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Brown Skua (Catharacta Antarctica) on Sablesable Island,Island, Novanova Scotiascotia

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Brown Skua (Catharacta Antarctica) on Sablesable Island,Island, Novanova Scotiascotia A possible Brown Skua (Catharacta antarctica) on SableSable Island,Island, NovaNova ScotiaScotia Ian McLaren mer (June–August) specimens in Canadian articles and material posted on the World- museums collected prior to 1980 in At- WideWeb, has indicated to us that the Sable Biology Department lantic Canada have been critically identified Island bird strongly resembled Brown Skua, as South Polar Skuas (Michel Gosselin, Na- although deficiencies in this information Dalhousie University tional Museum of Canada, pers. comm.; lead us to leave it formally unidentified. University of Western Ontario [see below]). Halifax, Nova Scotia B3H 4J1 All Great Skua specimens have come from BACKGROUND ON CONFIRMED AND the Grand Banks: an immature in late Au- POSSIBLE BROWN SKUAS IN THE ([email protected]) gust 1961 (D. McAlpine, New Brunswick NORTH ATLANTIC Museum, pers. comm.) and several adults Even after Chilean (chilensis) and South in September (M. Gosselin, pers. comm.). Polar Skuas were recognized as distinct Zoe Lucas The Great Skuas listed for Sable Island by species, the three other skua taxa in the P.O. Box 64, Halifax CRO McLaren (1980) were not critically identi- Southern Hemisphere continued to be clas- fied. Nova Scotian observers have attempt- sified as forms of Great Skua (e.g., Devil- Halifax, Nova Scotia B3J 2L4 ed to distinguish the two species since at liers 1977). Brown Skua, however, has be- least the early 1980s. Since 1992, Lucas has come generally recognized as a distinct studied six live Great Skuas on Sable Island species with three subspecies (e.g., Olsen at fairly close range, and has found five and Larsson 1997): Falkland Skua, nomi- ABSTRACT tideline carcasses (two salvageable as speci- nate antarctica, of the Falklands Islands and This paper considers of a skua pho- mens), all between September and January. southern Argentina; Tristan Skua, hamil- tographed 13 May 1996 on Sable Island, These were identified by their bulk, pres- toni, of Tristan da Cunha and Gough Is- Nova Scotia, which the authors identify as a ence of distinctly cinnamon or rufous un- lands in the South Atlantic; and Subantarc- probable Brown Skua (Catharacta antarctica derparts, and pale or rufous streaks on tic Skua, lonnbergi, of the Antarctic [Stercorarius antarctica]), a species not yet backs. She has also found two dead South Peninsula and islands from South Georgia confirmed in the western North Atlantic Polar Skuas and closely observed a live one, across the Southern Ocean to New Zealand. but recently documented by molecular evi- all in late May. These were typical light- or The future may bring further rearrange- dence in the eastern North Atlantic. intermediate-morph birds, dark-backed, ments in systematics of the southern skuas evenly brownish gray below, and with pale (Blechschmidt et al. 1993), and the evolu- FIELD ENCOUNTER nuchal areas. McLaren has similarly identi- tionary relationship between Great Skua In mid-day on 13 May 1996, Lucas found a fied South Polar Skuas off Seal Island in Au- and Pomarine Jaeger (Stercorarius pomari- skua sitting on the beach at Sable Island, a gust1983, Great Skuas during a pelagic trip nus) is apparently close enough (Braun and 45-km long crescentic sandbar some 160 off Halifax, Nova Scotia, in October 1990, Brumfield 1998) that the American Or- km from the nearest mainland of Nova Sco- and both species during a pelagic trip off nithologists’ Union (Banks 2000) has re- tia. This skua was approachable, to within Halifax in late September 1990, and on a re- cently merged genus Catharacta into genus about 5 m, allowing two photographs to be search cruise on the Scotian Shelf in late Stercorarius, although this reclassification taken in full sunlight before it stood up, September 1997. has not been widely accepted in the world took off, circled and flew away. Because of Because the photographs of the May ornithological community. Because Sterco- its overall bulk and dark ventral plumage— 1996 bird on Sable Island do not easily ac- rarius refers to the jaegers (called skuas in with what appeared to be warmer tones cord with either of the two regularly occur- many countries), we make use of Catharac- than had been observed on South Polar ring skua species, we have been led to con- ta below to limit our discussion to the larg- Skuas (Stercorarius maccormicki)—it was sider other possibilities. For several er taxa. initially identified in the field as a Great decades, skuas outwardly similar to the Brown Skua has long been known to Skua (S. skua). Sable Island bird have led observers in the wander north of the equator in the Indian North Atlantic to suspect that a third Ocean (Higgins and Davies 1996), but in DISCUSSION species, Brown Skua (Catharacta antarctica the western North Atlantic, our lack of SKUAS IN CANADIAN [Stercorarius antarcticus]) wanders to the specimens and limited criteria for the dis- ATLANTIC WATERS North Atlantic. This suspicion has been criminating Catharacta skuas within the Balch (1981) focussed attention of birders supported by recent molecular evidence antarctica/lonnbergi/hamiltoni complex have on summer presence of South Polar Skuas from the United Kingdom (Scott 2002, thwarted identification of several suspected in North Atlantic waters. Although earlier Moon and Carrington 2002). Information Brown Skuas. Single skuas photographed at literature indicates otherwise, three sum- on skua identification, in both published the Hudson Canyon off New York/New Jer- 622 NORTH AMERICAN BIRDS BROWN SKUA sey on 28 May 1977 (photograph in Ameri- can Birds 32: 1108; Balch 1981) and 29 May 1987 (Brady 1988) were thought to be Brown Skuas, the former a probable Sub- antarctic Skua, while the latter bird was identified by world authorities at the time as a Tristan Skua. Off North Carolina, records from 22 May 1992 (Brinkley 1994) and 29 May 1993 (M. O’Brien, pers. comm.; see Figures 6, 7) perhaps pertain to Sub- antarctic Skua. Several more recent candi- dates from the Gulf Stream off North Car- olina have also been posted to websites (<http://www.patteson.com/skuas/skuas.ht m>, and <http://www.magikcircle.com/ birds/image.asp?title_id=457>). At present, no North American bird records committee has admitted any taxon of skua other than maccormicki and skua to its official list, al- though the North Carolina Bird Records Committee has considered reports of Figure 1. Skua on Sable Island, Nova Scotia, 13 May 1996. Photograph by Zoe Lucas. Brown Skua (H. LeGrand, pers. comm.). In the eastern North Atlantic, Brown Numbers from that region have been pho- servations or photographs, morphometric Skua has now been confirmed twice: on the tographically identified as South Polar or genetic determination from birds in the Scilly Islands, 7 October 2001 (Scott 2002) Skuas (Newell et al. 1997), but others have hand remains necessary. The May 1996 bird and in Glamorgan, 3 February 2002 (Moon raised the possibility of Brown Skuas on Sable Island is therefore described here and Carrington 2002). The identification of among some of the more robust, intermedi- as a plausible but unconfirmed record, in these skuas from mitochondrial DNA did ate- and dark-plumaged individuals (Jiguet part to advance the question of skua identi- not exclude the possibility of either being a 1997). fication. hybrid with paternal South Polar Skua, but We can no longer assume that all skuas this was deemed very unlikely from popu- in the North Atlantic are either Great or APPEARANCE OF THE lation estimates and morphology (Votier et South Polar, and records committees SABLE ISLAND SKUA al. 2004). There are no other confirmed should now review all past reports of In diagnosing the Sable Island bird (Figure records of Brown Skuas from North Atlantic Catharacta, just as they did in the 1970s, 1, 2), we consulted photographs, illustra- waters, but more may come from the east- when South Polar Skua was accorded tions, and descriptions of the various ern Atlantic Ocean off Senegal, where only species status. Until more is known about species and subspecies of skuas by James Great Skuas had been thought to occur. the identification of Brown Skua from ob- (in Higgins and Davies 1996), Olsen and Larsson (1997), and Shirihai (2002), along with numerous web-posted photographs. • Ageing The reference taxa for the Sable Island skua were naturally Great and South Polar Skuas. At this time of year, a Great Skua born in 1995 would be at least almost one year old, whereas any southern-hemi- sphere skua could be considerably younger. The lack of pale feathering around the bill, the uniformly dark bill (some paleness apparently from light re- flection), and the blackish rather than grayish or mottled tarsi indicate that it is a post-juvenile. No matter what the species, there are no indications that this individual was under one year of age. Beyond that, it is not possible to age this bird on present knowledge, although we suspect that this individual is not in the “definitive” plumage of an adult of any taxon, given its rather uniform dorsal plumage. The bird might therefore best be considered a subadult, an imprecise label but one suited to the uncertainties surrounding skua Figure 1. Skua on Sable Island, Nova Scotia, 13 May 1996. Photograph by Zoe Lucas. plumages (see comments based on molt below). VOLUME 58 (2004) • NUMBER 4 623 feathers (Figure 4) suggests that primaries, secondaries, and median coverts are moder- ately fresh, but some scapular and lesser and greater covert feathers are variably ragged and others fresh. This indicates that the bird had only partially renewed its wing plumage, which would better fit a Southern Hemisphere schedule, although timing of molt is not fully understood, especially among subadult skuas. (Steve N. G. Howell [pers.
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