlo, B. (2000) A Field-Guide to the Birds of the Indian Subcontinent. UK: Pica Press
  • BirdLife International (2004).
  • IUCN Red List of Threatened Species. IUCN 2006
  • 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 Phylogenetics and Evolution
  • Harrison, Peter (1988): Seabirds (2nd edition). Christopher Helm, London
  • Peterson, R.T., Mountfort, G. & Hollom, P.A.D. (1993) Collins Field Guide- Birds of Britain and Europe.