Forster's tern (Sterna forsteri) is a tern in the family Laridae. The genus name Sterna is derived from Old English "stearn", "tern", and forsteri commemorates the naturalist Johann Reinhold Forster.

It breeds inland in North America in the northern United States and southern Canada, and migrates south to winter in the southern United States, Mexico, the Caribbean, and northern Central America. It is also a rare but annual 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, long with a wingspan and a weight ranging from 130 to 190 g (4.6-6.7 oz ). 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

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.

In its non-breeding plumage, the crown is white and a black comma-shaped patch covers the eye and the ear-covert. The wings are gray with the primaries being dark silvery gray, while the underside is white. 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.

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. Due to the instability of its nesting habitat, Forster's tern exhibits a high annual turnover rate.

It is a colonial nesting species that builds a shallow nest using marsh vegetation and often competes with gulls for nesting sites. A breeding colony may vary in numbers from a few couples to a thousand individuals. Males tend to guard the nest more often during the day while the female is more present at night. When disturbed, newborn chicks tend to crouch and remain silent. Forster's tern is a single prey loader and provision chicks with prey correlated to their size.

Before breeding, males practice courtship feeding. whereas pompano, herring, menhaden and shiner perch are often consumed in brackish or marine habitats. Insects such as dragonflies, caddisflies and grasshoppers are often consumed, but aquatic insect larvae, crustaceans and amphibian can complement the diet.

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. The incubation period may last 24 or 25 days after laying. The young are semi-precocial with shell removal being done by the parents.

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. 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

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.

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. American bittern, great blue heron, and black-crowned night heron are also possible predators while gulls and Caspian terns notably prey on the eggs of the Forster's tern. American mink are also one of few mammalian predators that can venture in the marsh and prey on eggs and young. Biomethylation of mercury is increased in marshes and salt ponds, hence increasing the susceptibility of the Forster's tern. High levels of selenium may also have deleterious effects on their health. Organochlorine contaminants such as PCBs may also diminish their breeding success.

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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

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References

  • Forster's Tern Species Account - Cornell Lab of Ornithology
  • Forster's Tern - Sterna forsteri - USGS Patuxent Bird Identification InfoCenter
  • Field Guide Page on Flickr