The California least tern, ''Sternula antillarum browni'', is a

subspecies In biological classification, subspecies is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics ( morphology), but that can successfully interbreed. Not all specie ...

least tern The least tern (''Sternula 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 of the Old World. Ot ...

that breeds primarily in bays of the

Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the conti ...

within a very limited range of

Southern California Southern California (commonly shortened to SoCal) is a geographic and cultural region that generally comprises the southern portion of the U.S. state of California. It includes the Los Angeles metropolitan area, the second most populous urban ...

, in

San Francisco Bay San Francisco Bay is a large tidal estuary in the U.S. state of California, and gives its name to the San Francisco Bay Area. It is dominated by the big cities of San Francisco, San Jose, and Oakland. San Francisco Bay drains water f ...

and in northern regions of

Mexico Mexico (Spanish language, Spanish: México), officially the United Mexican States, is a List of sovereign states, country in the southern portion of North America. It is borders of Mexico, bordered to the north by the United States; to the so ...

. This migratory bird is a U.S. federally listed

endangered An endangered species is a species that is very likely to become extinct in the near future, either worldwide or in a particular political jurisdiction. Endangered species may be at risk due to factors such as habitat loss, poaching and in ...

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 population decline through natural disasters, predation, and human disturbance.

Nesting and breeding behavior

Wintering locations are actually unknown, but suspected to include the

South American South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...

Pacific Coast. The California least tern arrives at its breeding grounds in late April.

Courtship Courtship is the period wherein some couples get to know each other prior to a possible marriage. Courtship traditionally may begin after a betrothal and may conclude with the celebration of marriage. A courtship may be an informal and private m ...

typically takes place removed from the nesting colony site, usually on an exposed tidal flat or beach. Only after

courtship Courtship is the period wherein some couples get to know each other prior to a possible marriage. Courtship traditionally may begin after a betrothal and may conclude with the celebration of marriage. A courtship may be an informal and private m ...

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 San Diego County (), officially the County of San Diego, is a county in the southwestern corner of the U.S. state of California. As of the 2020 census, the population was 3,298,634, making it California's second-most populous county and the f ...

, densities of 200 nests per acre have been observed. The

clutch A clutch is a mechanical device that engages and disengages power transmission, especially from a drive shaft to a driven shaft. In the simplest application, clutches connect and disconnect two rotating shafts (drive shafts or line shafts). ...

size can be anywhere from one to four eggs, but most commonly consists of two or three. Both female and male incubate the eggs for a period of about three weeks, and both parents tend the

semi-precocial In biology, altricial species are those in which the young are underdeveloped at the time of birth, but with the aid of their parents mature after birth. Precocial species are those in which the young are relatively mature and mobile from the mome ...

young. Young birds can fly at age four weeks. After formation of the new families, groupings of birds may appear at

lacustrine A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes lie on land and are not part of the ocean, although, like the much larger ...

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.

Feeding and roosting characteristics

The California least tern hunts primarily in shallow

estuaries An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environme ...

and

lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into '' coastal lagoons'' (or ''barrier lagoons ...

s, or beyond the breakers, even beyond 24 km offshore in areas of upwelling, and where smaller fishes are abundant. They hover until spotting

prey Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill ...

, and then plunge into the water without full submersion to extract prey. In the bays and lagoons of

and northern Mexico, the favored prey include

anchovy An anchovy is a small, common forage fish of the family Engraulidae. Most species are found in marine waters, but several will enter brackish water, and some in South America are restricted to fresh water. More than 140 species are placed in 1 ...

smelt Smelt may refer to: * Smelting, chemical process * The common name of various fish: ** Smelt (fish), a family of small fish, Osmeridae ** Australian smelt in the family Retropinnidae and species ''Retropinna semoni'' ** Big-scale sand smelt ''A ...

, silversides, shiner surfperch and small

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapoda, decapods, ostracoda, seed shrimp, branchiopoda, branchiopods, argulidae, fish lice, krill, remipedes, isopoda, isopods, barnacles, copepods, ...

s. The terns often feed near shore in the open ocean, especially in proximity to lagoons or bay mouths (Baird 2010). Adults do not require cover, so 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 owl The burrowing owl (''Athene cunicularia''), also called the shoco, is a small, long-legged owl found throughout open landscapes of North and South America. Burrowing owls can be found in grasslands, rangelands, agricultural areas, deserts, or an ...

s and

American kestrel The American kestrel (''Falco sparverius''), also called the sparrow hawk, is the smallest and most common falcon in North America. It has a roughly two-to-one range in size over subspecies and sex, varying in size from about the weight of ...

s (Collins, 1980). Depredation by domestic cats has been observed in at least one colony (California Wildlife, 1990).

Relation to other least terns

The California least tern is a subspecies of the

(''Sternula antillarum'') that breeds on the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...

Atlantic The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...

coast from

Maine Maine () is a state in the New England and Northeastern regions of the United States. It borders New Hampshire to the west, the Gulf of Maine to the southeast, and the Canadian provinces of New Brunswick and Quebec to the northeast and nor ...

Florida Florida is a state located in the Southeastern region of the United States. Florida is bordered to the west by the Gulf of Mexico, to the northwest by Alabama, to the north by Georgia, to the east by the Bahamas and Atlantic Ocean, and ...

and

Texas Texas (, ; Spanish: ''Texas'', ''Tejas'') is a state in the South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2020, it is the second-largest U.S. state by ...

. ''Sternula antillarum'' is also found in breeding grounds along rivers in the midwest and

Great Plains The Great Plains (french: Grandes Plaines), sometimes simply "the Plains", is a broad expanse of flatland in North America. It is located west of the Mississippi River and east of the Rocky Mountains, much of it covered in prairie, steppe, a ...

of the United States. Both the Californian subspecies and the nominate race of least tern are approximately 23 centimeters in length. Both have conspicuous black markings on their outermost primaries and fly over water with a distinctive hunchback appearance, with bills pointing slightly downward. The least tern is itself often considered conspecific with the

little tern The little tern (''Sternula albifrons'') is a seabird of the family Laridae. It was formerly placed into the genus ''Sterna'', which now is restricted to the large white terns. The genus name is a diminutive of '' Sterna'', "tern". The specific ' ...

. ''Sternula albifrons'', which has an extensive Old World distribution. The little tern differs from the least tern mainly in that its rump and tail are white, not grey, and it has a different call.

Bibliography

* ''Audubon Handbook: Western Birds'', McGraw Hill Book Company (1988) * P. Baird, "Foraging Study of California Least Terns in San Diego Bay and Near Ocean Waters," 2010. Unpubl. Comp. Rept.

United States Navy The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage ...

. * B. Massey, ''Breeding Biology of the California least tern'', Proceedings Linn. Society, New York 72:1-24 (1974) * ''California Wildlife, Volume II,

Bird Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweig ...

s'', ed, by David C. Zeiner, William F. Laudenslayer and Kenneth E. Meyer, published by the

California Department of Fish and Game The California Department of Fish and Wildlife (CDFW), formerly known as the California Department of Fish and Game (CDFG), is a state agency under the California Natural Resources Agency. The Department of Fish and Wildlife manages and protec ...

, November 1988. * Gary Deghi, C. Michael Hogan et al., ''Biological Assessment for the Proposed

Tijuana Tijuana ( ,"Tijuana"
(US) and < ...

/San Diego Joint International

Wastewater Treatment Wastewater treatment is a process used to remove contaminants from wastewater and convert it into an effluent that can be returned to the water cycle. Once returned to the water cycle, the effluent creates an acceptable impact on the environm ...

Plant'', Publication of the

United States Environmental Protection Agency The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it ...

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 Naval Air Station Alameda (NAS Alameda) was a United States Navy Naval Air Station in Alameda, California, on San Francisco Bay. NAS Alameda had two runways: 13–31 measuring and 07-25 measuring . Two helicopter pads and a control tower wer ...

'', 1980 Admin. Rep. 25pp. * Olsen and Larsson, ''Terns of Europe and North America'' {{Taxonbar, from=Q5020664 California least tern Native birds of the West Coast of the United States Fauna of the California chaparral and woodlands Fauna of the San Francisco Bay Area Natural history of Baja California Natural history of San Diego County, California Endangered fauna of California Endangered fauna of North America Endangered biota of Mexico California least tern Taxa named by Edgar Alexander Mearns