Letter - British Birds · 2019-11-23 · British Birds101 • August 2008 • 439–441 441...
3
439 © British Birds 101 • August 2008 • 439–441 It is now over a decade since the first of three recent controversial large skuas turned up in Britain, and the debate on their identity con- tinues to ebb and flow. Jiguet (2007) was among the more recent to discuss the three birds – in Dorset, in January 1996 (Millington 2000); on the Isles of Scilly, in October 2001 (Scott 2002); and in Glamorgan, in February 2002 (Moon & Carrington 2002) – and commented that the Scilly and Glamorgan birds resembled South Polar Skua Stercorarius maccormicki. Suspicion that the Scilly and Glamorgan birds were not Great Skuas S. skua, but more likely South Polar Skuas, led to DNA samples being taken. These samples confirmed that the birds were certainly not Great Skuas. However, their initial identification as Brown Skuas S. antarcticus (Votier et al. 2004) was not satis- factory; it was based on a reference DNA sample from a single South Polar Skua used in an earlier study (Cohen et al. 1997) that differed in just a single base from other southern skua DNA samples, including other South Polar Skuas analysed later (Votier et al. 2007). After that initial identification as Brown Skua, other evidence, that might have con- flicted, was quietly overlooked. Who was going to argue with a DNA test? Instead, a ‘wild skua chase’ to test other specimens, some labelled as South Polar Skuas, from the North Atlantic ensued. All turned out to have mitochondrial- DNA sequences that eliminated Great Skua, dif- fered from the South Polar used in Cohen et al. (1997) and were actually more similar to those of the two disputed British birds. This led to further tests of known South Polars, from King George Island, courtesy of Markus Ritz. Subse- quently, Votier et al. (2007) considered that the DNA samples of the two disputed British birds could not distinguish between South Polar and Brown Skua, or any other southern skua for that matter. However, fig. 1 in Votier et al. (2007) showed that the Scilly and Glamorgan birds clustered more closely with South Polar specimens from King George Island than with any other skuas. While the differences involved are small and not sufficient to identify the birds conclusively, at the very least the DNA evidence should not sway the identification away from South Polar. Status of southern skuas in the northern hemisphere To date, there are no confirmed records of Brown Skua in the North Atlantic, and there is only a handful from anywhere else in the northern hemisphere, in the Indian Ocean, though none farther north than Oman (~22°N) (Malling Olsen & Larsson 1997). There is a report of a possible Brown Skua in the North Pacific, among the many South Polars that occur there (Howell 2005), as well as a bird (not accepted) showing the characters of Brown Skua off North Carolina in May 1993 (Mike Tove pers. comm.). A German researcher, Mathias Kopp, fitted Global Location Sensing loggers to 11 breeding Brown Skuas on the Antarctic Peninsula, but none was recorded north of 25°S (Markus Ritz in litt.). Of course, younger birds may well disperse farther north. In contrast, South Polar Skua appears to be a regular migrant to both the North Atlantic and the North Pacific. Biometrics and structure Unfortunately, the Scilly bird was not measured, and the Dorset bird was not trapped, but the measurements of the Glamorgan bird exclude everything apart from ‘Falkland’ Brown Skua S. a. antarcticus and South Polar Skua. ‘Falkland Skua’ has a high mass to wing-area ratio and, with Tristan Skua S. a. hamiltoni, is the least likely long-distance migrant of all the southern skuas (Furness 1987, pp. 34–37). Moreover, Devillers (1977) claimed that a bill to tarsus ratio greater than 0.7 separates a high propor- tion of South Polars from Falkland Skuas (though based on a small sample). The Glamorgan bird has a ratio of at least 0.72. A provisional analysis of bill shape (bill length to depth ratio) of full-grown birds shows a complete separation between (small samples of) Antarctic Peninsula South Polars and Falkland Skuas (Peter Hayman pers. comm. plus my own work based on measurements from photographs). All three of the UK birds were full-grown – the Glamorgan bird was about one year old; the Scilly bird was at least 19 months and probably at least 31 months old; and the Dorset bird was judged to be in its third or fourth calendar-year (Millington 2000; Recent records of southern skuas in Britain Letter
Transcript of Letter - British Birds · 2019-11-23 · British Birds101 • August 2008 • 439–441 441...
439© British Birds 101 • August 2008 • 439–441
It is now over a decade since the first of threerecent controversial large skuas turned up inBritain, and the debate on their identity con-tinues to ebb and flow. Jiguet (2007) was amongthe more recent to discuss the three birds – inDorset, in January 1996 (Millington 2000); onthe Isles of Scilly, in October 2001 (Scott 2002);and in Glamorgan, in February 2002 (Moon &Carrington 2002) – and commented that theScilly and Glamorgan birds resembled SouthPolar Skua Stercorarius maccormicki.
Suspicion that the Scilly and Glamorganbirds were not Great Skuas S. skua, but morelikely South Polar Skuas, led to DNA samplesbeing taken. These samples confirmed that thebirds were certainly not Great Skuas. However,their initial identification as Brown Skuas S. antarcticus (Votier et al. 2004) was not satis-factory; it was based on a reference DNAsample from a single South Polar Skua used inan earlier study (Cohen et al. 1997) that differedin just a single base from other southern skuaDNA samples, including other South PolarSkuas analysed later (Votier et al. 2007).
After that initial identification as BrownSkua, other evidence, that might have con-flicted, was quietly overlooked. Who was goingto argue with a DNA test? Instead, a ‘wild skuachase’ to test other specimens, some labelled asSouth Polar Skuas, from the North Atlanticensued. All turned out to have mitochondrial-DNA sequences that eliminated Great Skua, dif-fered from the South Polar used in Cohen et al.(1997) and were actually more similar to thoseof the two disputed British birds. This led tofurther tests of known South Polars, from KingGeorge Island, courtesy of Markus Ritz. Subse-quently, Votier et al. (2007) considered that theDNA samples of the two disputed British birdscould not distinguish between South Polar andBrown Skua, or any other southern skua forthat matter. However, fig. 1 in Votier et al.(2007) showed that the Scilly and Glamorganbirds clustered more closely with South Polarspecimens from King George Island than withany other skuas. While the differences involvedare small and not sufficient to identify the birdsconclusively, at the very least the DNA evidenceshould not sway the identification away fromSouth Polar.
Status of southern skuas in the northernhemisphereTo date, there are no confirmed records ofBrown Skua in the North Atlantic, and there isonly a handful from anywhere else in thenorthern hemisphere, in the Indian Ocean,though none farther north than Oman (~22°N)(Malling Olsen & Larsson 1997). There is areport of a possible Brown Skua in the NorthPacific, among the many South Polars thatoccur there (Howell 2005), as well as a bird (notaccepted) showing the characters of BrownSkua off North Carolina in May 1993 (MikeTove pers. comm.). A German researcher,Mathias Kopp, fitted Global Location Sensingloggers to 11 breeding Brown Skuas on theAntarctic Peninsula, but none was recordednorth of 25°S (Markus Ritz in litt.). Of course,younger birds may well disperse farther north.In contrast, South Polar Skua appears to be aregular migrant to both the North Atlantic andthe North Pacific.
Biometrics and structureUnfortunately, the Scilly bird was not measured,and the Dorset bird was not trapped, but themeasurements of the Glamorgan bird excludeeverything apart from ‘Falkland’ Brown Skua S. a. antarcticus and South Polar Skua. ‘FalklandSkua’ has a high mass to wing-area ratio and,with Tristan Skua S. a. hamiltoni, is the leastlikely long-distance migrant of all the southernskuas (Furness 1987, pp. 34–37). Moreover,Devillers (1977) claimed that a bill to tarsusratio greater than 0.7 separates a high propor-tion of South Polars from Falkland Skuas(though based on a small sample). The Glamorgan bird has a ratio of at least 0.72.
A provisional analysis of bill shape (billlength to depth ratio) of full-grown birds showsa complete separation between (small samplesof ) Antarctic Peninsula South Polars and Falkland Skuas (Peter Hayman pers. comm.plus my own work based on measurementsfrom photographs). All three of the UK birdswere full-grown – the Glamorgan bird wasabout one year old; the Scilly bird was at least19 months and probably at least 31 months old;and the Dorset bird was judged to be in its thirdor fourth calendar-year (Millington 2000;
Recent records of southern skuas in Britain
Letter
Markus Ritz pers. comm.) – and all three had alength to depth ratio well within the range ofSouth Polar and outside the range of FalklandSkua. This analysis would benefit from a largersample of measurements, and there is also thepossibility that bill shape may be age-related,with younger birds having a more slender bill.
Photographs suggest that South Polar has alonger primary projection than Brown Skua(Steve Votier pers. comm.) although, like all
structural features, this must be assessed withcare. The Scilly skua has three primary tips wellclear of the tail and five primaries visiblebeyond the tertials, thus suggesting South Polarrather than Falkland. Both of these extensionsare notably longer than a series of measure-ments made of specimens of both FalklandSkua and the subantarctic Brown Skua S. a.lonnbergi by Peter Hayman (pers. comm.). Thisfeature is difficult to assess on photographs ofeither the Dorset or the Glamorgan skua, espe-cially as both have worn outer primaries. Wing-width to tail-length ratio can sometimes bejudged from photographs. South Polars have arelatively shorter tail and photographs of theDorset skua fall within the range of South Polar(Peter Hayman pers. comm.).
PlumageThe plumage of both the Glamorgan and theScilly skua is consistent with South Polars thatappear on the west coast of North America(Howell 2004; Steve Howell pers. comm.). Thefine neck-hackles of the Scilly bird are entirelyconsistent with South Polar and unlike BrownSkua. Some images of the Dorset skua exhibit afrosty bloom typical of South Polar Skua,though the photographic record is ambiguous asto its actual plumage tones (Millington 2000).The neck-streaking on this bird is also ratherfine, again indicating South Polar Skua (MarkusRitz pers. comm.). Interestingly, the Scilly bird is
closely matched in appear-ance by a skua taken atYarmouth, Norfolk, inOctober 1869, the specimennow in the Castle Museum,Norwich. This was identifiedby Bill Bourne as a SouthPolar and has since beenconfirmed by DNA as not aGreat Skua (MartynKennedy pers. comm.), butit was not accepted as theformer, perhaps wrongly, onthe grounds of provenance(Bourne & Curtis 1994;Bourne & Lee 1994).
MoultBoth the Dorset and theGlamorgan bird were inwing moult, both havingalmost completed their
440 British Birds 101 • August 2008 • 439–441
LetterD
ave
Hat
ton
Dav
e H
atto
n
221 & 222. The Scilly skua (see text), in care, St Mary’s, Isles of Scilly,October 2001. Plate 222 shows the long primary projection of this
individual, with three primary tips well beyond the tail tip; this supports its identification as South Polar Skua Stercorarius maccormicki rather
than Brown Skua S. antarcticus.
441British Birds 101 • August 2008 • 439–441
LettersLetter
primary moult. Jiguet (2007) claimed that thestate of moult of both birds eliminates SouthPolar Skua. Although there are few data onSouth Polar Skua moult after October (e.g.Howell 2004), of two immatures in the BallenyIslands (66°50’S 163°30’E) in February, one wasbeginning and the other finishing its primarymoult (James 1996) and therefore consistentwith the Dorset and Glamorgan birds. Morerecently, Howell (2008) also showed that theprimary moult stage of the Dorset bird (andtherefore also the Glamorgan bird) is notinconsistent with South Polar Skua. The Scillybird was not moulting.
HybridsOf course, the question of hybrids is bound toarise. Mixed pairs occur at a frequency of around15% at Potter Peninsula, King George Island(Ritz et al. 2005), and ringing recoveries demon-strate that hybrids may migrate north, includinga South Polar x Chilean Skua S. chilensis (3⁄4 South Polar), in the western North Atlantic(Koeppen & Scheil 2001). The wing-length of theGlamorgan Skua is below the minimum forSouth Polar given in James (1996), althoughwithin the range given by Ritz et al. (2005). DNAconfirmed that it was a female (which tend to belarger than males), so a very small bird indeed.Given that hybrids between South Polar and theusually larger Brown Skua are intermediate insize (Hemmings 1984; Parmalee 1988; Jiguet etal. 1999), this points to a pure South Polar. As forthe Scilly skua, seasoned observers, looking at thebird in a box, debated whether it was a Great or aPomarine Skua S. pomarinus, suggesting that itwas neither large nor heavy (Martin Scott pers.comm.). This bird showed no structural orplumage features, including its cold plumagetones, near to Brown Skua, so it is probably puretoo.
Whether the points highlighted above, someof which need more data to be fully corrob-orated, are sufficient for any of the three to beaccepted onto the British List as South PolarSkua is a matter for BBRC and BOURC.Personally, I see no evidence to argue against allthree being South Polar Skuas and the evidencefor the Scilly and Glamorgan birds is especiallycompelling.
Dick NewellOld Beach Farm, Green End, Landbeach, Cambridge CB25 9FD
Acknowledgments
I would like to thank Peter Hayman, Steve Howell, FrédéricJiguet, Richard Porter, Markus Ritz, Alan Rogers, MartinScott and Angus Wilson for helpful comments on an earlydraft of this letter.
References
Bourne,W. R. P., & Curtis,W. F. 1994. Bonxies, barnaclesand bleached blondes. Brit. Birds 87: 289–298.
— & Lee, D. S. 1994.The first record of McCormick’s (orSouth Polar) Skua for Europe and the NorthernHemisphere. Sea Swallow 43: 74–76.
Cohen, B. L., Baker,A. J., Blechschmidt, K., Dittman, D. L.,Furness, R.W., Gerwin, J. A., Helbig,A. J., de Korte, J.,Marshall, H. D., Palma, R. L., Peter, H.-U., Ramli, R.,Siebold, I.,Willcox, M. S.,Wilson, R. H., & Zink, R. M.1997. Enigmatic phylogeny of skuas (Aves:Stercorariidae). Proc. Roy. Soc. Lond. B 264: 181–190.
Devillers, P. 1977.The skuas of the North American Pacificcoast. Auk 94: 417–429.
Furness, R.W. 1987. The Skuas. Poyser, Calton.Hemmings,A. D. 1984.Aspects of the breeding biology of
McCormick’s Skua Catharacta maccormicki at SignyIsland, South Orkney Islands. Brit. Antarctic Survey Bull.65: 65–79.
Howell, S. 2004. South Polar Skuas off California. BirdingWorld 17: 288–297.
— 2005. Revisiting an old question: how many species ofskua occur in the North Pacific? Western Birds 36: 71–73.
— 2008. Moult of southern skuas. Birding World 21: 28.James, D. 1996. In: Higgins, P. J., & Davies, S. J. J. F. (eds.),
Handbook of Australian, New Zealand, and Antarctic Birds,Vol. 3, pp. 412–438. OUP, Melbourne.
Jiguet, F. 2007. Brown Skua in the North Atlantic. BirdingWorld 20: 327–333.
—, Chastel, O., & Barbraud, C. 1999.A hybrid South Polarx Brown Skua. Birding World 12: 118–122.
Koeppen, U., & Scheil, S. 2001. Beringungsbericht derVogelwarte Hiddensee 1999/2000: Skua hybridCatharacta maccormicki x (C. maccormicki x C. chilensis).Berichte der Vogelwarte Hiddensee 16: 26–27.
Malling Olsen, K., & Larsson, H. 1997. Skuas and Jaegers: aguide to the skuas and jaegers of the world. Pica Press,Robertsbridge.
Millington, R. 2000.An interesting skua in Dorset. BirdingWorld 13: 336–339.
Moon, S., & Carrington, D. 2002.A Brown Skua inGlamorgan. Birding World 15: 387–389.
Parmalee, D. F. 1988.The hybrid skua: a southern oceanenigma. Wilson Bull. 100: 345–356.
Ritz, M. S., Hahn, S., Janicke,T., & Peter, H.-U. 2005.Hybridisation between South Polar Skua (Catharactamaccormicki) and Brown Skua (C. antarctica lonnbergi) inthe Antarctic Peninsula region. Polar Biol. 29: 153–159.
Scott, M. 2002.A Brown Skua on the Isles of Scilly – thefirst for Europe? Birding World 15: 383–386.
Votier, S. C., Bearhop, S., Newell, R. G., Orr, K., Furness,R.W., & Kennedy, M. 2004a.The first record of BrownSkua Catharacta antarctica in Europe. Ibis 146: 95–102.
—, Kennedy, M., Bearhop, S., Newell, R. G., Griffiths, K.,Whitaker, H., Ritz, M. S., & Furness, R.W. 2007.Supplementary DNA evidence fails to confirm presenceof Brown Skuas Stercorarius antarctica in Europe: aretraction of Votier et al. (2004). Ibis 149: 619–621.
