The Prime Meridian and the opposite 180th meridian (at 180° longitude), which the International Date Line generally follows, form a great circle that divides the Earth into the Eastern and Western Hemispheres.

Unlike the parallels of latitude, which are defined by the rotational axis of the Earth (the poles being 90° and the Equator 0°), the Prime Meridian is arbitrary. By international convention, the modern Prime Meridian is one passing through Greenwich, London, United Kingdom, known as the International Meridian or Greenwich Meridian. Historically, various meridians have been used, including four different ones through Greenwich

The name skua comes from Faroese skúgvur (Stercorarius skua), and the island of Skúvoy is renowned for its colony of that bird. Jaeger is derived from the German word Jäger, meaning hunter.

Skuas nest on the ground in temperate and Arctic regions and are long-distance migrants.

Outside the breeding season, skuas take fish, offal and carrion. Many are partial kleptoparasites, chasing gulls, terns and other seabirds to steal their catches; the larger species, such as Great Skua also regularly kill and eat adult birds, such as puffins and gulls, and have been recorded as killing birds of the size of a Grey Heron. On the breeding grounds they commonly eat lemmings, and the eggs and young of other birds

They are in general medium to large birds, typically with grey or brown plumage, often with white markings on the wings. They have longish bills with a hooked tip, and webbed feet with sharp claws. They look like large dark gulls, but have a fleshy cere above the upper mandible. They are strong, acrobatic fliers.

Taxonomy
Skuas are related to gulls, waders, auks and skimmers. In the three smaller species (all Holarctic), breeding adults have the two central tail feathers obviously elongated and at least some adults have white on the underparts and pale yellow on the neck, characteristics that the larger species (all native to the Southern Hemisphere except for the Great Skua) do not share. Therefore the skuas are often split into two genera with only the smaller species retained in Stercorarius, and the large species placed in Catharacta. However, there is no genetic basis for this separation. The Pomarine and Great Skuas' mitochondrial DNA (which is inherited from the mother only) is in fact more closely related to each other than it is to either Arctic or Long-tailed Skuas, or to the Southern Hemisphere species. Thus, hybridization must have played a considerable role in the evolution of the diversity of Northern Hemisphere skuas

Species

Long-tailed Skua or Long-tailed Jaeger, Stercorarius longicaudus
Arctic Skua or Parasitic Jaeger, Stercorarius parasiticus
Pomarine Skua or Pomarine Jaeger, Stercorarius pomarinus
Chilean Skua, Stercorarius chilensis
South Polar Skua, Stercorarius maccormicki
Brown Skua, Stercorarius antarctica
Great Skua Stercorarius skua

Both of the sooties have distinctive black plumage over the head, wings and bellies. The Dark-mantled Sooty has a dark back and mantle as well, whereas the Light-mantled Sooty has an ashy-grey mantle, back and rump. The two species can also be told apart by the narrow yellow line on the Dark-mantled's bill. Despite the differences between the two species they can be hard to tell apart at sea, especially in poor light. Both species have a white incomplete eye-ring, dark bills and grey feet. They are among the smallest albatrosses, with wingspans of 200 cm (83 in) and are very narrow as well. The Light-mantled, at 2.5-3.7 kg (5.5-8.2 lbs) and sometimes to 4.6 kg (10.1 lbs), is larger than the Dark-mantled, at 2.4-2.7 kg (5.4-6 lbs). Unique amongst the albatrosses they have long stiff wedge shaped tails, the purpose of which is unclear but seems to be related to their ability to dive for food

The sooties, like most seabirds, are colonial, although sooties are less colonial than the other species of albatross. In fact, on some breeding islands (like Tristan da Cunha) sooties nest in very small groups or clusters of two to five nests, and the Light-mantled Sooties will even nest singly. This is in part due to the influence of humans, and in part due to their tendency to nest on cliffs, unlike the flatter ground preferred by other albatrosses. Sooties build cone shaped nests and lay a single egg. Eggs are incubated for 70 days, by both parents, the male taking the first stint after laying (lasting 11 days) thereafter both parents taking it in turns of 7 days. After hatching the chick is brooded for 20 days until it is able to thermoregulate on its own, after which both parents undertake the task of feeding it, on average bringing food to the chick every three days. The chick is fed for about 160 days, until it is able to fledge. There is no parental care after fledging. Sooties are able to complete a breeding cycle in under a year, but do not breed in consecutive years, instead taking a year off and returning to breed every two years. Around 22% of Dark-mantled Sooties survive until adulthood (there are no figures for Light-mantled). Both species return to the breeding colony after 7-10 years of fledging, and begin to breed a few years later.

Dark-mantled Sooty Albatrosses nest on islands in the South Atlantic (Tristan da Cunha and Gough Island) and islands in the South Indian Ocean (the Crozet Islands to Kerguelen Island). At sea they forage from South America to Australia, with a few records of birds reaching New Zealand. The Light-mantled Sooty Albatross has a wider distribution, nesting on South Georgia in the Atlantic, many of the same islands in the Indian Ocean, Macquarie Island and New Zealand's sub-Antarctic islands. At sea it forages further south than the Dark-mantled to Antarctica, and around the Southern Ocean as far north as Chile, Tasmania and South Africa. At sea they often eat more fish as opposed to squid than other albatross species, and the sooties also readily take carrion and particularly other seabirds. They also are the deepest diving of the albatross, often diving to 5m and once being recorded as deep as 12m.

The two species face similar threats, introduced species that attack chicks and eggs, and falling victim to long-line fisheries. These threats, combined with some historic harvesting of the birds and chicks, has led to an estimated 75% population decline in the Dark-mantled Sooty Albatross over the last 90 years (to around 40,000 birds), which has led to it being listed as an endangered species by the IUCN. The Light-mantled Sooty Albatross has not been as badly affected, and is considered near-threatened.