South Polar Skua

Cf. Brown Skua.  Throughout the southern oceans, South Polar Skua overlaps with Brown Skua—which itself comprises four forms that are possibly separate species.  South Polar is highly variable and each form of Brown has some internal variation.  Adults have some plumage tendencies that can be useful, especially when they are consistent with geographical probability, but immatures are variable and similar enough to be effectively unidentifiable in most cases—except by pure application of geographical probability.

Three of the four Brown Skua forms are localized: “Falkland” (antarcticus) around the Falkland Islands, southern Argentina, and elsewhere in the southwestern Atlantic north to southern Brazil; “Subtropical” (hamiltoni) around Tristan da Cunha, Gough, Amsterdam, and St. Paul Islands; and “Chatham” (ssp. nova) around the Chatham, Snares, and possibly other islands south of New Zealand.  In each of those areas, especially adjacent to the breeding grounds, the local form presumably predominates most of the time.

Around the more southerly islands, and generally at mid-latitudes of the Southern Ocean, “Subantarctic” (lonnbergi) predominates.  Farther south, near Antarctica, South Polar predominates—but they overlap along the Antarctic Peninsula, where they have been found to interbreed at least occasionally.  Both “Subantarctic” and South Polar range widely at sea, and they regularly overlap across most of the southern oceans.

South Polars vary from mostly pale-brown to mostly blackish.  The palest morphs are distinctive and do not resemble any form of Brown Skua.  Dark morph South Polars (including immatures) can be either mostly blackish-brown or a warmer shade of brown.  Compared to similar forms of Brown Skua, the typical plumage features of dark adult South Polars are: (1) mostly uniform upperparts with very little pale streaking; (2) often with blond-streaked feathering on the head, either limited to the nape, or forming a full collar, or a pale hood; but (3) sometimes with a dark-brown hood, contrasting with paller or lightly blond-streaked feathering on the neck, extending onto the upperparts and underparts.  Subadult South Polars often show more streaking on the upperparts than adults do, which can cause confusion with Brown.

Brown Skuas are difficult to generalize.  The widespread “Subantarctic” form, which overlaps widely with South Polar, has fairly consistent features.  Immatures are usually all-dark with tones that vary from warm to cold to blackish.  Adults usually have fairly prominent pale streaking on the back (including the scapulars), sometimes extending down to the rump and up to the nape.  “Subantarctic” often shows a contrastingly darker hood.

Among the three more localized forms of Brown Skua, “Falkland” is fairly distinctive.  Adults are typically a medium-dark shade of brown, with extensive blond streaking that is especially dense on the neck and usually extends across most of the upperparts and underparts.  Many have a darker, mostly unstreaked crown, forecrown, or face.  (Adult “Falkland” typically has approximately the same pattern as the North Atlantic Great Skua, but with blonder, less cinnamon streaks.)  Immatures are generally unstreaked, usually darker overall than adults, but variable in tone. 

“Subtropical” is less distinctive, and generally resembles the subadults of other forms, including South Polar.  It typically has pale streaks sparsely but evenly distributed across its upperparts, including the wings, and often has a somewhat paler belly.  Like “Subantarctic”, “Subtropical” often shows a dark hood.

“Chatham” is usually unstreaked, and varies from blackish to a medium-pale brown—much like an intermediate South Polar.  “Chatham” often has a pale nape—much like many dark morph South Polars.