By Kari Kirschbaum
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, Dobkin, and Wheye, 1988; McNicholl, Lowther, and Hall, 2001)
Biogeographic Regions:
nearctic
(native
); neotropical
(native
).
Habitat
Forster's terns are found in fresh, brackish, and saltwater marshes, including marshy borders along lakes, islands, and streams. (Bent, 1921; McNicholl, Lowther, and Hall, 2001)
These animals are found in the following types of habitat:
temperate
; terrestrial
; saltwater or marine
; freshwater
.
Aquatic Biomes:
lakes and ponds; rivers and streams; coastal
; brackish water
.
Wetlands: marsh
.
Other:
estuarine
.
Physical Description
(4.58 to 6.69 oz; avg. 5.63 oz)
(12.99 to 14.17 in)
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, Lowther, and Hall, 2001; Udvardy and Farrand, 1998)
Some key physical features:
endothermic
; bilateral symmetry
.
Sexual dimorphism:
sexes alike.
Reproduction
Forster's terns breed once per year.
April and May
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, Dobkin, and Wheye, 1988; McNicholl, Lowther, and Hall, 2001)
Mating systems:
monogamous
.
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, Lowther, and Hall, 2001; Udvardy and Farrand, 1998)
Key reproductive features:
iteroparous
; seasonal breeding
; gonochoric/gonochoristic/dioecious (sexes separate); sexual
; oviparous
.
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, Dobkin, and Wheye, 1988; Fraser, 1997; McNicholl, 1971; McNicholl, Lowther, and Hall, 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, Lowther, and Hall, 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, Williams, and Sooter, 1942; McNicholl, 1980; McNicholl, Lowther, and Hall, 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, Lowther, and Hall, 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, Lowther, and Hall, 2001)
Animal Foods:
amphibians; fish; insects.
Predation
- black-crowned night-herons (Nycticorax nycticorax)
- herring gulls (Larus argentatus)
- raccoons (Procyon lotor)
- great horned owls (Bubo virginianus)
- short-eared owls (Asio flammeus)
- marsh rice rats (Oryzomys palustris)
- minks (Mustela)
- snapping turtles (Chelydra serpentina)
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, Lowther, and Hall, 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, Dobkin, and Wheye, 1988)
Economic Importance for Humans: Negative
There are no known adverse affects of Forster's terns on humans.
Economic Importance for Humans: Positive
We do not have information on the economic importance of this species for humans.
Conservation Status
IUCN Red List: [link]:
Least Concern.
US Migratory Bird Act: [link]:
Protected.
US Federal List: [link]:
No special status.
CITES: [link]:
No special status.
State of Michigan List: [link]:
Special Concern.
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, Lowther, and Hall, 2001; Mossman, 1989)
Contributors
Kari Kirschbaum (author, editor), Animal Diversity Web Staff.
Jackson Lynch (author), University of Arizona.
Todd McWhorter (editor), University of Arizona. Alaine Camfield (editor), Animal Diversity Web.
