Forster's Tern

识别特征

Forster's tern is a medium-sized tern with a slender body, deeply forked long tail and relatively long legs.<p>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 grey with the primaries being dark silver grey, while the underside is white. The bill is black and the legs are a dull brownish red.<p>When breeding, an intense black cap extending down the neck appears. The wings and the back are pale grey while the underside is bright white. It has a black-tipped orange bill and bright orange legs.<p>The juveniles have coloring similar to a non-breeding adult but often have darker primaries.

分类信息

Forster's tern is a member of the Sternidae. They were previously considered as a subfamily of the Laridae (gull) but have been classified as a distinct family since 1838. Linnaeus described the tern using the term rustrum subulatum in reference to their awl-shaped bill. Forster's tern was named in honor of Johann Reinhold Forster, a German naturalist.

学名

Sterna forsteri

命名信息

Nuttall, 1834

目

Charadriiformes

科

Laridae

属

Sterna

分布与栖息地

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. Main habitats are marshes, estuaries, islands, salt marshes and marshy areas surrounding lakes and streams.<p>Forster's tern is usually restricted to North America. It nests in marshes during the summer, either on the Atlantic or Pacific coast, but also in the Prairies or along the Great Lakes in Canada and the US. Due to the instability of its nesting habitat, Forster's tern exhibits a high annual turnover rate.<p>Forster's tern also winters in marshes along the southern coast of the US and Mexico but can sometimes reach the northern extremity of Central America. It is also common for the tern to winter in the Caribbean.<p>It near annual vagrant to Western Europe and has occasionally wintered in Great Britain and Ireland.

繁殖区

NA, MA : inland c Alberta to c Manitoba (sc Canada) patchily to California and n Baja California (nw Mexico), Colorado, and the Great Lakes; coastal w North Atlantic from New Jersey to North Carolina; Gulf of Mexico coast from Alabama to n Tamaulipas (ne Mexico)

非繁殖区

to Gulf of Mexico coast, Bahamas, Cuba and southern Mexico

迁徙

Complete migrants

行为

Forster's tern is often found in marshes over shallow open water. It is a shallow plunge-diver that often hovers before attacking. When hunting, its head is pointed downward whereas when travelling, it is pointed forward.<p>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. In many occasions, Forster's tern will share nesting sites with the Yellow-headed blackbird.<p>Both parents are involved in brood caring and Forster's tern does not exhibit sex-specific differences in space use. 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.<p>Before breeding, males practice courtship feeding.<p>The common call of the Forster's tern is a descending kerr. The threat call used in defensive attack is a low harsh zaar. A succession of kerrs is used by the female as a begging call during courtship.<p>The major constituent of Forster's tern diet is fish. Carp, minnow, sunfish, trout-perch, trout, perch, killifish, stickleback, shiner are common prey in freshwater whereas pompano, herring, menhaden and shiner perch are often consumed in brackish or marine habitats. On the West Coast of the United States and Canada, Forster's tern is also known to prey on Pacific lamprey juveniles. Insects such as dragonflies, caddisflies and grasshoppers are often consumed, but aquatic insect larvae, crustaceans and amphibian can complement the diet.<p>The Forster's tern is a shallow plunge-diver, having its head pointing downward when hunting. The attack usually starts in a hovering position before initiating a headfirst dive with wings partially folded backward. Insects may occasionally be caught by the wing and preys are swallowed in the air. Prey handling behavior may include dropping and re-catching fish before swallowing them. In some areas, Forster's tern tends to prefer forage to turbid water. This may prevent detection but it may also be a sign of higher prey density and increased presence near the surface. Preferences for water clarity may depend on prey availability.<p>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. Forster's tern is a colonial nester with colony size ranging from one to a thousand nests. Adults establish a very small territory around the nest and nests are usually clumped together. Males will practice courtship feeding and females will beg for food using a kerr kerr kerr call.<p>A typical clutch off eggs ranges from 2 to 4. The incubation period may last 24 or 25 days after laying. The young are semi-precocial with shell removal being done by the parents. The chicks have upper and lower mandible egg teeth, which they lose 3 to 5 days after hatching. The chicks usually leave the nest with the parents 4 days post hatching and move into areas of denser vegetation. Fledging occurs 28 days after hatching. After a few weeks of fledging, young terns leave the natal colony but join the group for roosting, while migrating towards the wintering ground.<p>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. Reproductive success varies from year to year and from colony to colony.<p>Forster's tern exhibits very aggressive behavior when threatened by nest predators; if a nest is disturbed, the colony mobs the aggressor, diving towards it and issuing loud calls. 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 <p>Yellow-headed blackbirds sharing nesting sites have been known to actively join tern mobs against predators. Western grebes recognize the tern's alarm call; this can be interpreted as information parasitism.

保护状况

According to the IUCN, the status of the Forster's tern is of little concern, however, degradation of marsh habitat may be threatening. Boating activity may also affect nest vegetation and increase erosion, which may lead to further degradation of tern nesting grounds. Excessive noise may also have caused nest desertion and chick mortality. This species is listed under the Migratory Birds Treaty act in the U.S. It is endangered in Illinois and Wisconsin while being of special concern in Michigan and Minnesota. Preservation of wetlands and introduction of artificial nesting sites may help preserve the species in high-risk areas ().<p>Increasing populations of carp in drainage systems, causing damages to marsh vegetation may limit habitat availability for Forster's tern. There have also been anecdotal reports of intense spawning activity of carp damaging tern's floating nests.<p>As with many species of piscivorous birds, Forster's tern is susceptible to bioaccumulation of pollutants. High mercury concentration may induce biochemical stress, reducing the overall health of terns. 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 PCB may also diminish their breeding success.

种群趋势

rising