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Stercorarius antarcticus (Lesson, 1831)

provided by EG-BAMM, Matthias Kopp

Description

The Brown Skua is the biggest of all southern hemisphere skua species. The current phylogenetic classification identifies three families in the order Lariformes in the high latitudes of the southern hemisphere: gulls, terns and skuas. Herein the skuas (Stercorariidae) for their part occur with three species: S. maccormicki, S. chilensis and S. antarcticus. The latter of which is subdivided into three sub-species: S. a. antarcticus, S. a. lonnbergi and S. a. hamiltoni (Ritz et al. 2008).

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Species details

Photos

  • Stercorarius antarcticus - Brown skua - EGBAMM - Matthias Kopp
  • Stercorarius antarcticus - A pair of brown skua vocalizing in King George Island - EGBAMM - Matthias Kopp
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Species distribution

Brown Skuas have a huge area of distribution, which is circumpolar at high latitudes of the southern hemisphere. Their prevailing breeding sites are at Sub-Antarctic Islands within the Antarctic Convergence. But they also breed at islands near New Zealand, representing the northernmost breeding areas. On the other extreme side, Brown Skuas are breeding on islands near the Antarctic Peninsula (not further south than Anvers Island archipelago 64°46′ S 64°03′ W) (Ritz et al. 2006). An outlying, but however constant, breeding record (the only published one) is a single Brown Skua female which breeds at the western edge of the Ross sea (Port Martin) at the Antarctic Continent/ East Antarctic. That female breeds in a mixed pair constellation with a south polar skua (Barbraud et al. 1999). The zone of sympatric occurrence with C. maccormicki: In the breeding range of the Brown Skua, there is a zone characterized by an alongside occurrence with another skua species, the South Polar Skua S. maccormicki. A 500km wide hybrid zone is located in the West Antarctic, in the area of the Antarctic Peninsula representing the southern and northern extremes of the breeding ranges of Brown- and South Polar Skuas, respectively. The zone ranges from the South Orkney Islands (60°45’S) in the north to the Anvers Island archipelago (about 65°S) in the south. Pairs formed by S. maccormicki x S. maccormicki and S. a. lonnbergi x S. a. lonnbergi are dominating the numbers and are occurring side by side. The characteristic feature of the hybrid zone is the occurrence of mixed species pairs, formed by S. maccormicki and S. a. lonnbergi. The percentage of such mixed species pairs varies within that zone and is highest in the northern part; like on Fildes Peninsula/ King George Island were 12 % of all breeding pairs are mixed pairs. These pairs are always formed by a South Polar Skua male and a Brown Skua female. The offspring of mixed species pairs is fertile (Ritz et al. 2006). The hybrid zone has been intensively studied; amongst others in terms of species foraging ecology. In the area of sympatric occurrence, a foraging pattern different from the pattern of circumpolar allopatric occurrence has evolved which is due to species competition. However, Brown Skuas are dominating all terrestrial resources over South Polar Skuas by outcompeting them – so a change in the foraging behavior can be observed only in the South Polar Skua, which is forced to prey on marine resources.

Within their huge breeding range, Brown Skuas experience a wide variety of climatic conditions, having consequences for the non-breeding period distribution. Brown Skuas do not necessarily migrate big distances but, typically for pelagic birds, they usually do leave the breeding grounds and return to land only for breeding. How far they move depends on the breeding area, whereas the northern breeding populations stay close to the breeding ground and the southern populations migrate further north. Herein the migration pattern and wintering areas are largely unknown (Olsen and Larsson 1997). Museum skins and colour slights of C. a. lonnbergi-specimens found/sighted at the northern hemisphere appeared to be misidentified being rather South Polar Skuas (Devillers 1977). Phillips et al. (2007) firstly used tracking devices for getting insight into migration patterns and wintering areas of that species. The study showed that Brown Skuas breeding at Bird Island/ South Georgia are leaving the breeding area and wintering over deep oceanic water in the Argentine basin between the Antarctic Polar Front and the northern sub-tropical-front.

Nests are built at places which are free of snow in early spring compared to the surrounding facilitated by landscape features e.g. small hills or moraines. As nest material they use lichens, grasses and/or mosses depending on the local availability. Like other skua species, the Brown Skua also occupies a territory around the nest which is defended against each intruder by the territory owners, and most vigorously against conspecifics (but also against scientists and unsuspecting tourists) (Trivelpiece et al. 1980). Herein, the size of the territory varies a lot and depends amongst others factors on the breeding location and landscape features. The territory may firstly be: a pure nest territory, defended to protect the brood or secondly an all-purpose territory which includes beneath the nest as well prey organisms (like a penguin rookery). It is a brood defense but also for all feeding needs within the whole breeding season (Hahn and Peter 2003).

Brown Skuas are feed on eggs and penguin chicks, burrowing petrels and on seal carcasses or placentae (Reinhardt et al. 2000) so they have a predominantly predatory lifestyle, with the focus on terrestrial prey. Herein individuals could be highly specialized depending on a single food source (Hahn and Peter 2003). Offshore foraging is believed to be performed to a minor extent (Hahn and Bauer 2008).
The occupancy of feeding territories is like in the other skua species as well in Brown Skuas very common (Trivelpiece et al. 1980, Maxson and Bernstein 1982, Moncorps et al. 1998). These territories were defended against invaders and usually guarantee sufficient amount of food within the whole breeding season.

Breeding starts in November and spans until the beginning of December. Brown Skuas which breed in more southern sites, the clutch initiation happens in some seasons even later depending primarily on conditions in the territory; Skuas need snow free places for egg laying. Brown Skuas breed in loose colonies, resulting in the nests being quite separated from each other (highest recorded density 132 pairs/km2 (Phillips et al. 2004)). A normal pair formation with one male and one female is usual but cooperative breeding in trios or in bigger groups is regionally not seldom (e.g. Young 1998). Brown Skuas lay up to two well camouflaged brown-green-black speckled eggs which they incubate for 28 to 32 days. The chicks down feathers are uniformly colored and light grey to brown. They are able to leave the nest after a few days but stay always in their parent’s territory and fledge after around 50 days. After fledging they depend for about one more month on their parents, mainly for food provision. Brown Skuas reach sexual maturity at 5+ years but usually after 8 years (Young 1998). The mortality rate of chicks is high, attributed amongst others to brood loss because of predation by other Skuas, starvation or post fledging cases of death. Brown Skuas show a fidelity to the place of birth (Young 1998). They tend to return to this place each year for breeding activities throughout their entire lives; predominantly pairs even occupy the same territory.

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