Least Tern
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| Image:California Least Tern.jpg Least Tern, California subspecies
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| Sternula antillarum (Lesson, 1847) |
The Least Tern (Sternula antillarum, formerly Sterna antillarum) is a species of tern that breeds in North America and locally in northern South America. It is closely related to, and was formerly often considered conspecific with, the Little Tern S. albifrons of the Old World. Other close relatives include the Yellow-billed Tern S. superciliaris and Peruvian Tern S. lorata, both from South America.
It is a small tern, 22-24 cm long, with a wingspan of 50 cm, and weighing 39-52 g. The upper parts are a fairly uniform pale gray, and the underparts white. The head is white, with a black cap and line through the eye to the base of the bill, and a small white forehead patch above the bill; in winter, the white forehead is more extensive, with a smaller and less sharply defined black cap. The bill is yellow with a small black tip in summer, all blackish in winter. The legs are yellowish. The wings are mostly pale gray, but with conspicuous black markings on their outermost primaries. In behavior, it flies over water with fast, jerky wingbeats and a distinctive hunchback appearance, with the bill pointing slightly downward.
It is migratory, wintering in Central America, the Caribbean and northern South America. It has occurred as a vagrant to Europe, with one record in Great Britain.
It differs from Little Tern mainly in that its rump and tail are gray, not white, and it has a different, more squeaking call; from Yellow-billed Tern in being paler gray above and the bill having a short (not no) black tip; and from Peruvian Tern in being paler gray above and white (not pale gray) below and a shorter black tip to the bill.
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[edit] Subspecies
- Sternula antillarum antillarum (Lesson, 1847). Breeds on the Atlantic coast of North America, from Maine south along the east and south coasts of the United States, Bermuda, the Caribbean, and Venezuela; winters south to northern Brazil.
- Sternula antillarum athalassos (Burleigh & Lowery, 1942). Breeds on the inland rivers of the Mississippi River basin; winters south to northern Brazil.
- Sternula antillarum browni (Mearns, 1916) California Least Tern. Breeds on the Pacific coast of North America, from central California south to western Mexico; winters mainly in Central America.
[edit] Conservation and status
[edit] S. a. antillarum
The population is about 21,500 pairs; it is not considered threatened, though is considered vulnerable, due to disturbance at the breeding sites by beach development and severe recreational pressure (HBW).
[edit] S. a. athalassos
The interior subspecies, with a population of about 1000 pairs, was listed as an endangered subspecies in 1985, due to loss of habitat caused by dams, reservoirs, channelization, and other changes to river systems.
[edit] S. a. brownii
The western population, the California Least Tern, was listed as an endangered species in 1972 with a population of about 600 pairs. With aggressive management, the Californian population has rebounded in recent years to about 4,500 pairs, a marked recent increase from 582 pairs in 1974 when census work began, though it is still listed as an endangered subspecies. The California subspecies breeds on bays of the Pacific Ocean within a very limited range of southern California, in San Francisco Bay and in northwestern Mexico. While numbers have gradually increased with its protected status, it is still vulnerable to natural disasters or further disturbance by man.
[edit] Nesting and breeding behavior
The Least Tern arrives at its breeding grounds in late April. The breeding colonies are not dense and may appear along either marine or estuarine shores, or on sand bar islands in large rivers, in areas free from humans or predators. Courtship typically takes place removed from the nesting colony site, usually on an exposed tidal flat or beach. Only after courtship has confirmed mate selection does nesting begin by mid-May and is usually complete by mid-June. Nests are situated on barren to sparsely vegetated places near water, normally on sandy or gravelly substrates. In the San Francisco Bay region, breeding typically takes place on abandoned salt flats. Where the surface is hard, this species may use an artificial indentation (such as a deep dried footprint) to form the nest basin.
The nest density may be as low as several per acre, but in San Diego County, densities of 200 nests per acre have been observed. Most commonly the clutch size is two or three, but it is not rare to consist of either one or four eggs. Both female and male incubate the eggs for a period of about three weeks, and both parents tend the semiprecocial young. Young birds can fly at age four weeks. After formation of the new families, groupings of birds may appear at lacustrine settings in proximity to the coast. Late season nesting may be renests or late season arrival activity. In any case, the bulk of the population has left the breeding grounds by the end of August.
[edit] Feeding and roosting characteristics
The Least Tern hunts primarily in shallow estuaries and lagoons, where smaller fishes are abundant. They hover until spotting prey, and then plunge into the water without full submersion to extract dinner. In the bays and lagoons of Southern California and northern Mexico, the favored prey include anchovy, smelt, silversides, shiner surfperch and small crustaceans. Not infrequently the terns feed near shore in the open ocean, especially in proximity to lagoons or bay mouths.
Adults do not require cover, so that they commonly roost on the open ground. After young chicks are three days old, they are brooded less frequently by parents and require wind blocks and shade. Notable disruption of colonies can occur from predation by Burrowing Owls and American Kestrels (Collins, 1980). Predation by domestic cats has been observed in at least one colony (California Wildlife, 1990).
[edit] Bibliography
- BirdLife International (2004). Sterna antillarum. 2006 IUCN Red List of Threatened Species. IUCN 2006. Retrieved on 11 May 2006. Database entry includes justification for why this species is of least concern
- National Geographic Society (2002): Field Guide to the Birds of North America. National Geographic, Washington DC. ISBN 0-792-26877-6
- Audubon Handbook: Western Birds, McGraw Hill Book Company (1988) ISBN 0-07-019977-9
- B. Massey, Breeding Biology of the California least tern, Proceedings Linn. Society, New York 72:1-24 (1974)
- California Wildlife, Volume II, Birds, ed, by David C. Zeiner, William F. Laudenslayer and Kenneth E. Meyer, published by the California Department of Fish and Game,Nov , 1988.
- Gary Deghi, C. Michael Hogan et al., Biological Assessment for the Proposed Tijuana/San Diego Joint Innternational Wastewater Treatment Plant, Publication of the U.S. Environmental Protection Agency Region IX, Earth Metrics Incorporated, Burlingame, CA with Harvey and Stanley, Alviso, CA
- L. Collins and S. Bailey, California least tern nesting season at Alameda Naval Air Station, 1980 Admin. Rep. 25pp.
- Olsen and Larsson, Terns of Europe and North America ISBN 0-7136-4056-1nl:Amerikaanse dwergstern
