Editing Forster's tern

{{short description|Species of bird}}
{{speciesbox
| image = Forster's Tern, Horicon NWR, Wisconsin.jpg
| image_caption = Breeding plumage, at [[Horicon Marsh]] in Wisconsin
| image2 = Forster's Tern.jpg
| image2_caption = Nonbreeding plumage, at [[Bolsa Chica Ecological Reserve|Bolsa Chica]] in California
| status = LC
| status_system = IUCN3.1
| status_ref = <ref name="iucn status 12 November 2021">{{cite iucn |author=BirdLife International |date=2018 |title=''Sterna forsteri'' |volume=2018 |page=e.T22694646A132564836 |doi=10.2305/IUCN.UK.2018-2.RLTS.T22694646A132564836.en |access-date=12 November 2021}}</ref>
| genus = Sterna
| species = forsteri
| authority = [[Thomas Nuttall|Nuttall]], 1834
| range_map = Sterna forsteri map.svg
| range_map_caption = {{leftlegend|#FF7F2A|Breeding}}{{leftlegend|#FFDD55|Migration}}{{leftlegend|#7137C8|Year-round}}{{leftlegend|#5F8DD3|Non-breeding}}
}}

'''Forster's tern''' ('''''Sterna forsteri''''') is a [[tern]] in the family [[Laridae]]. The genus name ''Sterna'' is derived from [[Old English]] "stearn", "tern",<ref name=OED>{{OED|Sterna}}</ref> and ''forsteri'' commemorates the naturalist [[Johann Reinhold Forster]].<ref name=job>{{cite book | last= Jobling | first= James A | year= 2010| title= The Helm Dictionary of Scientific Bird Names | url= https://archive.org/details/Helm_Dictionary_of_Scientific_Bird_Names_by_James_A._Jobling | publisher= Christopher Helm | location = London | isbn = 978-1-4081-2501-4 | page = [https://archive.org/details/Helm_Dictionary_of_Scientific_Bird_Names_by_James_A._Jobling/page/n163 163]}}</ref>

It breeds inland in [[North America]] in the northern [[United States]] and southern [[Canada]], and [[bird migration|migrates]] south to winter in the southern United States, [[Mexico]], the [[Caribbean]], and northern [[Central America]]. It is also a rare but annual [[vagrancy (biology)|vagrant]] in western [[Europe]], and has wintered in [[Ireland]] and [[Great Britain]] 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 offering of fish by the male to the female is part of the courtship display.

This is a medium-small tern, {{convert|33|–|36|cm|in|abbr=on}} long with a {{convert|64|–|70|cm|in|abbr=on}} wingspan and a weight ranging from 130 to 190 g (4.6-6.7 oz ).<ref>{{Cite web|title=Forster's Tern Identification, All About Birds, Cornell Lab of Ornithology|url=https://www.allaboutbirds.org/guide/Forsters_Tern/id|access-date=2020-09-25|website=www.allaboutbirds.org|language=en}}</ref> It is most similar to the [[common tern]], 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]].

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==
[[File:Juvenile Forster's tern calling for its parent (96328).jpg|thumb|Juvenile calling in Queens, New York]]
Forster's tern is a medium-sized tern with a slender body, deeply forked long tail and relatively long legs.<ref name="ARKive">{{cite web|title=Forster's tern|url=http://www.arkive.org/forsters-tern/sterna-forsteri/|website=ARKive|access-date=13 October 2016|archive-url=https://web.archive.org/web/20161013153633/http://www.arkive.org/forsters-tern/sterna-forsteri/|archive-date=13 October 2016|url-status=dead|df=dmy-all}}</ref><ref name="Cornell">{{cite web|title=Forster's tern|url=https://www.allaboutbirds.org/guide/Forsters_Tern/id|website=The Cornell Lab of Ornithology|access-date=12 October 2016}}</ref>

In its non-breeding plumage, the crown is white and a black comma-shaped patch covers the eye and the ear-covert.<ref name="ARKive" /><ref name="Cornell" /><ref name="Sibley">{{cite book|last1=Sibley|first1=David Allen|title=The Sibley Field Guide to Birds of Eastern North America|publisher=Knopf|location=New York|edition=2nd}}</ref> The wings are gray with the [[Flight feather|primaries]] being dark silvery gray, while the underside is white.<ref name="ARKive" /><ref name="Cornell" /> The bill is black and the legs are a dull brownish red.<ref name="ARKive" />

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.<ref name="ARKive" /><ref name="Sibley" />

The juveniles have coloring similar to a non-breeding adult but often have darker primaries.<ref name="ARKive" />

==Taxonomy==

Forster's tern is a member of the gull and tern family [[Laridae]]; though some taxonomists treat Laridae as a [[Superfamily (biology)|superfamily]], in which terns are placed in their own family, [[Sternidae]].<ref>{{Cite book |last=Burger |first=J |title=Encyclopedia of Ocean Sciences |last2=Gochfeld |first2=M. |date=2019 |isbn=978-0-12-813082-7 |edition=3rd |doi=10.1016/B978-0-12-409548-9.11521-0}}</ref> Forster's tern was named by [[Thomas Nuttall]] in honor of [[Johann Reinhold Forster]], the German naturalist who first suggested it differed from the common tern.<ref name="ARKive" /> Its closest relative is the [[snowy-crowned tern]] (''S. trudeaui'') of South America; the two are [[sister species]] and basal to the rest of the genus ''Sterna''.<ref name=Bridge2005>{{cite journal |last1=Bridge |first1=Eli S. |last2=Jones |first2=Andrew W. |last3=Baker |first3=Allan J. |year=2005 |title=A phylogenetic framework for the terns (Sternini) inferred from mtDNA sequences: implications for taxonomy and plumage evolution |doi=10.1016/j.ympev.2004.12.010 |journal=Molecular Phylogenetics and Evolution |volume=35 |pages=459–469 |pmid=15804415 |issue=2}}</ref>

==Habitat and distribution==

Forster's tern is a [[marsh]] dwelling species. It can be found either in freshwater, brackish or saltwater. It is often found over shallow open water deep in the marsh.<ref name="Sibley" /> Main habitats are marshes, estuaries, islands, salt marshes and marshy areas surrounding lakes and streams.<ref name="ARKive" />

Forster's tern is usually restricted to [[North America]].<ref name="Cornell" />  It nests in marshes during the summer, on both the Atlantic and Pacific coast, and also in the [[Prairies]] and on the [[Great Lakes]] in Canada and the US.<ref name="ARKive" /><ref name="Cornell" /><ref name="Visser94" >Visser JM and Peterson GW. 1994. Breeding populations and colony site dynamics of seabirds nesting in Louisiana. Colonial Waterbirds. 17(2): 146–152</ref> Due to the instability of its nesting habitat, Forster's tern exhibits a high annual turnover rate.<ref name="Visser94" />

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.<ref name="Cornell" />

It is a near-annual vagrant to Western Europe and has occasionally wintered in Great Britain and Ireland.<ref name="ARKive" />

==Behavior==

Forster's tern is often found in marshes over shallow open water.<ref name="Sibley" /> It is a shallow [[Seabird|plunge-diver]] that often hovers before attacking. When hunting, its head is pointed downward whereas when travelling, it is pointed forward.<ref name="ARKive" /><ref name="Salt" >Salt GW and Willard DE. 1971. The hunting behavior and success of Forster's Tern. Ecology. 52(6): 989–998.</ref>

It is a colonial nesting species that builds a shallow nest using marsh vegetation and often competes with gulls for nesting sites.<ref name="McNicholl71" >McNicholl MK. 1971. The breeding biology and ecology of forster's tern (Sterna forsteri) at delta, Manitoba. Thesis. Department of Zoology. University of Manitoba.</ref><ref name="McNicholl82" >McNicholl MK. 1982. Factors affecting reproductive success of Forster's Terns at Delta Marsh, Manitoba. Colonial Waterbirds. 5(1): 32–38.</ref><ref name="Strong" >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.</ref> A breeding colony may vary in numbers from a few couples to a thousand individuals.<ref name="McNicholl71" /> In many occasions, Forster's tern will share nesting sites with the [[yellow-headed blackbird]].<ref name="McNicholl82" />

Both parents are involved in brood care and Forster's tern does not exhibit sex-specific differences in space use.<ref>{{cite journal|last1=Bluso-Demers|first1=J.|last2=Colwell|first2=M.A.|last3=Takekawa|first3=J.Y.|last4=Ackerman|first4=J.T.|year=2008|title=Space use by Forster's Terns breeding in South San Francisco Bay|journal=Waterbirds|volume=31|issue=3|pages=357-364|jstor=25148344}}</ref> Males tend to guard the nest more often during the day while the female is more present at night.<ref name="Bluso10" >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.</ref> When disturbed, newborn chicks tend to crouch and remain silent.<ref name="Hall" >Hall JA. 1988. Early chick mobility and brood movements in the Forster's Tern (Sterna forsteri). Journal of Field Ornithology. 59(3): 247–251.</ref> Forster's tern is a single prey loader and provision chicks with prey correlated to their size.<ref name="Fraser" >{{cite journal|last=Fraser|first=G.|year=1997|title=Feeding Ecology of Forster's Terns on Lake Osakis, Minnesota|journal=Colonial Waterbirds|volume=20|issue=1|pages=87-94|doi=10.2307/1521767}}</ref>

Before breeding, males practice [[Nuptial gift|courtship feeding]].<ref name="Fraser" />

===Vocalization===

The common call of the Forster's tern is a descending kerr.<ref name="ARKive" /><ref name="Cornell" /><ref name="Sibley" /> The threat call used in defensive attack is a low harsh zaar.<ref name="ARKive" /><ref name="Cornell" /> A succession of kerrs is used by the female as a begging call during courtship.<ref name="ARKive" />

===Diet and feeding===

The major constituent of Forster's tern diet is fish. Carp, minnow, [[Centrarchidae|sunfish]], trout, perch, [[killifish]], [[stickleback]] and [[Shiner (fish)|shiner]] are common prey in freshwater,<ref name="Fraser" /><ref>{{cite journal|last1=Taylor|first1=R.A.|last2=Wurtsbaugh|first2=W.A.|year=1991|title=Predation risk and the importance of cover for juvenile rainbow trout in lentic systems|journal=Transactions of the American Fisheries Society|volume=120|issue=6|pages=728-738|doi=10.1577/1548-8659(1991)120<0728:PRATIO>2.3.CO;2}}</ref> whereas [[pompano]], [[herring]], [[menhaden]] and [[shiner perch]] are often consumed in brackish or marine habitats.<ref name="McNicholl71" /><ref name="Fraser" /> On the West Coast of the United States and Canada, Forster's tern is also known to prey on Pacific lamprey juveniles.<ref><Close DA, Fitzpatrick MS and Li HW. 2002. The ecological and cultural importance of a species at risk of extinction, pacific lamprey. Fisheries. 27(7): 19–25.</ref> Insects such as dragonflies, [[caddisflies]] and grasshoppers are often consumed, but aquatic insect larvae, crustaceans and amphibian can complement the diet.<ref name="ARKive" /><ref name="McNicholl71" />

The Forster's tern is a shallow plunge-diver, having its head pointing downward when hunting.<ref name="ARKive" /><ref name="Salt" /> The attack usually starts in a hovering position before initiating a headfirst dive with wings partially folded backward.<ref name="Salt" /> Insects may occasionally be caught by the wing and prey are swallowed in the air.<ref name="Salt" /><ref name="Fraser" /> Prey handling behavior may include dropping and re-catching fish before swallowing them.<ref name="Salt" /> In some areas, Forster's tern tends to prefer forage to [[turbid]] 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.<ref>Henkel LA. 2006. Effects of water clarity on the distribution of marine birds in nearshore waters of Monterey Bay, California. Journal of Field Ornithology. 77(2): 151–156</ref>

===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.<ref name="ARKive" /> Forster's tern is a colonial nester with colony size ranging from one to a thousand nests.<ref name="McNicholl71" /> Adults establish a very small territory around the nest and nests are usually clumped together.<ref name="ARKive" /><ref name="McNicholl71" /><ref name="McNicholl82" /> Males will practice courtship feeding and females will beg for food using a kerr kerr kerr call.<ref name="ARKive" /><ref name="Fraser" />

A typical clutch of eggs ranges from two to four.<ref name="Dakin" >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/></ref> The incubation period may last 24 or 25 days after laying.<ref name="Dakin" /><ref name="McNicholl83" >McNicholl, MK. 1983. Hatching of Forster's Terns. The Condor. 85(1): 50–52.</ref> The young are [[precocial|semi-precocial]] with shell removal being done by the parents.<ref name="McNicholl71" /> The chicks have upper- and lower-mandible egg teeth, which they lose three to five days after hatching.<ref name="McNicholl83" /> The chicks usually leave the nest with the parents four days post hatching and move into areas of denser vegetation.<ref name="Hall" /><ref name="Dakin" /> Fledging occurs 28 days after hatching.<ref name="Dakin" /> After a few weeks of fledging, young terns leave the natal colony but join the group for roosting, while migrating towards the wintering ground.<ref name="ARKive" /><ref>Ackerman JT, Bluso-Demers JD and Takekawa JY. 2009. Postfledging Forster's tern movements, habitat selection, and colony attendance in San Francisco Bay. The Condor. 111(1): 100–110.</ref>

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.<ref name="Bluso10" /> Reproductive success varies from year to year and from colony to colony.<ref name="McNicholl82" />

===Mobbing===

Forster's tern exhibits very aggressive behavior when threatened by nest predators; if a nest is disturbed, the colony [[mobbing (animal behavior)|mobs]] the aggressor, diving towards it and issuing loud calls.<ref>Siglin RJ and Weller MW. 1963. Comparative nest defense behavior of four species of marsh birds. The Condor. 65(5): 432–437.</ref> 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 <ref name="McNicholl71" />

Yellow-headed blackbirds sharing nesting sites have been known to actively join tern mobs against predators.<ref name="McNicholl82" /> [[Western grebe]]s recognize the tern's alarm call; this can be interpreted as information [[parasitism]].<ref>{{cite journal|last1=Nuechterlein |first1=GL.|year= 1981|title='Information parasitism' in mixed colonies of western grebes and Forster's terns. |journal=Animal Behaviour|volume=29|issue=4|pages= 985–989|doi=10.1016/S0003-3472(81)80051-6 |s2cid=53152999 }}</ref>

==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.<ref name="McNicholl71" /><ref name="Strong" /> 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]] houses. They are, in fact, highly solicited nesting grounds. Also in Manitoba, ''[[Scirpus]]'' and often ''[[Typha]]'' are the main plants used for nest building.<ref name="McNicholl71" /><ref name="McNicholl82" />

In the case of large colonies, nesting area availability decreases. Forster's tern will then nest on sand, gravel or mud.<ref name="McNicholl71" /> 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.<ref name="Dakin" />

Floating nests are usually tolerant to a slight increase or decrease of water levels but re-nesting is common.<ref name="McNicholl71" /> 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.<ref name="McNicholl71" /><ref name="McNicholl82" />

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.<ref>{{cite web|title=Green Bay Tern Nesting Platform a Success|url=https://www.fws.gov/FieldNotes/regmap.cfm?arskey=35323|website=U.S. Fish and Wildlife Service|access-date=18 December 2017}}</ref>

===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.<ref name="McNicholl71" /> Whitish or cream colored eggs have also been reported.<ref name="ARKive" /><ref name="McNicholl71" />

==Predator==

[[Bird of prey|Raptors]] such as falcons and hawks, as well as owls and [[corvidae|crows]] may prey on adults and young.<ref name="McNicholl71" /> There have also been anecdotal reports of [[snapping turtle]]s preying on young still in the nest.<ref>Pryor GS. 1996. Observations of shorebird predation by snapping turtles in Eastern Lake Ontario. The Wilson Bulletin. 108(1): 190–192.</ref> [[American bittern]], [[great blue heron]], and [[black-crowned night heron]] are also possible predators while gulls and [[Caspian tern]]s notably prey on the eggs of the Forster's tern.<ref name="McNicholl71" />

When their ranges overlap, [[marsh rice rat]]s are possibly the most efficient Forster's tern egg predator.<ref>{{cite journal|last1=Brunjes|first1=J.H.|last2=Webster|first2=W.D.|year=2003|title=Marsh rice rat, ''Oryzomus palustris'', predation on Forster's tern, ''Sterna forsteri'', eggs in coastal North Carolina|journal=Canadian Field-Naturalist|volume=117|issue=4|pages=654-655|doi=10.22621/cfn.v117i4.820|doi-access=free}}</ref> [[American mink]] are also one of few mammalian predators that can venture in the marsh and prey on eggs and young.<ref name="McNicholl82" />

Predator success usually remains low during breeding season due to the aggressive mobbing response of adults.<ref name="McNicholl71" />

==Status and conservation==

According to the IUCN, the status of the Forster's tern is of little concern,<ref name="iucn status 12 November 2021" /> 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 (<ref name="ARKive" />).

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.<ref name="McNicholl82" />

As with many species of piscivorous birds, Forster's tern is susceptible to [[bioaccumulation]] of pollutants. High [[mercury (element)|mercury]] concentrations may induce biochemical stress, reducing the overall health of terns.<ref>Hoffman DJ, Eagles-Smith CA, Ackerman JT, Adelsbach TL, Stebbins KR. 2011. Oxidative stress response of Forster's terns (Sterna forsteri) and Caspian terns (Hydroprogne caspia) to mercury and selenium bioaccumulation in liver, kidney, and brain. Environmental Toxicology. 30(4): 920–929.</ref> [[Biomethylation]] of mercury is increased in marshes and salt ponds, hence increasing the susceptibility of the Forster's tern.<ref>Ackerman JT, Eagles-Smith CA, Takekawa JY, Bluso JD and Adelsbach TL. 2008. Mercury concentrations in blood and feathers of prebreeding forster's terns in relation to space use of san Francisco bay, California, usa, habitats. Environmental Toxicology. 27(4): 897–908.</ref> High levels of [[selenium]] may also have deleterious effects on their health.<ref>Ackerman JT and Eagles-Smith CA. 2009. Selenium bioaccumulation and body condition in shorebirds and terns breeding in San Francisco Bay, California, USA. Environmental Toxicology. 28(10): 2134–2141.</ref> [[Organochlorine]] contaminants such as [[Polychlorinated biphenyl|PCB]]s may also diminish their breeding success.<ref>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.</ref>

==Gallery==
<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]]
File:Forster's Tern from the Crossley ID Guide Britain and Ireland.jpg|ID composite
</gallery>

==References==
{{Reflist}}

==External links==
{{Commons category|Sterna forsteri|Forster's tern}}
{{Wikispecies|Sterna forsteri}}
* [http://www.birds.cornell.edu/AllAboutBirds/BirdGuide/Forsters_Tern.html Forster's Tern Species Account] - Cornell Lab of Ornithology
* [http://www.mbr-pwrc.usgs.gov/id/framlst/i0690id.html Forster's Tern - ''Sterna forsteri''] - USGS Patuxent Bird Identification InfoCenter
* [https://www.flickr.com/groups/birdguide/pool/tags/Sterna%20forsteri Field Guide Page on Flickr]
* {{InternetBirdCollection|forsters-tern-sterna-forsteri|Forster's Tern}}
* {{VIREO|Forster's+Tern|Forster's Tern}}
* {{IUCN_Map|22694646/132564836|Sterna forsteri}}

{{Taxonbar|from=Q430529}}

{{Use dmy dates|date=March 2017}}

[[Category:Sterna|Forster's tern]]
[[Category:Birds of North America]]
[[Category:Birds described in 1834|Forster's tern]]
[[Category:Taxa named by Thomas Nuttall|Forster's tern]]