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Sterna forsteriForster's tern

By Kari Kirschbaum and Jackson Lynch

Geographic Range

Forster's terns, Sterna forsteri, winter in California, on the Atlantic Coast of the United States (south of New Jersey), in the Bahamas, Guatemala, the Greater Antilles and on the coast of the Gulf of Mexico. They breed in scattered patches throughout North America. (Ehrlich, et al., 1988; McNicholl, et al., 2001)

Habitat

Forster's terns are found in fresh, brackish, and saltwater marshes, including marshy borders along lakes, islands, and streams. (Bent, 1921; McNicholl, et al., 2001)

Physical Description

Forster's terns are medium-sized birds. They are white with a pale gray back and wings and a black cap. They have very deeply forked tails with long, streamer-like outer-retrices. Their legs are orange, and their bills are orange with a black tip. In winter they lack the black cap, but have a distinctive black mark behind each eye. Male and female adults are similar in appearance. Adults weigh 130 to 190 g, and are 33 to 36 cm long. Immature birds are similar in appearance to adults, but generally have darker primaries.

Forster's terns that breed in western and interior North America appear slightly larger and having darker upperparts than those that breed along the Atlantic and Gulf coasts. These groups have been designated as subspecies of Forster's terns, Sterna forsteri forsteri and Sterna forsteri litoricola. However, the designation of these groups as subspecies is not universally accepted. some ornithologists consider Sterna forsteri to be monotypic. (McNicholl, et al., 2001; Udvardy and Farrand, 1998)

  • Sexual Dimorphism
  • sexes alike
  • Range mass
    130 to 190 g
    4.58 to 6.70 oz
  • Range length
    33 to 36 cm
    12.99 to 14.17 in

Development

Young are semi-precocial at birth. Their eyes are open, down is present, and they are able to walk upon hatching. Although they can walk, they are nidiculous (stay in the nest). (Nice 1962) Egg-teeth are lost within 3-5 days of hatching. Young are known to leave the nest as early as 4 days, but are not capable of flight until 4 or 5 weeks. (Hall 1988, 1989)

Reproduction

Forster's terns breed in colonies, and are monogamous. Pair formation appears to occur around the time arrival on the breeding grounds in April. Courtship includes a number of displays and postures, including courtship feeding. (Ehrlich, et al., 1988; McNicholl, et al., 2001)

Forster's terns breed in April and May. The female lays 1 to 6 (usually 2 or 3) buff, spotted eggs on a nest dead vegetation. The nest is built on marshy shoreline, on top of a muskrat lodge, or on a mat of floating vegetation. The male and female both participate in incubation, which lasts 20 to 28 days. The chicks are semi-precocial when they hatch; they have open eyes, down, and are able to walk but remain in the nest and are fed by adults. The chicks are brooded by both adults for the first 3 days or so, and are fed for at least 4 weeks. They are able to fly 4 to 5 weeks after hatching. The age of first breeding is unknown, but is believed to be at least 2 years.

Forster's terns breed in April and May. The female lays 2 or 3 eggs that are buff-colored and spotted. The nest is made of dead plant material, and is built on the shoreline, or on top of a muskrat lodge or a floating mat of plants. The parents take turns incubating the eggs for 20 to 28 days. When the chicks hatch, they can walk, but they still depend on the parents to feed them. The parents brood the chicks for about 3 days, and feed the chicks for at least 4 weeks. The chicks can fly when they are 4 to 5 weeks old. They probably do not begin breeding until they are at least 2 years old. (McNicholl, et al., 2001; Udvardy and Farrand, 1998)

  • Breeding interval
    Forster's terns breed once per year.
  • Breeding season
    April and May
  • Range eggs per season
    1 to 6
  • Average eggs per season
    3
  • Average eggs per season
    3
    AnAge
  • Range time to hatching
    20 to 28 days
  • Range age at sexual or reproductive maturity (female)
    2 (low) years
  • Range age at sexual or reproductive maturity (male)
    2 (low) years

Brooding behavior is poorly documented in Forster's terns, but adults appear to brood young less than 3 days old and during stormy periods. Both adults feed young at least until they are able to fly, and most likely longer. (Ehrlich, et al., 1988; Fraser, 1997; McNicholl, 1971; McNicholl, et al., 2001)

  • Parental Investment
  • no parental involvement
  • precocial
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • protecting
      • male
      • female
  • pre-weaning/fledging
    • provisioning
      • male
      • female
  • pre-independence
    • provisioning
      • male
      • female

Lifespan/Longevity

Not much is known about the lifespan of Forster's terns. The oldest banded Forster's tern on record was 12 years old when it died. (McNicholl, et al., 2001)

Behavior

Forster's terns are diurnal and migratory. They are graceful fliers with a cruising speed up to 16 km/h.

Forster's terns are colonial. They do not defend a territory, only the nest itself during the breeding season. (Cottam, et al., 1942; McNicholl, 1980; McNicholl, et al., 2001)

Home Range

We do not have information on home range for this species at this time.

Communication and Perception

Forster's terns use calls and visual displays to communicate. (McNicholl, et al., 2001; Udvardy and Farrand, 1998)

Food Habits

Forster's terns eat many species of small fish, arthropods and occasionally frogs.

Forster's terns hunt by flying back and forth over the water with their bill pointing downward and their feet folded against their body. They typically fly about 6 to 8 m above water. When they spot prey, they either plunge directly into the water toward the prey or hover briefly before diving. They occasionally hunt from perches, such as posts, bridges and telephone wires. (McNicholl, et al., 2001)

  • Animal Foods
  • amphibians
  • fish
  • insects

Predation

Known predators of Forster's terns include black-crowned night-herons (Nycticorax nycticorax), herring gulls (Larus argentatus), great horned owls (Bubo virginianus), short-eared owls (Asio flammeus), snapping turtles (Chelydra serpentina), marsh rice rats (Oryzomys palustris) and mink (genus Mustela).

Floating nest sites and nests on muskrat lodges isolate and protect Forster's terns from some predators. Terns respond to predators that enter a colony by diving and swooping at them, sometimes striking the predator's back. (McNicholl, et al., 2001)

Ecosystem Roles

Forster's terns have an impact on populations of the prey they eat and are an important food source for their predators. Their nests are also occasionally parasitized by red-necked grebes (Podiceps grisegena) and American coots (Fulica americana). Forster's terns host several species of external parasites, including at least three species of lice. (Ehrlich, et al., 1988)

Economic Importance for Humans: Positive

We do not have information on the economic importance of this species for humans.

Economic Importance for Humans: Negative

There are no known adverse affects of Forster's terns on humans.

Conservation Status

Forster's terns are not protected under the Endangered Species Act or CITES. However, they are designated as a species of special concern in Michigan and Minnesota and are endangered in Illinois and Wisconsin. They are also protected under the US Migratory Bird Treaty Act.

There are an estimated 120,000 Forster's terns worldwide. The most important causes of mortality in this species are probably predation and egg loss due to storms, heavy rains and high waves. (Alvo and McNicholl, 1996; Cuthbert and Louis, 1993; Fraser, 1994; McNicholl, et al., 2001; Mossman, 1989)

Contributors

Alaine Camfield (editor), Animal Diversity Web.

Kari Kirschbaum (author, editor), Animal Diversity Web.

Jackson Lynch (author), University of Arizona, Todd McWhorter (editor), University of Arizona.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

acoustic

uses sound to communicate

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

brackish water

areas with salty water, usually in coastal marshes and estuaries.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

coastal

the nearshore aquatic habitats near a coast, or shoreline.

colonial

used loosely to describe any group of organisms living together or in close proximity to each other - for example nesting shorebirds that live in large colonies. More specifically refers to a group of organisms in which members act as specialized subunits (a continuous, modular society) - as in clonal organisms.

diurnal
  1. active during the day, 2. lasting for one day.
endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

estuarine

an area where a freshwater river meets the ocean and tidal influences result in fluctuations in salinity.

freshwater

mainly lives in water that is not salty.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

marsh

marshes are wetland areas often dominated by grasses and reeds.

migratory

makes seasonal movements between breeding and wintering grounds

monogamous

Having one mate at a time.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

piscivore

an animal that mainly eats fish

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

tactile

uses touch to communicate

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

terrestrial

Living on the ground.

visual

uses sight to communicate

young precocial

young are relatively well-developed when born

References

Alvo, R., M. McNicholl. 1996. Status report on Forster's Tern (Sterna forsteri) in Canada. Comm. Status Endangered Wildlife Canada, Ottawa, ON.

Bent, A. 1921. Life histories of North American gulls and terns. U.S. National Museum Bulletin: 113.

Cottam, C., C. Williams, C. Sooter. 1942. Flight and running speeds of birds. Wilson Bulletin, 54: 121-131.

Cuthbert, F., M. Louis. 1993. The Forster's tern in Minnesota: Status, distribution, and reproductive success. Wilson Bulletin, 105: 184-187.

Ehrlich, P., D. Dobkin, D. Wheye. 1988. The Birder's Handbook: A Field Guide to the Natural History of North American Birds. New York: Simon and Schuster.

Fraser, G. 1994. Feeding and nesting behaviors of the Forster's Tern on Lake Osakis, Minnesota. M.S. Thesis, North Dakota State University, Fargo.

Fraser, G. 1997. Feeding ecology of Forster's Terns on Lake Osakis, Minnesota. Colonial Waterbirds, 20(1): 87-94.

Hall, J. 1989. Aspects of Forster's Tern (*Sterna forsteri*) reproduction on cobblestone islands in south central Washington. Northwest Sci., 63: 90-95.

Hall, J. 1988. Early chick mobility in brood movements in Forster's Tern (*Sterna forsteri*). Journal of Field Ornithology, 59(3): 247-251.

Hall, J. 1998. Vocal repertoire of Forster's Tern. Colonial Waterbirds, 21: 388-405.

McNicholl, M., P. Lowther, J. Hall. 2001. Forster's Tern (*Sterna forsteri*). Pp. 1-24 in Birds of North America, Vol. 595. Philadelphia, PA: The Birds of North America.

McNicholl, M. 1980. Territories of Forster's Terns. Proc. Colonial Waterbirds Group, 3: 196-203.

McNicholl, M. 1971. The breeding biology and ecology of Forster's Tern (Sterna forsteri) at Delta, Manitoba. M.Sc. thesis, University of Manitoba, Winnipeg.

Mossman, M. 1989. Wisconsin's Forster's Tern recovery plan. Passanger Pigeon, 51: 171-186.

Nice, M. 1962. Development of behavior in precocial birds. Transactions of the Linnaean Society New York, 8: 1-212.

Udvardy, M., J. Farrand. 1998. National Audubon Society Field Guide to North American Birds (Western Region). New York: Alfred A. Knopf.

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