HBW 3 - Species accounts: Brown Skua (Catharacta antarctica)
3. Brown Skua
Other common names: Southern (Great)/Subantarctic/Falkland Skua; Lönnberg's Skua (lonnbergi)
Taxonomy. Lestris antarcticus Lesson, 1831, Falkland Islands.
Sometimes considered conspecific with all three congeners, and all four probably form superspecies; genus has frequently been merged into Stercorarius. Often considered conspecific with C. skua, but differs more from C. skua than from C. maccormicki and C. chilensis (see page 552). Occasional hybridization reported with C. maccormicki and with C. chilensis, but hybrids very rare. Race hamiltoni differs only slightly from nominate antarctica. Race lonnbergi often considered a separate species. Proposed races clarkei and intercedens invalid, and nowadays included in lonnbergi. Name lonnbergi sometimes erroneously spelt loennbergi, but removal of Scandinavian umlaut does not justify addition of "e". Three subspecies currently recognized.
Subspecies and Distribution.
C. a. antarctica (Lesson, 1831) - Falkland Is and SE Argentina; winters SE South America.
C. a. hamiltoni (Hagen, 1952) - Tristan da Cunha and Gough I; winters near breeding areas.
C. a. lonnbergi Mathews, 1912 - Antarctic Peninsula, subantarctic islands of Atlantic, Indian and Pacific Oceans; winters near or slightly dispersed from breeding areas.
Descriptive notes. 52-64 cm; 1200-2100 g; wingspan 126-160 cm. Brown plumage, but highly variable from uniformly dark brown to light brown with pale flecking; relatively heavy bill, and bulky body. Lacks distinct capped appearance of C. skua and C. chilensis; has dark lesser and median underwing-coverts. In all races, juveniles darker and more uniform in colour with more chestnut in body plumage and with less distinct white wing flashes than in adults. Race hamiltoni has less white or golden flecking, and little rufous on body, so appears uniformly dark with few markings on head and little contrast between head and collar; larger than nominate race in all linear measurements. Race lonnbergi much larger than either of the other subspecies, especially around New Zealand, showing less pale flecking than nominate race but more than hamiltoni.
Habitat. Marine, or on and around subantarctic islands populated by burrow-nesting seabirds or penguins. Nests mainly on islands free from human disturbance, though can be attracted to scientific bases that provide scavenging opportunities. May use territory just for breeding, or for breeding and all feeding needs, or may hold separate feeding territories. Breeding territories may be on grass, gravel or bare rock.
Food and Feeding. Highly predatory, feeding mainly on burrow-nesting prions, gadfly-petrels, storm-petrels, shearwaters and penguins, and scavenging on penguin eggs, chicks and adults, afterbirth and carcasses of seals. Will also scavenge around fishing boats and ships, and feed at sea, but some populations are essentially resident with seabird prey available all year round, and so rarely venture to sea. Burrow-nesting seabirds are mainly caught at night on the ground as they seek burrows, and remains are often put onto a "midden", a characteristic prey storage site within the territory.
Breeding. Starts Oct-Nov. Loosely colonial, highly territorial. May breed in trios or larger groups, with several males but only one female per territory; communal breeding associated with all year round residency, and may occur in more than 30% of territories at some New Zealand colonies. Nest scrape unlined or scantily lined with dead grass. Usually 2 eggs, but only 1 laid by some inexperienced birds; incubation 28-32 days; chick has uniformly coloured light pinkish brown down; leaves nest 1-2 days after hatching; fledging 40-50 days. Breeding success usually high. Sexual maturity at 6+ years; adult survival 0·9-0·96.
Movements. Some birds at colonies in New Zealand, Tristan da Cunha and Gough I remain resident throughout year, with others dispersing to sea locally. In harsher environments all birds leave colony during winter and disperse at sea. Probably does not normally reach Northern Hemisphere, and several earlier northern records now reassigned to C. maccormicki. Usually breeds close to birthplace, facilitating racial divergences.
Status and Conservation. Not globally threatened. Total population c. 13,000-14,000 pairs: 3000-5000 pairs of nominate race, in Falklands and Argentina; 2500 pairs of hamiltoni, predominantly on Gough I with c. 200 pairs on Tristan da Cunha; and c. 7000 pairs of lonnbergi, with 1000-2000 pairs in Kerguelen Is, 1000 pairs on South Georgia, 550 pairs on Macquarie I, 460 pairs on Marion and Prince Edward Is, 400-1000 pairs in Crozet Is, 420 pairs in South Shetlands, 300 pairs in South Orkneys, 190 pairs on Elephant I, 100's of pairs at South Sandwich Is and Heard I, 150 pairs on Antarctic Peninsula, 105 pairs in Chatham Is, 100 pairs on Auckland I, c. 50 pairs each on Snares, Campbell and Antipodes Is, and a few pairs on Stewart I, Bouvetoya and Amsterdam I. Numbers have been greatly reduced by human persecution in Chathams and Tristan da Cunha, and possibly in parts of Falklands, but increases and range expansion on Antarctic Peninsula may be partly due to provision of increased feeding opportunities at Antarctic research bases, with skuas benefiting from refuse and disturbed seabird colonies. Many populations would be adversely affected by declines in numbers of prions, gadfly-petrels and storm-petrels breeding on islands, but populations of those long-lived birds are normally very stable.
Bibliography. Ali & Ripley (1982), Barré (1976), Barton (1982), Beaman (1994), Blake (1977), Bourne & Curtis (1994), Bruemmer (1993), Burton (1968a, 1968b), Court & Davis (1990), Croxall, McInnes & Prince (1984), Croxall, Prince et al. (1984), Devillers (1977c, 1978), Fraser (1984), Hagen (1952), Hamilton (1934), Hemmings (1989, 1990), Johnson (1965), Jones & Skira (1979), Jouventin (1994), Lamey (1992, 1995), Langrand (1990), Marchant & Higgins (1996), Millar, Anthony et al. (1994), Millar, Lambert et al. (1992), Milon et al. (1973), Moors (1980), Osborne, B.C. (1985), Parmalee (1985), Parmalee & Pietz (1987), Peter et al. (1990), Pietz (1985, 1986, 1987), Richardson (1984), Ryan & Moloney (1991), Sick (1985c, 1993), de Silva (1991), Sinclair (1980), Skira (1984), Stonehouse (1956), Swales (1965), Trivelpiece & Volkman (1982), Trivelpiece et al. (1980), Urban et al. (1986), Williams (1980a, 1980b, 1980c), Woehler (1991), Young (1977, 1978).