Forster's tern (Sterna forsteri)

Description

Forster's tern fact file

• Description
• Biology
• Range
• Habitat
• Status
• Threats
• Conservation
• Find out more
• Glossary
• References
• Print factsheet

Forster's tern description

Named after the German naturalist Johann R. Forster, Forster’s tern (Sterna forsteri) is a graceful and highly manoeuvrable seabird with a slender body, long, pointed wings and a deeply forked tail ^{(2) (3) (4)}. During the breeding season, Forster’s tern has a distinctive black cap which extends to the forehead, crown and back of the neck. The shoulders, back, wings and tail are pale grey, contrasting with the white neck and underparts. The flight feathers have pale, silvery grey-white tips, with darker tips on the outermost primaries. A white margin is visible on the outer margin of the tail feathers. The bill is orange with a black tip, and the legs and feet are bright orange or orange-red ^{(2) (3) (4) (5) (6) (6)}.

Outside of the breeding season, Forster’s tern lacks the black cap, which is instead replaced with a white forehead, crown and a characteristic black mask which covers the eyes and ear-coverts. The primaries become a dark silver-grey, the bill becomes black and the legs turn dull red-brown ^{(2) (3) (4) (5)}. The juvenile Forster’s tern generally appears similar to the non-breeding adult, but it typically has darker primary feathers ^{(2) (4)}.

The most common calls of Forster’s tern are a simple, raspy, descending "kerrrr", a begging "kerr kerr kerr" during courtship and a very low "zaaaar" during defensive attacks ⁽³⁾.

Size

Length: 33 - 36 cm ⁽²⁾

Weight

130 - 190 g ⁽²⁾

Forster's tern biology

Forster’s tern feeds primarily on small fish, arthropods and crustaceans ^{(2) (3) (4) (5) (8)}. While foraging, Forster’s tern will typically fly back and forth over the water, with the bill pointing downwards and the feet folded against the body ⁽²⁾. Forster’s tern performs direct, shallow ‘plunge-dives’, generally only submerging the bill and front of the head when catching prey. Occasionally the bird will hover for several seconds above the surface before diving. Forster’s tern may also feed by flying low and skimming above the water, ‘dipping’ to pluck out prey, or it may hawk insects in the air while in flight. Forster’s tern may also sometimes forage from perches such as posts, bridges, telephone wires, or floating debris ^{(2) (3) (4) (8)}. 

This species breeds between May and mid-June, although breeding may begin as early as April on the Gulf Coast of the U.S. ^{(3) (8)}. Generally, loose breeding colonies of around 5 to 250 pairs are formed, and individual pairs are thought to maintain small breeding territories around the nest ^{(2) (3) (5)}. Both adults build the nest, which is typically positioned close to open water among floating and emergent vegetation. Most often the nest is little more than a cup constructed from plant material. Forster’s tern may also nest on mud or sand in a unlined or sparsely lined scrape in the ground, and, more rarely, on boards, spoil, fine shell and coarse gravel islands ^{(2) (5) (8)}.

Forster’s tern usually lays 3 eggs, although clutch size may range from 2 to 5, and the eggs are incubated by both adults for around 23 to 25 days ^{(3) (4)}. Like most tern species, Forster’s tern will fiercely defend its nest and young against intruders ^{(4) (5)}. The young chicks are fed and guarded closely by the adults until they fledge at three to four weeks, after which both the adults and juveniles migrate from the breeding grounds to their wintering habitat ^{(3) (4)}.  

Forster's tern range

The breeding range of Forster’s tern is almost entirely restricted to a number of scattered locations throughout North America ^{(2) (5) (8)}. It nests along the Atlantic, Gulf and Pacific coasts of the U.S., and in the prairie and Great Lakes region of the U.S. and Canada ⁽⁴⁾. Forster’s tern generally winters in the south-east and south-west U.S., along the coasts of Mexico, and in Central America and the Caribbean, including the Bahamas and the Greater Antilles ^{(1) (2) (4) (5) (8)}. This species is a rare but annual visitor to Western Europe, and has been recorded wintering in Great Britain and Ireland ⁽⁵⁾.

Forster's tern habitat

Typically associated with marsh habitats, Forster’s tern may be found in fresh, brackish or saltwater marshes, the marshy borders of lakes, islands or streams, and the marsh areas close to beaches and estuaries. It is generally found in the open, deeper areas of marsh, where it nests among floating and emergent vegetation ^{(2) (4) (8)}. 

Forster's tern status

Forster’s tern is classified as Least Concern (LC) on the IUCN Red List ⁽¹⁾.

Forster's tern threats

The nesting and foraging behaviour of Forster’s tern makes it particularly vulnerable to changes in land or water usage. As such, the main threat to Forster’s tern is habitat loss due to the degradation, development, draining, and filling or flooding of wetland habitats ⁽²⁾. In areas where recreational boating is permitted, the wake from boats is thought to have a detrimental effect on the floating vegetation used by Forster’s tern for nesting, while excessive noise at nesting sites is also likely to cause nest desertion and increased mortality in newly hatched chicks ⁽⁴⁾.

Forster's tern conservation

Forster’s Tern is listed under the Migratory Birds Treaty Act in the U.S., making it illegal to pursue, hunt, take, capture, kill or sell this species. It is also designated a species of Special Concern in Michigan and Minnesota, and is considered Endangered in Illinois and Wisconsin ⁽⁴⁾. 

Several conservation measures have been proposed which would benefit Forster’s tern, including preserving and restoring wetland habitats, especially in areas used for nesting. In Wisconsin, wooden platforms have been installed in areas of suitable habitat to serve as artificial nesting sites. Other recommendations include educational outreach programmes to address the problems caused by recreational boating, and the placement of signs near breeding colonies to further raise awareness ⁽⁴⁾. The need for additional studies on the biology and ecology of Forster’s tern has also been suggested ⁽²⁾.

ARKive is supported by OTEP, a joint programme of funding from the UK FCO and DFID which provides support to address priority environmental issues in the Overseas Territories, and Defra

Find out more

Find out more about Forster’s tern and other birds:

• BirdLife International:
  http://www.birdlife.org/datazone/speciesfactsheet.php?id=3274

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Glossary

Arthropods

A very diverse phylum (a major grouping of animals) that includes crustaceans, insects and arachnids. All arthropods have paired jointed limbs and a hard external skeleton (exoskeleton).

Coverts

Small feathers concealing the bases of larger flight feathers, usually on the wings or tail.

Crustaceans

Diverse group of arthropods (a phylum of animals with jointed limbs and a hard chitinous exoskeleton) characterised by the possession of two pairs of antennae, one pair of mandibles (mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps, woodlice and barnacles.

Emergent

Aquatic plants whose stems and leaves extend beyond the water’s surface.

Flight feathers

The feathers at the end of the wing, involved in flight.

Incubate

To keep eggs warm so that development is possible.

Primaries

In birds, the main flight feathers projecting along the outer edge of the wing.

Territory

An area occupied and defended by an animal, a pair of animals or a colony.

References

1. IUCN Red List (March, 2011)
   http://www.iucnredlist.org/
2. Mcnicholl, M.K., Lowther, P.E. and Hall, J.A. (2001) Forster's tern (Sterna forsteri). In:  Poole, A. (Ed.) The Birds of North America Online. Cornell Lab of Ornithology, Ithaca. Available at:
   http://bna.birds.cornell.edu/bna/species/595
3. Montana Field Guide - Forster's Tern (Sterna forsteri) (March, 2011)
   http://fieldguide.mt.gov/detail_ABNNM08090.aspx
4. Michigan State University. (2004) Sterna forsteri Forster’s tern. Michigan Natural Features Inventory, Lansing, Michigan. Available at:
   http://web4.msue.msu.edu/mnfi/abstracts/zoology/Sterna_forsteri.pdf  
5. Mongabay - Sterna forsteri (March, 2011)
   http://biodiversity.mongabay.com/animals/s/Sterna_forsteri.html
6. Kaufmann, K. (2001) Birds of North America. Houghton Mifflin, Boston.
7. Raffaele, H. A. (2003) Birds of the West Indies. Princeton University Press, New Jersey.
8. BirdLife International (March, 2011)
   http://www.birdlife.org/datazone/speciesfactsheet.php?id=3274