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Forster's tern

Sterna forsteri

What do they look like?

Forster's terns are medium-sized birds. They are mostly white, with a pale gray back and wings and a black cap. They have long tails that form a deep “V” shape, and their outer tail feathers are long, like streamers. Their legs are orange, and their bills are orange with a black tip.

In winter, Forster’s terns do not have a black cap. Instead, they have a black mark behind each eye. Male and female Forster’s terns look the same. They weigh 130 to 190 g, and are 33 to 36 cm long. Young Forster’s terns look like adults, but have darker wing feathers. (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

Where do they live?

Forster's terns, Sterna forsteri breed in North America. They are most common in south-central Manitoba, northern California, southern Oregon and along the Gulf Coast. They winter along the Pacific Coast in California, on the Atlantic Coast of the United States (south of New Jersey), in Mexico, the Bahamas, Guatemala, the Greater Antilles and on the coast of the Gulf of Mexico. (Ehrlich, et al., 1988; McNicholl, et al., 2001)

What kind of habitat do they need?

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)

How do they grow?

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)

How do they reproduce?

Forster's terns breed in colonies. They are monogamous. Males and females look for mates around the time that they arrive on the breeding grounds in the spring. Males try to attract a female by performing displays, and bringing food (fish) to the female. This is called courtship feeding. (Ehrlich, et al., 1988; McNicholl, et al., 2001)

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)

  • How often does reproduction occur?
    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

Both parents incubate the eggs and brood the chicks for at least three days after they hatch. After the chicks hatch, both parents feed them until they are able to fly. (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

How long do they live?

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)

How do they behave?

Forster's terns are active during the day (diurnal). They are also migratory; they fly south in the fall to spend winter in warmer climates. They are graceful fliers, and can fly up to 16 km/hour.

Forster's terns are colonial. They do not defend a territory. During the breeding season, they defend only the nest. (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.

How do they communicate with each other?

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

What do they eat?

Forster's terns eat small fish, arthropods and occasionally frogs. They hunt by flying back and forth above the water, searching for food. When they spot prey, they either dive directly into the water toward the prey or hover for a few seconds and then plunge head-first toward the water. Sometimes, Forster's terns hunt from perches, such as posts, bridges and telephone wires. (McNicholl, et al., 2001)

  • Animal Foods
  • amphibians
  • fish
  • insects

What eats them and how do they avoid being eaten?

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) are all predators of Forster's terns.

Forster's terns are protected from some predators by their nest sites, which are often surrounded by water. When a predator does enter a colony, the terns dive and swoop at the predator, sometimes striking the predator on the back. (McNicholl, et al., 2001)

What roles do they have in the ecosystem?

Forster's terns affect the populations of the animals they eat. They are also an important food source for their predators. Forster's terns provide habitat for many different parasites, including at least three species of lice.

Red-necked grebes (Podiceps grisegena) and American coots (Fulica americana) sometimes lay eggs in the nests of Forster's terns. If the terns care for these eggs, they help the grebe and coot populations. (Ehrlich, et al., 1988)

Do they cause problems?

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

How do they interact with us?

We do not know if this species affects humans.

Are they endangered?

Forster's terns are protected by the federal government under the US Migratory Bird Treaty Act. They are also designated as a "species of special concern" in Michigan and Minnesota and are "endangered" in Illinois and Wisconsin.

There are about 120,000 Forster's terns in the world. Most deaths in this species are probably due to predation and eggs that are lost during storms, heavy rains and high waves.

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.

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 generate their own body heat through metabolic processes.

estuarine

an area where a freshwater river meets the ocean and the fresh and saltwater mixes

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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Lynch, J. 2002. "Sterna forsteri" (On-line), Animal Diversity Web. Accessed April 23, 2014 at http://www.biokids.umich.edu/accounts/Sterna_forsteri/

BioKIDS is sponsored in part by the Interagency Education Research Initiative. It is a partnership of the University of Michigan School of Education, University of Michigan Museum of Zoology, and the Detroit Public Schools. This material is based upon work supported by the National Science Foundation under Grant DRL-0628151.
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