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Forster's tern (''Sterna forsteri'') is a
tern Terns are seabirds in the family Laridae, subfamily Sterninae, that have a worldwide distribution and are normally found near the sea, rivers, or wetlands. Terns are treated in eleven genera in a subgroup of the family Laridae, which also ...
in the family
Laridae Laridae is a family of seabirds in the order Charadriiformes that includes the gulls, terns (including white terns), noddies, and skimmers. It includes around 100 species arranged into 22 genera. They are an adaptable group of mostly aerial bird ...
. The genus name ''Sterna'' is derived from
Old English Old English ( or , or ), or Anglo-Saxon, is the earliest recorded form of the English language, spoken in England and southern and eastern Scotland in the Early Middle Ages. It developed from the languages brought to Great Britain by Anglo-S ...
"stearn", "tern", and ''forsteri'' commemorates the naturalist
Johann Reinhold Forster Johann Reinhold Forster (; 22 October 1729 – 9 December 1798) was a German Reformed pastor and naturalist. Born in Tczew, Dirschau, Pomeranian Voivodeship (1466–1772), Pomeranian Voivodeship, Polish–Lithuanian Commonwealth (now Tczew, Po ...
. It breeds inland in
North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...
in the northern
United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...
and southern
Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...
, and migrates south to winter in the southern United States,
Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...
, the
Caribbean The Caribbean ( , ; ; ; ) is a region in the middle of the Americas centered around the Caribbean Sea in the Atlantic Ocean, North Atlantic Ocean, mostly overlapping with the West Indies. Bordered by North America to the north, Central America ...
, and northern
Central America Central America is a subregion of North America. Its political boundaries are defined as bordering Mexico to the north, Colombia to the southeast, the Caribbean to the east, and the Pacific Ocean to the southwest. Central America is usually ...
. It is also a rare but annual
vagrant Vagrancy is the condition of wandering homelessness without regular employment or income. Vagrants usually live in poverty and support themselves by travelling while engaging in begging, scavenging, or petty theft. In Western countries, ...
in western
Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...
, and has wintered in
Ireland Ireland (, ; ; Ulster Scots dialect, Ulster-Scots: ) is an island in the North Atlantic Ocean, in Northwestern Europe. Geopolitically, the island is divided between the Republic of Ireland (officially Names of the Irish state, named Irelan ...
and
Great Britain Great Britain is an island in the North Atlantic Ocean off the north-west coast of continental Europe, consisting of the countries England, Scotland, and Wales. With an area of , it is the largest of the British Isles, the List of European ...
on a number of occasions. This species breeds in colonies in marshes. It nests in a ground scrape and lays two or more eggs. Like all white terns, it is fiercely defensive of its nest and young. The Forster's tern feeds by plunge-diving for fish, but will also hawk for insects in its breeding marshes. It usually feeds from saline environments in winter, like most ''Sterna'' terns. It usually dives directly, and not from the "stepped-hover" favored by the
Arctic tern The Arctic tern (''Sterna paradisaea'') is a tern in the family Laridae. This bird has a circumpolar breeding distribution covering the Arctic and sub-Arctic regions of Europe (as far south as Brittany), Asia, and North America (as far south ...
. The offering of fish by the male to the female is part of the courtship display. This is a medium-small tern, long with a wingspan and a weight ranging from 130 to 190 g (4.6-6.7 oz ). It is most similar to the
common tern The common tern (''Sterna hirundo'') is a seabird in the family Laridae. This bird has a circumpolar distribution, its four subspecies breeding in Temperateness, temperate and subarctic regions of Europe, Asia and North America. It is stron ...
, with pale gray upperparts and white underparts. Its legs are red and its bill is red, tipped with black. In winter, the forehead becomes white and a characteristic black eye mask remains. Juvenile Forster's terns are similar to the winter adult. The call is similar to that of common terns, but also some harsher sounds suggestive of a small gull like
Bonaparte's gull Bonaparte's gull (''Chroicocephalus philadelphia'') is a member of the gull family Laridae found mainly in northern North America. At in length, it is one of the smallest species of gull. Its plumage is mainly white with grey upperparts. Durin ...
. This species is unlikely to be confused with the common tern in winter because of the black eye mask, but is much more similar in breeding plumage. Forster's has a gray center to its white tail, and the upperwings are whiter, without the darker primary wedge of the common tern.


Description

Forster's tern is a medium-sized tern with a slender body, deeply forked long tail and relatively long legs. In its non-breeding plumage, the crown is white and a black comma-shaped patch covers the eye and the ear-covert. The wings are gray with the
primaries Primary elections or primaries are elections held to determine which candidates will run in an upcoming general election. In a partisan primary, a political party selects a candidate. Depending on the state and/or party, there may be an "open pri ...
being dark silvery gray, while the underside is white. The bill is black and the legs are a dull brownish red. When breeding, an intense black cap extending down the neck appears. The wings and the back are pale gray while the underside is bright white. It has a black-tipped orange bill and bright orange legs. The juveniles have coloring similar to a non-breeding adult but often have darker primaries.


Taxonomy

Forster's tern is a member of the gull and tern family
Laridae Laridae is a family of seabirds in the order Charadriiformes that includes the gulls, terns (including white terns), noddies, and skimmers. It includes around 100 species arranged into 22 genera. They are an adaptable group of mostly aerial bird ...
; though some taxonomists treat Laridae as a superfamily, in which terns are placed in their own family, Sternidae. Forster's tern was named by
Thomas Nuttall Thomas Nuttall (5 January 1786 – 10 September 1859) was an English botanist and zoologist who lived and worked in America from 1808 until 1841. Nuttall was born in the village of Long Preston, near Settle in the West Riding of Yorkshire a ...
in honor of
Johann Reinhold Forster Johann Reinhold Forster (; 22 October 1729 – 9 December 1798) was a German Reformed pastor and naturalist. Born in Tczew, Dirschau, Pomeranian Voivodeship (1466–1772), Pomeranian Voivodeship, Polish–Lithuanian Commonwealth (now Tczew, Po ...
, the German naturalist who first suggested it differed from the common tern. Its closest relative is the snowy-crowned tern (''S. trudeaui'') of South America; the two are
sister species In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...
and basal to the rest of the genus ''Sterna''.


Habitat and distribution

Forster's tern is a
marsh In ecology, a marsh is a wetland that is dominated by herbaceous plants rather than by woody plants.Keddy, P.A. 2010. Wetland Ecology: Principles and Conservation (2nd edition). Cambridge University Press, Cambridge, UK. 497 p More in genera ...
dwelling species. It can be found either in freshwater, brackish or saltwater. It is often found over shallow open water deep in the marsh. Main habitats are marshes, estuaries, islands, salt marshes and marshy areas surrounding lakes and streams. Forster's tern is usually restricted to
North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...
. It nests in marshes during the summer, on both the Atlantic and Pacific coast, and also in the
Prairies Prairies are ecosystems considered part of the temperate grasslands, savannas, and shrublands biome by ecologists, based on similar temperate climates, moderate rainfall, and a composition of grasses, herbs, and shrubs, rather than trees, as the ...
and on the
Great Lakes The Great Lakes, also called the Great Lakes of North America, are a series of large interconnected freshwater lakes spanning the Canada–United States border. The five lakes are Lake Superior, Superior, Lake Michigan, Michigan, Lake Huron, H ...
in Canada and the US.Visser JM and Peterson GW. 1994. Breeding populations and colony site dynamics of seabirds nesting in Louisiana. Colonial Waterbirds. 17(2): 146–152 Due to the instability of its nesting habitat, Forster's tern exhibits a high annual turnover rate. Forster's tern also winters in marshes along the southern coast of the US and Mexico but can sometimes reach the northern extremity of Central America. It is also common for the tern to winter in the Caribbean. It is a near-annual vagrant to Western Europe and has occasionally wintered in Great Britain and Ireland.


Behavior

Forster's tern is often found in marshes over shallow open water. It is a shallow plunge-diver that often hovers before attacking. When hunting, its head is pointed downward whereas when travelling, it is pointed forward.Salt GW and Willard DE. 1971. The hunting behavior and success of Forster's Tern. Ecology. 52(6): 989–998. It is a colonial nesting species that builds a shallow nest using marsh vegetation and often competes with gulls for nesting sites.McNicholl MK. 1971. The breeding biology and ecology of forster's tern (Sterna forsteri) at delta, Manitoba. Thesis. Department of Zoology. University of Manitoba.McNicholl MK. 1982. Factors affecting reproductive success of Forster's Terns at Delta Marsh, Manitoba. Colonial Waterbirds. 5(1): 32–38.Strong CM, Spear LB, Ryan TP and Dakin RE. 2004. Forster's Tern, Caspian Tern, and California Gull colonies in San Francisco Bay: Habitat use, numbers and trends, 1982-2003. Waterbirds. 27(4): 411–423. A breeding colony may vary in numbers from a few couples to a thousand individuals. In many occasions, Forster's tern will share nesting sites with the yellow-headed blackbird. Both parents are involved in brood care and Forster's tern does not exhibit sex-specific differences in space use. Males tend to guard the nest more often during the day while the female is more present at night.Bluso-Demers JD, Ackerman JT and Takekawa JY. 2010. Colony attendance patterns by mated Forster's Terns Sterna forsteri using an automated data-logging receiver system. Ardea. 98(1): 59–65. When disturbed, newborn chicks tend to crouch and remain silent.Hall JA. 1988. Early chick mobility and brood movements in the Forster's Tern (Sterna forsteri). Journal of Field Ornithology. 59(3): 247–251. Forster's tern is a single prey loader and provision chicks with prey correlated to their size. Before breeding, males practice courtship feeding.


Vocalization

The common call of the Forster's tern is a descending kerr. The threat call used in defensive attack is a low harsh zaar. A succession of kerrs is used by the female as a begging call during courtship.


Diet and feeding

The major constituent of Forster's tern diet is fish. Carp, minnow, sunfish, trout, perch,
killifish A killifish is any of various oviparous (egg-laying) cyprinodontiform fish, including families Aplocheilidae, Pantanodontidae, Cyprinodontidae, Fundulidae, Nothobranchiidae, Profundulidae, Aphaniidae and Valenciidae. All together, there ar ...
,
stickleback The sticklebacks are a family of ray-finned fishes, the Gasterosteidae which have a Holarctic distribution in fresh, brackish and marine waters. They were thought to be related to the pipefish and seahorses but are now thought to be more close ...
and shiner are common prey in freshwater, whereas
pompano Pompanos ( ) are saltwater fish, marine fish in the genus ''Trachinotus'' in the family (biology), family Carangidae (better known as "slabs"). Pompano may also refer to various other, similarly shaped members of the Carangidae, or the Order (bi ...
,
herring Herring are various species of forage fish, belonging to the Order (biology), order Clupeiformes. Herring often move in large Shoaling and schooling, schools around fishing banks and near the coast, found particularly in shallow, temperate wate ...
,
menhaden Menhaden, also known as mossbunker, bunker, and "the most important fish in the sea", are forage fish of the genera ''Brevoortia'' and ''Ethmidium'', two genera of marine fish in the order Clupeiformes. ''Menhaden'' is a blend of ''poghaden'' ...
and shiner perch are often consumed in brackish or marine habitats. On the West Coast of the United States and Canada, Forster's tern is also known to prey on Pacific lamprey juveniles. Insects such as dragonflies,
caddisflies The caddisflies (order Trichoptera) are a group of insects with aquatic larvae and terrestrial adults. There are approximately 14,500 described species, most of which can be divided into the suborders Integripalpia and Annulipalpia on the basis ...
and grasshoppers are often consumed, but aquatic insect larvae, crustaceans and amphibian can complement the diet. The Forster's tern is a shallow plunge-diver, having its head pointing downward when hunting. The attack usually starts in a hovering position before initiating a headfirst dive with wings partially folded backward. Insects may occasionally be caught by the wing and prey are swallowed in the air. Prey handling behavior may include dropping and re-catching fish before swallowing them. In some areas, Forster's tern tends to prefer forage to
turbid Turbidity is the cloudiness or haziness of a fluid caused by large numbers of individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of both water clarity and wate ...
water. This may prevent detection but it may also be a sign of higher prey density and increased presence near the surface. Preferences for water clarity may depend on prey availability.


Reproduction

The breeding season for Forster's tern can start as early as April on the Gulf Coast of the United States and extend from May to mid-June depending on latitude. Forster's tern is a colonial nester with colony size ranging from one to a thousand nests. Adults establish a very small territory around the nest and nests are usually clumped together. Males will practice courtship feeding and females will beg for food using a kerr kerr kerr call. A typical clutch of eggs ranges from two to four.Dakin RE. 2000. Nest site selection by Forster's terns (Sterna forsteri). Master's Theses. San Jose State University. http://scholarworks.sjsu.edu/cgi/viewcontent.cgi?article=2980&context=etd_theses/> The incubation period may last 24 or 25 days after laying.McNicholl, MK. 1983. Hatching of Forster's Terns. The Condor. 85(1): 50–52. The young are semi-precocial with shell removal being done by the parents. The chicks have upper- and lower-mandible egg teeth, which they lose three to five days after hatching. The chicks usually leave the nest with the parents four days post hatching and move into areas of denser vegetation. Fledging occurs 28 days after hatching. After a few weeks of fledging, young terns leave the natal colony but join the group for roosting, while migrating towards the wintering ground. There is a similar involvement from both male and female in incubation and chick rearing. Males tend to incubate the eggs diurnally and females, mostly nocturnally. Reproductive success varies from year to year and from colony to colony.


Mobbing

Forster's tern exhibits very aggressive behavior when threatened by nest predators; if a nest is disturbed, the colony mobs the aggressor, diving towards it and issuing loud calls. Aggressiveness increases immediately prior to and during hatching of the chicks. Ducks and grebes nesting in the same area often benefit from the tern's aggressive behavior toward potential predators Yellow-headed blackbirds sharing nesting sites have been known to actively join tern mobs against predators.
Western grebe The western grebe (''Aechmophorus occidentalis'') is a species in the grebe family of water birds. Folk names include "dabchick", "swan grebe" and "swan-necked grebe". Western grebe fossils from the Late Pleistocene of southwest North America we ...
s recognize the tern's alarm call; this can be interpreted as information
parasitism Parasitism is a close relationship between species, where one organism, the parasite, lives (at least some of the time) on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The en ...
.


Nest

Forster's terns tend to nest in marshy areas, either in freshwater or in estuaries. The nests are usually located deep within the marsh, either on tidal islands or evaporation pond islands, but also on manmade dikes. Nests are composed of adjacent marsh vegetation. Many nests are considered floating and are made of marsh grasses, then can be set on top of the vegetation or deposited on floating rafts of vegetation. In Manitoba, there is a strong association between Forster's tern nests and
muskrat The muskrat or common muskrat (''Ondatra zibethicus'') is a medium-sized semiaquatic rodent native to North America and an introduced species in parts of Europe, Asia, and South America. The muskrat is found in wetlands over various climates ...
houses. They are, in fact, highly solicited nesting grounds. Also in Manitoba, ''
Scirpus ''Scirpus'' is a genus of grass-like species in the sedge family Cyperaceae many with the common names club-rush, wood club-rush or bulrush. They mostly inhabit wetlands and damp locations. Description ''Scirpus'' are rhizomatous perennial herbs ...
'' and often ''
Typha ''Typha'' is a genus of about 30 species of monocotyledonous flowering plants in the family Typhaceae. These plants have a variety of common names, in British English as bulrushStreeter D, Hart-Davies C, Hardcastle A, Cole F, Harper L. 2009. ' ...
'' are the main plants used for nest building. In the case of large colonies, nesting area availability decreases. Forster's tern will then nest on sand, gravel or mud. The nests will consist of a hollow in the substrates, either lined with grass or not and driftwood, shells, dried fish, bones and feathers are also often used. Floating nests are usually tolerant to a slight increase or decrease of water levels but re-nesting is common. Strong wave action, wind or flooding, usually induced by storms, can often damage the nest and eggs. Weather is the main explanation for nest failure and egg loss. Unsheltered nests are more prone to destruction than sheltered ones. Nest made on higher ground are also more shielded from flooding but are more exposed to the wind. Forster's Terns have been recorded using man-made platforms, most notably in Wisconsin, where they were built to substitute for the Cat Island Chain. They demonstrated overall success, with Forster's Terns preferring to use them to avoid the vulnerabilities that come with a natural nest.


Eggs

The egg's primary color ranges from a greenish to a brownish hue. They are evenly spotted with dark brown, almost black or gray spots. There are color variations between and within clutches; earlier eggs are usually paler, greener and larger. Coloration of the eggs may vary depending on location. Whitish or cream colored eggs have also been reported.


Predator

Raptors such as falcons and hawks, as well as owls and
crows The Common Remotely Operated Weapon Station (CROWS) is a series of remote weapon stations used by the US military on its armored vehicles and ships. It allows weapon operators to engage targets without leaving the protection of their vehicle. ...
may prey on adults and young. There have also been anecdotal reports of
snapping turtle The Chelydridae is a family of turtles that has seven extinct and two extant genera. The extant genera are the snapping turtles, ''Chelydra'' and ''Macrochelys''. Both are endemic to the Western Hemisphere. The extinct genera are '' Acherontemys' ...
s preying on young still in the nest.
American bittern The American bittern (''Botaurus lentiginosus'') is a species of wading bird in the heron family. It has a Nearctic distribution, breeding in Canada and the northern and central parts of the United States, and wintering in the U.S. Gulf Coast ...
,
great blue heron The great blue heron (''Ardea herodias'') is a large wading bird in the heron family Ardeidae, common near the shores of open water and in wetlands over most of North and Central America, as well as far northwestern South America, the Caribbea ...
, and
black-crowned night heron The black-crowned night heron (''Nycticorax nycticorax'') r black-capped night heron commonly shortened to just night heron in Eurasia, is a medium-sized heron found throughout a large part of the world, including parts of Europe, Asia, and Nort ...
are also possible predators while gulls and
Caspian tern The Caspian tern (''Hydroprogne caspia'') is a species of tern, with a subcosmopolitan but scattered distribution. Despite its extensive range, it is monotypic of its genus, and has no accepted subspecies. The genus name is from Ancient Greek '' ...
s notably prey on the eggs of the Forster's tern. When their ranges overlap,
marsh rice rat The marsh rice rat (''Oryzomys palustris'') is a semiaquatic North American rodent in the family Cricetidae. It usually occurs in wetland habitats, such as swamps and salt marshes. It is found mostly in the eastern and southern United States, f ...
s are possibly the most efficient Forster's tern egg predator.
American mink The American mink (''Neogale vison'') is a semiaquatic species of Mustelidae, mustelid native to North America, though human introduction has expanded its range to many parts of Europe, Asia, and South America. Because of range expansion, the Am ...
are also one of few mammalian predators that can venture in the marsh and prey on eggs and young. Predator success usually remains low during breeding season due to the aggressive mobbing response of adults.


Status and conservation

According to the IUCN, the status of the Forster's tern is of little concern, however, degradation of marsh habitat may be threatening. Boating activity may also affect nest vegetation and increase erosion, which may lead to further degradation of tern nesting grounds. Excessive noise may also have caused nest desertion and chick mortality. This species is listed under the Migratory Birds Treaty act in the U.S. It is endangered in Illinois and Wisconsin while being of special concern in Michigan and Minnesota. Preservation of wetlands and introduction of artificial nesting sites may help preserve the species in high-risk areas (). Increasing populations of carp in drainage systems, causing damage to marsh vegetation, may limit habitat availability for Forster's tern. There have also been anecdotal reports of intense spawning activity of carp damaging tern's floating nests. As with many species of piscivorous birds, Forster's tern is susceptible to
bioaccumulation Bioaccumulation is the gradual accumulation of substances, such as pesticides or other chemicals, in an organism. Bioaccumulation occurs when an organism absorbs a substance faster than it can be lost or eliminated by catabolism and excretion. T ...
of pollutants. High mercury concentrations may induce biochemical stress, reducing the overall health of terns. Biomethylation of mercury is increased in marshes and salt ponds, hence increasing the susceptibility of the Forster's tern. High levels of
selenium Selenium is a chemical element; it has symbol (chemistry), symbol Se and atomic number 34. It has various physical appearances, including a brick-red powder, a vitreous black solid, and a grey metallic-looking form. It seldom occurs in this elem ...
may also have deleterious effects on their health.
Organochlorine Organochlorine chemistry is concerned with the properties of organochlorine compounds, or organochlorides, organic compounds that contain one or more carbon–chlorine bonds. The chloroalkane class (alkanes with one or more hydrogens substituted ...
contaminants such as
PCB PCB may refer to: Science and technology * Polychlorinated biphenyl, an organic chlorine compound, now recognized as an environmental toxin and classified as a persistent organic pollutant * Printed circuit board, a board used in electronics * P ...
s may also diminish their breeding success.Kubiak TJ, Harris HJ, Smith LM, Schwarts TR, Stalling DL, Trick JA, Sileo L, Docherty DE and Erdman TC. 1989. Microcontaminants and reproductive impairment of the Forster's tern on Green Bay, Lake Michigan 1983. Archives of Environmental Contamination and Toxicology. 18(5): 706–727.


Gallery

Image:Forster's Tern (Sterna forsteri) RWD1.jpg, Breeding Image:Forster's Tern (Sterna forsteri) RWD2.jpg, Nonbreeding Image:Forsters_Tern_Fishing.jpg, Forster's tern fishing on
Lake Mattamuskeet Lake Mattamuskeet is the largest natural lake in North Carolina. It is a shallow coastal lake, averaging 2–3 ft (0.61–0.91 m) feet in depth, and stretches long and wide. Lake Mattamuskeet lies on the Albemarle-Pamlico Peninsula. Lak ...
File:Forster's Tern from the Crossley ID Guide Britain and Ireland.jpg, ID composite


References


External links


Forster's Tern Species Account
- Cornell Lab of Ornithology

- USGS Patuxent Bird Identification InfoCenter
Field Guide Page on Flickr
* * * {{Use dmy dates, date=March 2017 Forster's tern Birds of North America Forster's tern Forster's tern