California Least Tern
Encyclopedia
The Least Tern 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
. Other close relatives include the Yellow-billed Tern
and Peruvian Tern
, 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. Many spend their whole first year in their wintering area (Thompson et al. 1997). It has occurred as a vagrant to Europe
, with one record in Great Britain
.
It differs from the Little Tern mainly in that its rump and tail are gray, not white, and it has a different, more squeaking call; from the Yellow-billed Tern in being paler gray above and having a black tip to the bill; and from Peruvian Tern in being paler gray above and white (not pale gray) below and having a shorter black tip to the bill.
s, reservoirs, channelization, and other changes to river systems.
, 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 increase from 582 pairs in 1974 when census work began, though it is still listed as an endangered subspecies. The California subspecies breeds on beaches and 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 predators, natural disasters or further disturbance by humans.
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 sandbar 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 southeastern United States, many breeding sites are on white gravel rooftops. 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 the result of late arrivals. In any case, the bulk of the population has left the breeding grounds by the end of August.
The Least Tern hunts primarily in shallow estuaries and lagoons, where smaller fishes are abundant. It hovers until spotting prey, and then plunges into the water without full submersion to extract meal. In the bays and lagoons of Southern California
and northern Mexico, the favored prey include anchovy
, smelt, silversides
, shiner surfperch and small crustacean
s. Sometimes 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 Owl
s and American Kestrel
s (Collins, 1980). Predation by domestic cats has been observed in at least one colony (California Wildlife, 1990). Predation on inland breeding terns by Coyotes, Bobcats, feral dogs and cats, Great Blue Herons, Mississippi Kites, and owls has also been documented (Jones 1997-2006).
Tern
Terns are seabirds in the family Sternidae, previously considered a subfamily of the gull family Laridae . They form a lineage with the gulls and skimmers which in turn is related to skuas and auks...
that breeds in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
and locally in northern South America
South America
South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...
. It is closely related to, and was formerly often considered conspecific with, the Little Tern
Little Tern
The Little Tern, Sternula albifrons or Sterna albifrons, is a seabird of the tern family Sternidae. It was formerly placed into the genus Sterna, which now is restricted to the large white terns . The former North American and Red Sea S. a...
of the Old World
Old World
The Old World consists of those parts of the world known to classical antiquity and the European Middle Ages. It is used in the context of, and contrast with, the "New World" ....
. Other close relatives include the Yellow-billed Tern
Yellow-billed Tern
The Yellow-billed Tern is a species of tern in the Sternidae family.It is found in Argentina, Bolivia, Brazil, Colombia, Ecuador, French Guiana, Guyana, Panama, Paraguay, Peru, Suriname, Trinidad and Tobago, Uruguay, and Venezuela.Its natural habitats are rivers, swamps, and freshwater...
and Peruvian Tern
Peruvian Tern
The Peruvian Tern is a species of tern in the Sternidae family.It is found in northern Chile, Ecuador, and Peru.Its natural habitats are hot deserts, sandy shores, and coastal saline lagoons.It is threatened by habitat loss....
, 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
Flight feather
Flight feathers are the long, stiff, asymmetrically shaped, but symmetrically paired feathers on the wings or tail of a bird; those on the wings are called remiges while those on the tail are called rectrices . Their primary function is to aid in the generation of both thrust and lift, thereby...
. In behavior, it flies over water with fast, jerky wingbeats and a distinctive hunchback appearance, with the bill pointing slightly downward.
It is migratory
Bird migration
Bird migration is the regular seasonal journey undertaken by many species of birds. Bird movements include those made in response to changes in food availability, habitat or weather. Sometimes, journeys are not termed "true migration" because they are irregular or in only one direction...
, wintering in Central America
Central America
Central America is the central geographic region of the Americas. It is the southernmost, isthmian portion of the North American continent, which connects with South America on the southeast. When considered part of the unified continental model, it is considered a subcontinent...
, the Caribbean
Caribbean
The Caribbean is a crescent-shaped group of islands more than 2,000 miles long separating the Gulf of Mexico and the Caribbean Sea, to the west and south, from the Atlantic Ocean, to the east and north...
and northern South America. Many spend their whole first year in their wintering area (Thompson et al. 1997). It has occurred as a vagrant to Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
, with one record in Great Britain
Great Britain
Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...
.
It differs from the Little Tern mainly in that its rump and tail are gray, not white, and it has a different, more squeaking call; from the Yellow-billed Tern in being paler gray above and having a black tip to the bill; and from Peruvian Tern in being paler gray above and white (not pale gray) below and having a shorter black tip to the bill.
Subspecies
- Sternula antillarum antillarum (LessonRené-Primevère LessonRené Primevère Lesson was a French surgeon, naturalist, ornithologist, and herpetologist.Lesson was born at Rochefort, and at the age of sixteen he entered the Naval Medical School there...
, 1847). Breeds on the Atlantic coast of North America, from MaineMaineMaine is a state in the New England region of the northeastern United States, bordered by the Atlantic Ocean to the east and south, New Hampshire to the west, and the Canadian provinces of Quebec to the northwest and New Brunswick to the northeast. Maine is both the northernmost and easternmost...
south along the east and south coasts of the United StatesUnited StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
, BermudaBermudaBermuda is a British overseas territory in the North Atlantic Ocean. Located off the east coast of the United States, its nearest landmass is Cape Hatteras, North Carolina, about to the west-northwest. It is about south of Halifax, Nova Scotia, Canada, and northeast of Miami, Florida...
, the CaribbeanCaribbeanThe Caribbean is a crescent-shaped group of islands more than 2,000 miles long separating the Gulf of Mexico and the Caribbean Sea, to the west and south, from the Atlantic Ocean, to the east and north...
, and VenezuelaVenezuelaVenezuela , officially called the Bolivarian Republic of Venezuela , is a tropical country on the northern coast of South America. It borders Colombia to the west, Guyana to the east, and Brazil to the south...
; winters south to northern BrazilBrazilBrazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
. - Sternula antillarum athalassos (Burleigh & Lowery, 1942). Breeds on the rivers of the Mississippi RiverMississippi RiverThe Mississippi River is the largest river system in North America. Flowing entirely in the United States, this river rises in western Minnesota and meanders slowly southwards for to the Mississippi River Delta at the Gulf of Mexico. With its many tributaries, the Mississippi's watershed drains...
, Brazos RiverBrazos RiverThe Brazos River, called the Rio de los Brazos de Dios by early Spanish explorers , is the longest river in Texas and the 11th longest river in the United States at from its source at the head of Blackwater Draw, Curry County, New Mexico to its mouth at the Gulf of Mexico with a drainage...
, Trinity RiverTrinity River (Texas)The Trinity River is a long river that flows entirely within the U.S. state of Texas. It rises in extreme north Texas, a few miles south of the Red River. The headwaters are separated by the high bluffs on the south side of the Red River....
, and Rio GrandeRio GrandeThe Rio Grande is a river that flows from southwestern Colorado in the United States to the Gulf of Mexico. Along the way it forms part of the Mexico – United States border. Its length varies as its course changes...
basins (Thompson et al. 1997); winters south to northern BrazilBrazilBrazil , officially the Federative Republic of Brazil , is the largest country in South America. It is the world's fifth largest country, both by geographical area and by population with over 192 million people...
. - Sternula antillarum browni (MearnsEdgar Alexander MearnsEdgar Alexander Mearns was a notable American ornithologist and field naturalist....
, 1916) California Least TernCalifornia Least TernThe Least Tern 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...
. Breeds on the Pacific coast of North America, from central CaliforniaCaliforniaCalifornia is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
south to western MexicoMexicoThe United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
; winters mainly in Central AmericaCentral AmericaCentral America is the central geographic region of the Americas. It is the southernmost, isthmian portion of the North American continent, which connects with South America on the southeast. When considered part of the unified continental model, it is considered a subcontinent...
.
S. a. antillarum
The population is about 21,500 pairs; it is not currently considered federally threatened, though it is considered threatened in many of the states in which it breeds. Threats include egg and fledgling predators, high tides and recreational use of nesting beaches.S. a. athalassos
The interior subspecies, with a current population of about 7,000 pairs, was listed as an endangered subspecies in 1985 (estimated 1,000 breeding pairs), due to loss of habitat caused by damDam
A dam is a barrier that impounds water or underground streams. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees are used to manage or prevent water flow into specific land regions. Hydropower and pumped-storage hydroelectricity are...
s, reservoirs, channelization, and other changes to river systems.
S. a. brownii
The western population, the California Least TernCalifornia Least Tern
The Least Tern 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...
, 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 increase from 582 pairs in 1974 when census work began, though it is still listed as an endangered subspecies. The California subspecies breeds on beaches and bays of the Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
within a very limited range of southern California, in San Francisco Bay
San Francisco Bay
San Francisco Bay is a shallow, productive estuary through which water draining from approximately forty percent of California, flowing in the Sacramento and San Joaquin rivers from the Sierra Nevada mountains, enters the Pacific Ocean...
and in northwestern Mexico
Mexico
The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
. While numbers have gradually increased with its protected status, it is still vulnerable to predators, natural disasters or further disturbance by humans.
Nesting and breeding behavior
Courtship
Courtship is the period in a couple's relationship which precedes their engagement and marriage, or establishment of an agreed relationship of a more enduring kind. In courtship, a couple get to know each other and decide if there will be an engagement or other such agreement...
typically takes place removed from the nesting colony site, usually on an exposed tidal flat or beach. Only after courtship
Courtship
Courtship is the period in a couple's relationship which precedes their engagement and marriage, or establishment of an agreed relationship of a more enduring kind. In courtship, a couple get to know each other and decide if there will be an engagement or other such agreement...
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 southeastern United States, many breeding sites are on white gravel rooftops. 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
Clutch
A clutch is a mechanical device which provides for the transmission of power from one component to another...
size is two or three, but it is not rare to consist of either one or four eggs. Both female and male incubate
Avian incubation
Incubation refers to the process by which certain oviparous animals hatch their eggs, and to the development of the embryo within the egg. The most vital factor of incubation is the constant temperature required for its development over a specific period. Especially in domestic fowl, the act of...
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
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...
settings in proximity to the coast. Late-season nesting may be renests or the result of late arrivals. In any case, the bulk of the population has left the breeding grounds by the end of August.
Feeding and roosting characteristics
Southern California
Southern California is a megaregion, or megapolitan area, in the southern area of the U.S. state of California. Large urban areas include Greater Los Angeles and Greater San Diego. The urban area stretches along the coast from Ventura through the Southland and Inland Empire to San Diego...
and northern Mexico, the favored prey include anchovy
Anchovy
Anchovies are a family of small, common salt-water forage fish. There are 144 species in 17 genera, found in the Atlantic, Indian, and Pacific Oceans. Anchovies are usually classified as an oily fish.-Description:...
, smelt, silversides
Silverside (fish)
The Old World silversides are a family, Atherinidae, of fish in the order Atheriniformes. They occur worldwide in tropical and temperate waters...
, shiner surfperch and small crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
s. Sometimes 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 Owl
Burrowing Owl
The Burrowing Owl is a tiny but long-legged owl found throughout open landscapes of North and South America. Burrowing Owls can be found in grasslands, rangelands, agricultural areas, deserts, or any other open dry area with low vegetation. They nest and roost in burrows, such as those excavated...
s and American Kestrel
American Kestrel
The American Kestrel , sometimes colloquially known as the Sparrow Hawk, is a small falcon, and the only kestrel found in the Americas. It is the most common falcon in North America, and is found in a wide variety of habitats. At long, it is also the smallest falcon in North America...
s (Collins, 1980). Predation by domestic cats has been observed in at least one colony (California Wildlife, 1990). Predation on inland breeding terns by Coyotes, Bobcats, feral dogs and cats, Great Blue Herons, Mississippi Kites, and owls has also been documented (Jones 1997-2006).