Stercorariidae – Skuas or Jaegers

The Stercorariidae (Skuas or jaegers) are a group of seabirds in one genus, Stercorarius. The family is related to the waders, but merits its own family. The order Charadriiformes also includes Pluvianellidae (Magellanic Plover), Chionidae  (Sheathbills), Burhinidae (Thick-knees & Stone Curlews), Pluvianidae (Egyptian Plover), Ibidorhynchidae (Ibisbill), Haematopodidae (Oystercatchers), Recurvirostridae (Stilts & Avocets), Charadriidae (Plovers & Lapwings), Pedionomidae (Plains Wanderer), Thinocoridae (Seedsnipes), Rostratulidae (Painted Snipe), Jacanidae (Jacanas), Scolopacidae (Sandpipers), Dromadidae (Crab-Plover), Turnicidae (Buttonquails), Glareolidae (Coursers & Pratincoles), Alcidae (Auks) and Laridae (Gulls, Terns & Skimmers). Glareolidae contains two distinct groups, the pratincoles and the coursers.

They nest on the ground in temperate and Arctic regions, and are long-distance migrants. They have even been sighted at the South Pole.

Great Skua Stercorarius skua – ©Mike Pennington CC BY-SA 2.0 via Wikimedia Commons

Outside the breeding season, skuas take fish, offal, and carrion. Many are partial kleptoparasites, comprising up to 95% of the feeding methods of wintering birds, by chasing gulls, terns and other seabirds to steal their catches, regardless of the size of the species attacked (up to three times heavier than the attacking skua). The larger species, such as the Great Skua, also regularly kill and eat adult birds, such as puffins and gulls, and have been recorded as killing birds as large as a Grey Heron. On the breeding grounds, the three, more slender northern breeding species commonly eat lemmings. Those species that breed in the southern oceans largely feed on fish that can be caught near their colonies. The eggs and young of other birds are an important food source for most skua species during the nesting season.

Pomarine Skua (Jaeger) Stercorarius pomarinus – ©Dubi Shapiro

In the southern oceans and Antarctica region, some skua species (especially the South Polar Skua) will readily scavenge the carcasses at breeding colonies of penguins and pinnipeds, sometimes taking live penguin chicks. In these areas, skuas seem to defer to the considerably larger giant petrels.

Arctic Skua (Parasitic Jaeger) Stercorarius parasiticus – ©Dubi Shapiro

They are medium to large birds, typically with grey or brown plumage, often with white markings on the wings. The skuas range in size from the Long-tailed Skua Stercorarius longicauda, at 310 grams, to the Brown Skua Stercorarius antarcticus, at 1.63kg. On average, a skua is about 56cm long, and 121cm wingspan. 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.

Long-tailed Skua (Jaeger) Stercorarius longicaudus – ©Smudge 9000 BY-SA 2.0 via Wikimedia Commons

The skuas are strong, acrobatic fliers. They are generally aggressive in disposition. Potential predators who go near their nest will be quickly dived at by the parent bird, which usually targets the head of the intruder; a practice known as ‘dive bombing’.

South Polar Skua Stercorarius maccormicki – ©Dominic Sherony CC BY-SA 2.0 via Wikimedia Commons

Skuas are related to gulls, waders, auks, and skimmers. In the three smaller species, all nesting exclusively in the 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. These characteristics are not shared by the larger species, all native to the Southern Hemisphere except for the Great Skua. 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, based on genetics, behaviour, and feather lice, the overall relationship among the species is best expressed by placing all in a single genus. The Pomarine Skua’s and Great Skuas’ mitochondrial DNA (inherited from the mother) is in fact more closely related to each other than it is to either Arctic Skua or Long-tailed Skua, or to the Southern Hemisphere species. Thus, hybridisation must have played a considerable role in the evolution of the diversity of Northern Hemisphere skuas.