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California least tern
The California least tern, ''Sternula antillarum browni'', is a subspecies of least tern that breeds primarily in bays of the Pacific Ocean within a very limited range of Southern California, in San Francisco Bay and in and extreme northern Mexico. This migratory bird is a U.S. federally listed endangered subspecies. The total population of the subspecies amounted to 582 breeding pairs in 1974, when census work on this bird began. While numbers have gradually increased with its protected status, the species is still vulnerable to natural disasters or further disturbance by man.
==Nesting and breeding behavior==
Wintering locations are actually unknown, but suspected to include the South American Pacific Coast. The California least tern arrives at its breeding grounds in late April. 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 breeding colonies are not dense and may appear along marine or estuarine shores in areas free from humans or predators.
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 semi-precocial 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 California by the end of August.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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