BIRD-BANDING A JOURNAL OF ORNITHOLOGICAL INVESTIGATION VOL. XXXII OCTOBER,1961 No. 4 DISTRIBUTION AND LIFE HISTORY STUDIES OF THE SOUTH-POLAR SKUA By CARLR. EKLUND INTRODUCTION Skuasare in the Order Charadriiformes,and togetherwith jaegers of the genusStercorarius constitute the Family Stercorariidae.Taxon- omists disagree as to whether the skua comprisesa single bipolar speciesor whether it consistsof several speciesand sub-species.In this study four speciesare recognizedand all are classifiedin the bipolar genus Catharacta,with one speciesnormally inhabitifig the Northern Hemisphereand the other three the Southern. Theseinclude Catharactaskua, the great skuaof the NorthernHemisphere; 'C. chilen- sis t Saunders), the Chilean skua found generally in the temperate regions of South America; C. lonnbergi t Mathews), the brown skua which primarily inhabitsthe subantarcticareas; and C. maccormicki f Saunders},the south-polarskua which is confinedto Antarctica. This latter species,which is the subjectfor this study, has been commonly calledthe Antarcticskua as well as McCormick'sskua, but south-polar skua is the more common name and this term is followed. ACKNOWLEDGMENTS: This study was used as basis for a doctoral disserta- tion at the University of Maryland. Funds were initially provided by •he United Sta,tes National Committee--IGY of the National Academy of Sciences, and sub- sequent banding work was done through a grant provided by the National Science Foundati,on. Without this aid the study could never have been carried ont. Banding records are being kept by the United States ,Fish and Wildlife Service and this is gratefully acknowledged.Special thanks are due many personsfrom the United States as well as ot,her countries who so ably cooperatedin this interna- tional program with their banding and observationwork. Whenever possible, an attempt has been made to give proper credit in the text for this work. The south-polarskua was first collectedduring Captain James Clark Ross'sexpedition in the Erebusand Terror to Victoria Land in 1839-43 (McCormick, 1884, p. 154). Saunders(1893) proposedthe name Stercorariusmaccormicki, and first describedit, over 50 years after it had beencollected, as a distinctspecies, on the basisof five specimens forwardedto the British Museumby Dr. McCormick. Thesehad been procuredat PossessionIsland, latitude 71 ø 14t S., longitude171 ø 15t W. The south-polarskua nests chiefly on lands within the Antarctic Circle, and on the basis of presentrecords the breeding range is gen- erally confinedto the area southof the 65th parallel exceptin Palmer- land where it may extendnorthward to betweenlatitudes 62-64 ø S. This correspondsroughly to the 32øF isotherm ivan Rooy, 1957). Hamilton 11934,pp. 173-174) indicatesthat the speciesdoes not occur 187 ] 88 ] EKLUND,The South-Polar Skua Bird-BandingOctober ,40' 150' •ZO' •0' •OC' •O' SO' • ?O' 60' 50' 4•' PLATE Z ANTARCTICA SOUTHPOLAR SKUA'. RANGE AND RECORDS ß •_• ,•. , Pack- zce only part of the year and probable hmH of norma/winter range of SouthPolar Skuo ß t•ecord$ of South Polar Skua recoveries . north of latitude65 ø S. in Pahnerland.Gain (1914, p. 109) recorded it at the SouthShetland Islands, however, and foundit nestingas far north as Admiralty Bay, latitude 62ø 06' S. This record is further supportedby collectionsfrom the DancoCoast of Palmerland•Graham- land), latitude64 ø 30' S., longitude62 ø W. tMurphy,1936, p. 1010). Bagshawe(1939, p. 280) describesthe brown skua at Water Boat Point on the DancoCoast, latitude 64ø 48' S., longitude62 ø 43' W., but this appearsto be misidentified.He describesthe young as slate- colored. This being the case it must be the south-polarrather than the brown skua,since young of the latter are buff-colored. Four recordsof the occurrenceof the south-polarskua have been made in New Zealand at locations shown on Plate I. The first C. mac- cormickiresulted from a misidentifiedspecimen of C. lonnbergifrom Stewart Island •Falla, 1940). Falla recorded another occurrenceof C. maccormicki from the west coast of North Island, New Zealand, in 1940, and a third was recorded on the New Zealand coast in 1950. All these records were based on identification of dismembered birds. Fleming (1953) describesa fourth record of an exhaustedbird that came ashore on North Island. It was surmised that it had been blown into the Tasman Sea by a storm that originatedin the subantarctic winter range of the skua. Vol. XXXII 1961 EKLUND,The South-Polar Skua [ 189 Argentine naturalistsprocured a specimenof C. rnaccormickiat Laurie Island in the easternmostSouth Orkneys, latitude 60 ø 45' S., longitude44 ø 35t W., in November1904. Up to that time this was the most northern record of the species(Clark, 1906). A specimen collectedin the Sagami Sea off the coast of Japan was identified as C. maccormickiby Murphy (1936, p. 1007). Description Plumage. There is considerablevariation in the color plumageof south-polarskuas. The bird is muchpaler brownthan the brownskua, and it lacks the cinnamon or rufous-colored feathers so characteristic of most other forms. The ventral and dorsal surfacesare generally more light brown, with a somewhatstreaked appearance, becoming darker posteriorly. The head and neck have a light chamoisor straw- color tone. The darker brown of the wing feathers helps accentuate the large outstandingwhite patch on the primaries,which is so readily recognizedin flight. The feature that most clearly distinguishesC. maccormickifrom all othersis the downystage of the young. It is the only oneof the genus with young of a bluish or slate-graycolor. The nestlingsof all the other skuasare buff or rufous. Gain 11914, p. 115) maintainsthat this characteristicdistinguishes the south-polarfrom the brown skua of the subantarctic.In the courseof this studythe youngof the species have been observedat five different regionsof Antarcticaand all have been slate-colored.Buff-colored young have never been seenin these areas. Most of the fledgling down is shed by the seventhor eighth week, and the plumage at this stage is a uniform mouse-gray,which later becomesvery dark gray. Flight generallytakes place after about 9 weeksof age, during the latter half of February and the first half of March in the Windmill Islands. At this time the darker coloration of the young bird makesit quite distinguishableas it flies with the adults. Body Measurements.Measurements taken by Friedmann 1,1945, p. 311) of south-polarskuas collected in Palmerlandshow an average wing dimensionfor 8 males of 373.6 mm., comparedwith an average of 380.1 for 10 females. Tail measurementsaveraged 151.9 for males as against150.1 for females. Tarsus measurementsaveraged 62.0 for males and 62.6 for females. The culmen, measured from the base, averaged 59.6 for males and 60.4 for females. The average of the middle toe without the claw was 54.2 for males and 56.8 for females. Sex differentiationthrough measurementsmay be indicated after a larger collectionof specimenshas been examined. Falla {1937, p. 248) examined 15 specimensof C. maccormicki and in the measurements found indicationsof a greatersize in the female,although this was not so markedas in C. lonnbergi. He felt that in any carefullysexed series of breedingadults the length of the wing might be a dimensionin which females would have a decided advantage. Friedmann's exam- inationsof C. maccormickiindicated an averagedifference of 6.5 mm. in wing measurementsbetween the sexes. Wilson (1907, p. 75) says there seemsto be a fairly constantthough slight advantagein point of size in the female over that of the male. Weight. Weights taken of 20 adults in January-February1941 at 190] EKLUND,The South-Polar Skua Bird-BandingOctober the East Base in Palmerland (latitude 68 ø 12' S., longitude 67 ø 03' W.) averaged1,295 grams (Eklund, 1945). The average weight of 10 birds at the Hallett Station in January 1957 was 1,500; of 30 adults weighed at McMurdo Sound in the same month, 1,285; and of 50 adults taken at the Wilkes Station in February-March 1957, 1,390. The averageweight of these110 specimens,taken in thesefour different regionsin Antarctica,was 1,355 grams (2 lbs. 153/• oz.); maximum, 1,940; and minimum, 985. Body measurementsgiven by previouslycited authoritiesdo not in- clude weight. It would be interestingto obtain a truly representative sampleto determinewhether differencesexist here betweensexes, as in the caseof other polar-inhabitingspecies such as the giant petrel (Macronectesgiganteus) and the southernblack-backed or kelp gull (Larus dominicanus). Unfortunately,weights by sex of the above 110 specimenscould not be obtainedbecause all the birds, except5 of those taken at East Base, were banded and released for migration studies. Body Temperature. The south-polarskua seemsmore active in everythingit doesthan any other polar speciesobserved. The rapid physiologicor metabolicprocesses involved in such energy appear borne out by its high body temperatures.Temperature studies were carriedout in Palmerlandin 1939-41,and of 6 speciesthe skuashowed the highestaverage, as well as the maximum, of all recordingstaken (Eklund, 1942, p. 545). In the above cited study. 28 skuasshowed an average temperature of 106.14ø F., with a maximum of 108.40ø for a juvenile as ;veil as for an adult, and a minimum of 107.40ø for a juvenile and 103.40ø for an adult. This averagewas 2.84ø higher than that of the snow petrel(Pagodroma nivea); 1.11ø higherthan the southernblack-backed gull; 2.34ø higherthan the Adeliepenguin; and 0.51ø higherthan the antarctic tern (Sterna vittata). Eggs. A clutchusually consists of 2 eggs. The color varies slightly, but generallyis greenishin variousshades, with light-brownsplotches. Some eggs collectedin Palmerland in 1939-41 were more brownish than green,with dull, rather cloudedsplotches of brown. Many seen in the Windmill Islandsappeared much lighter greenthan others. From Cape Adare 26 collectionsmeasured between 65.5 and 78.7 mm. in length,and 47.7 and 52.0 in width (Oates,1901, p. 2261: and 8 eggscollected in AdelieLand (Etchecoparand Prevost,1054, p. 243) showedsomewhat greater uniformity, the maximum length, 73.7: the minimum, 70; and the mean, 71.9, respectively.