The Aleutian tern (Onychoprion aleuticus) is a migratory bird living in the subarctic region of the globe most of the year. It is frequently associated with the Arctic tern, which it closely resembles. While both species have a black cap, the Aleutian tern may be distinguished by its white forehead (although juvenile Arctic terns also have white foreheads). During breeding season, the Arctic terns have bright red bills, feet, and legs while those of the Aleutian terns are black.
Onychoprion aleuticus has not been thoroughly studied yet. If its winter migratory range was completely unknown until the late 1980s; it is now known that many Aleutian terns spend the winter near the Equator in the western Pacific. The Aleutian tern breeds in wide-ranging coastal colonies only in Alaska and eastern Siberia. Although Alaskan and Siberian populations are not well monitored, both are thought to be in significant decline.
Taxonomy
The Aleutian tern, Onychoprion aleuticus (Baird, 1869) is a bird in the family Laridae, a family of seabirds in the order Charadriiformes that includes the gulls, terns and skimmers. The generic name is from Ancient Greek onux, "claw" or "nail", and prion, "saw". The specific epithet aleuticus refers to the Aleutian Islands.
It was formerly named Sterna aleutica.. Indeed, in his 1758 Systema Naturae, Linnaeus placed the terns in the genus Sterna. However, a phylogenetic analysis found that the species in the Onychoprion clade, which includes O. aleuticus, O. fuscatus (Sooty tern), and O. anaethetus (Bridled tern), are related only distantly to the "typical" terns retained in a much-restricted Sterna. Still, in broader terms the genera Onychoprion and Sterna are sisters. Relationships between various tern species, and between the terns and the other Charadriiformes, were formerly difficult to resolve because of a poor fossil record and the misidentification of some finds. It weighs 84–140 g (2.9–5.0 oz).
Plumage
Breeding adult and juvenile are the only plumages likely to be encountered in North America. Breeding adult (definitive alternate plumage) has a white forehead, a black cap, a mid-grey mantle with darkish grey underparts, and white rump and tail. Its underwing is whitish, with dark-tipped primaries and a diagnostic dark bar on the secondaries. Whereas the forehead bar disappears in winter, the dark secondary bar remains.
Lee described juveniles with a white collar, extensive white forehead, white underparts, and gray tail with white outer web of outer tail-feather. They usually do not have clear dark bar on their secondaries.
thumb|323x323px|Adult Onychoprion aleuticus in breeding plumage in Nome, Alaska.
Molt strategies are very poorly known in the Aleutian tern but are presumably similar to those of other medium-sized, northern-breeding, migratory terns, such as the much better-studied common tern. The Aleutian tern exhibits a Complex Alternate Strategy:
Distribution and habitat
left|thumb|342x342px|Map of the Chukchi Sea (Western Alaska).
During breeding season, the colonies gather along Pacific coastlines of Alaska and Russia. More precisely, breeding colonies have been located along the coast of the Chukchi Sea (western Alaska), on the Seward Peninsula and Yukon-Kuskokwim River Delta along the Alaska Peninsula, in the Aleutian Islands, in the Kodiak Archipelago, on the Kenai Peninsula and Copper River delta, and along the Gulf of Alaska. However, within the last decade, there have been reports of colony declines and disappearances at individual sites in Alaska.
This species is strongly migratory, and although the wintering range is poorly known, it is believed to lie off Indonesia and Malaysia. The regular appearance of Aleutian terns in fall off Hong Kong suggests one possible route for southbound migrants. Only a very small number of Charadriiformes breeding in Alaska appear to have a connection to East Asia. In December 2017, they were spotted for the first time on Australia's eastern coast.
Habitat
The Aleutian tern usually lives in partially vegetated sandy beaches and grassy meadows, mossy boglands and marshes, either on isolated rocky islands or along coasts, often near river mouths. The Aleutian tern is pelagic when it is not breeding.
Status
Trends in the Aleutian tern distribution indicate that overall, the species seems to undergo rapid declines over generations. Indeed, the numbers at known colonies in Alaska have declined 8.1% annually since 1960, which corresponds to 92.9% over three generations, with large colonies experiencing greater declines than small colonies. Precise factors of declines are unclear but likely include habitat modification, predation, egg harvesting and human disturbance. Aleutian terns are very sensitive to disturbance at colonies and may seasonally or even permanently abandon their colonies in response to human disturbance. The lack of data on breeding biology and ecological behaviours limits the development of conservation actions for the species. Further research should focus on the monitoring of populations and colonies in Alaska, Russia and South Pacific regions and the understanding of the main causes of the recent decline of the species.
Behaviour
Breeding and Parental Behaviour
Aleutian terns breed in colonies, and are site-faithful if their habitat is sufficiently stable. Pairs form on the breeding area, shortly after arrival. The pairs build the nest during the last half of May and first half of June, shortly before egg-laying. and range from length. Their color ranges from a clay/olive green to a honey yellow, After a three-week incubation period, the eggs hatch from early June until late July. Several days after hatching, young birds move to taller vegetation before moving with adults to staging areas along coast (semiprecocial juvenils). After 4–5 weeks, the chicks start fledging. It sometimes nests among Arctic terns, which, like most white terns, are fiercely defensive of their nest and young and will attack large predators. The Aleutian tern, however, is not aggressive in defence of its nests or young.
Courtship
The courtship display of the Aleutian tern has never been thoroughly described, but all terns are thought to display the same strategies of courtship: ceremonial "fish flight," "low flight," "high flight," and ground "parade". observed Aleutian terns that were courting on surrounding beaches of the Copper River Delta (Alaska), away from colonies.
Diet and Foraging
Aleutian terns primarily feed on small fish, but their diet also includes crustaceans, insects and zooplankton. They forage mostly by flying, hovering low over water and swooping down or surface-dipping into the water to take their food from the surface. Only contact- and surface-dipping have been observed, even in places where other species have been seen plunge-diving. Occasionally adults and fledgling juveniles catch insects by hawking over freshwater ponds. Individuals occasionally attempt to steal fish from other adults bringing fish to chicks. Colony size usually ranges from 4–150 pairs, but up to 700 pairs on Sakhalin Island (Siberia). The Aleutian terns frequently nests with Arctic Terns in Alaska and Common Terns in Siberia. It also happens that colonies of Mew Gull (Larus canus) are close to the Aleutian tern nests.
Less aggressive than Arctic terns; Onychoprion aleuticus is frequently chased at colonies and often excluded from mixed-species foraging areas by Arctic terns. however, breeding success in the species is lower in the absence of Arctic terns as they often rely on their aggressivity to chase potential predators from the colony.