Polar Biol 2001) 24: 59±61 Ó Springer-Verlag 2001
SHORT NOTE
S. C. Baker á C. Barbraud Foods of the South Polar skua Catharacta maccormicki at Ardery Island, Windmill Islands, Antarctica
Accepted: 3 June 2000
Abstract South Polar skuas Catharacta maccormicki) includes clis, both steep and gentle slopes, and areas of breed on ArderyIsland in the absence of a local morainic boulder slopes. breeding population of Adelie penguins Pygoscelis In manyother Antarctic areas, feeding during the adeliae). Assessment was made of the food remains in breeding season bySouth Polar skuas, and hence their skua feeding territories in 1995/1996. The diet of South selection of breeding location, is largelydependent on Polar skuas largelyconsisted of fulmarine petrel species Adelie penguin rookeries in combination with avail- which bred on ArderyIsland. Southern fulmar Fulma- abilityof food at sea Young 1963; MuÈ ller-Schwarze and rus glacialoides) remains were the predominant prey MuÈ ller-Schwarze 1973; Trillmich 1978; Hull et al. 1994; items found, and skuas appeared to feed preferentially Norman et al. 1994). Ecklund 1961) estimated that at on this species. least 95% of skua nesting habitats in the Windmill Island group are close to Adelie penguin rookeries. The absence of breeding populations of Adelie penguins on ArderyIsland allows investigation of skua feeding where other bird species are likelyto be predominant in their Introduction diet. Other studies that have investigated the diet of skuas remote from Adelie penguin populations have ArderyIsland occurs in the Arderyand Odbert Island found food items of importance to be snow petrels SpeciallyProtected Area, Windmill Island group, near Zipan and Norman 1993), southern fulmars and CaseyStation 66 °22¢S, 110°27¢E), Antarctica. Ardery Antarctic petrels Green 1986). Island has breeding populations of South Polar skua This studyinvestigated the food of skuas on Ardery Catharacta maccormicki), Wilson's storm petrel Island in the summer of 1995/1996 in relation to the Oceanites oceanicus), and the fulmarine petrels, south- proximityof skua nest sites and feeding territories to ern fulmar Fulmarus glacialoides), Antarctic petrel petrel colonies. Thalassoica antarctica), Cape petrel Daption capense) and snow petrel Pagodroma nivea). The distribution and size of bird populations on ArderyIsland are de- tailed in Bonner and Lewis Smith 1985), van Franeker Materials and methods et al. 1990) and Barbraud and Baker 1998); dierent petrel species nest in discrete colonies. Adelie penguins The diet of South Polar skuas on ArderyIsland was studied be- Pygoscelis adeliae) do not breed on ArderyIsland; the tween 22 December 1995 and 29 February1996. The location of closest colonies occur on Odbert Island, 2.5 km distant. skua nests, feeding territories of non-breeding skua pairs, and non- territorial feeding areas used byseveral skuas were identi®ed and The island consists of charnockite rock and the terrain mapped Fig. 1). Sampling locations were rated according to their proximityto breeding colonies of avian preyspecies. Territories were visited regularlyon at least a weeklybasis. C. Barbraud &)1 The remains from skua preywere recorded and then removed to Tour du Valat, Le Sambuc, 13200 Arles, France avoid repeat records at subsequent visits. Items found included e-mail: [email protected] regurgitated pellets, egg shells, remains of adult birds, and chick remains. All items at ®rst visit were included in the analysis. S. C. Baker, Pellets were sorted and the remains identi®ed bycomparison with 451 Huon Road, South Hobart, material from known bird species. The species of bird and type of Tasmania 7004, Australia item egg, adult or chick) were recorded for all remains found. Present address: Chi-squared tests Steel and Torrie 1980) were used to test 1C.E.B.C./C.N.R.S. 79360 Villiers en Bois, France whether the counts of egg, adult and chick remains of each species 60 which 44 7.4%) were found during the ®rst visit. Southern fulmar material was byfar the most numerous, constituting 86.0% of items found. Material from snow petrels 7.1%) and Cape petrels 4.2%) was much less abundant, and was concentrated at a few sites. Material from Adelie penguins 2.4%) and Antarctic petrels 0.3%) appeared veryrarelyin the skuas' diet. No South Polar skua remains were found in the collection areas. However, observations on skua breed- ing success suggest that 12 skua eggs and 3 skua chicks were probablyconsumed during the observation period. Observation of Antarctic petrel colonies suggests that a low number of eggs and chicks mayalso have been consumed byskuas. No remains of Wilson's storm petrel were found at anyof the studysites. Fig. 1 Location of South Polar skua nests N ), territories of non- The incidence of egg, adult and chick remains breeders T ) and feeding areas F ) on ArderyIsland in 1995/1996 number of items) was compared to the population pairs) of each preyspecies nesting on ArderyIsland. are in proportion to the populations of each species on Ardery Island. Chi-squared tests found that the proportion of egg, The remains of adult birds were often detached from the body, adult and chick remains were dierent than would be e.g. head, wings, tarsii, or combinations of these. Anyseparated expected from the population size of each species: egg item of adult bird was recorded. Hence the possibilityexists that v2 79.56, df 3, P<0.005; adult v2 10.18, df 3, more than one separated item from an individual bird was 2 recorded; consequently, the percentages of adult birds of a prey P<0.025; chick v 22.36, df 3, P<0.005. species maybe slightlyexaggerated relative to egg and chick counts Comparison of the number of food items relative to of the same and other species. the nesting populations Table 1) suggests a dispropor- tionatelyhigh number of southern fulmar eggs and chicks compared to those of other preyspecies. The number of southern fulmar adult remains was also Results higher than the other species; however, there were also a relativelyhigh number of snow petrel adults while the A total of 594 food items was recorded from 18 South number of Antarctic petrel adult items was dispropor- Polar skua feeding areas on ArderyIsland Table 1), of tionatelylow. Eight of the feeding sites provided sucient material Table 1 Comparison of egg, adult and chick remains between prey for comparison of diet Table 2). Southern fulmar items species of South Polar skua; counts, frequencyof occurrence and were the most numerous at all sites, forming between the numbers of preyitems relative to the preyspecies' population size
Preyspecies and Item Count Frequency Preyitem/ Table 2 Frequencyof occurrence %) of food items and population size %) pair %) approximate numbers of nest sites of preyspecies within respective on ArderyIsland territories in parentheses) of eight South Polar skua feeding sites on ArderyIsland Southern fulmar Egg 267 44.9 6.9 3860 pairs Adult 175 29.5 4.5 Skua site Preyspecies Chick 69 11.6 1.8 Total 511 86.0 13.2 Southern Snow Cape Adelie Antarctic fulmar petrel petrel penguin petrel Snow petrel Egg 6 1.0 0.8 800 pairs Adult 33 5.6 4.1 N1 96.5 0.0 0.0 2.3 1.2 Chick 3 0.5 0.4 >500) 50) 50) 0) 30) Total 42 7.1 5.3 N3 94.7 0.0 5.3 0.0 0.0 Cape petrel Egg 9 1.5 1.6 >500) 50) 100) 0) 30) 550 pairs Adult 16 2.7 2.9 N6 100.0 0.0 0.0 0.0 0.0 Chick 0 0 0 >500) 50) 100) 0) 0) Total 25 4.2 4.5 N7 91.2 2.9 0.0 5.9 0.0 >500) 50) 100) 0) 0) Antarctic petrel Egg 0 0 0 N9 63.2 14.0 15.8 7.0 0.0 247 pairs Adult 2 0.3 0.8 >500) 100) 50) 0) 0) Chick 0 0 0 N10 88.1 8.9 0.0 3.0 0.0 Total 2 0.3 0.8 >500) 100) 50) 0) 0) Adelie penguin Egg 0 0 ± N12 62.2 25.6 11.0 1.2 0.0 0 pairs Adult 14 2.4 ± 150) 200) 50) 0) 0) Chick 0 0 ± F2 94.0 1.2 3.6 0.0 1.2 Total 14 2.4 ± >500) 50) 100) 0) 100) 61 62.2 and 100% of food remains found, and were the ArderyIsland seems to be the onlylocation in the most abundant preyspecies within each feeding territory Windmill Islands region where South Polar skuas are except N12. dependent on petrel species for food. Observations on Shirley, Holl, Odbert and Peterson Islands, all of which support Adelie penguin colonies, suggest that skuas are largelydependent on Adelie penguins as a food source. Discussion Acknowledgements This research was conducted under a permit The results of this studysuggest that ArderyIsland granted under Section 9 of the Antarctic Treaty Environment Protection) Act of Australia 1980). We wish to thank members of supports a breeding population of South Polar skuas the 49th ANARE expedition to Caseyfor logistical support and which is largelyindependent of Adelie penguins as a ®eld assistance during this project. In particular, we would like to food source, with southern fulmars providing the bulk of thank G.G. Robertson, H. Weimerskirch and P. Jouventin for their food. Our results support the concept of opportu- facilitating the project, R.P. Sco®eld for comments on a draft manuscript, T.A. Reid for statistical advice and U. Ryan for help nistic feeding bySouth Polar skuas Norman and Ward with mapping. Digital data were supplied byAUSLIG. 1990), which states that skuas appear to largelydepend on the most numerous and readilyobtainable prey species for food. On ArderyIsland, southern fulmar was the most numerous preyspecies and nests were more References accessible to skuas than snow petrel and Wilson's storm petrel nests. In addition, snow and Cape petrel adults Barbraud C, Baker SC 1998) Fulmarine petrels and South Polar skua Catharacta maccormickii populations on ArderyIsland, and chicks appeared more prone to spitting stomach oil Windmill Islands, Antarctica. Emu 98: 234±236 in defence S.C. Baker, personal observation) than either Bonner WN, Lewis Smith RI eds) 1985) Conservation areas in the southern fulmars or Antarctic petrels. Antarctic. SCAR/ICSU, Cambridge The results suggest an apparent preference for Ecklund CR 1961) Distribution and life historystudies of the southern fulmar adults and chicks compared to other South Polar skua. Bird Banding 32: 187±223 Franeker JA van, Bell PJ, Montague TL 1990) Birds of Ardery preyspecies. The main Cape and Antarctic petrels' egg- and Odbert Islands, Windmill Islands, Antarctica. Emu 90: laying periods were prior to commencement of this 74±80 study. This is likely to bias the egg counts and would Green K 1986) Observations on the food of the South Polar skua partiallyexplain the disproportionatelyhigh numbers of Catharacta maccormicki near Davis, Antarctica. Polar Biol 6: 185±186 southern fulmar eggs, although snow petrel eggs appear Hull C, Carter C, Whitehead MD 1994) Aspects of breeding to be preyed upon much less frequently. Although the chronologyand success of the Antarctic skua Catharacta overall rates of predation of snow and Cape petrels are maccormicki at Magnetic Island, Prydz Bay, Antarctica. Corella similar, there were a greater proportion of snow petrel 18: 37±40 MuÈ ller-Schwarze D, MuÈ ller-Schwarze C 1973) Dierential pre- adult remains but a lower proportion of chick remains dation bySouth Polar skuas in an Adelie penguin rookery. compared to Cape petrels. Condor 75: 127±131 Adelie penguin remains were found veryinfrequently Norman FI, Ward SJ 1990) Foods of the South Polar skua at Hop in the food remains of South Polar skuas. Leopard seal Island, Rauer Group, East Antarctica. Polar Biol 10: 489±493 Hydrurga leptonyx) faeces are also a source of Adelie Norman FI, McFarlane RA, Ward SJ 1994) Carcasses of Adelie penguins as a food source for South Polar skuas: some pre- penguin remains Green 1986). Skuas were observed liminaryobservations. Wilson Bull 106: 26±34 feeding on these on ice ¯ows close to ArderyIsland. Steel RGD, Torrie JH 1980) Principals and procedures of statis- Alternatively, skuas may have foraged on dead adult tics: a biometrical approach, 2nd edn. McGraw Hill, Sydney Adelie penguins from Odbert Island. Trillmich F 1978) Feeding territories and breeding success of South Polar skuas. Auk 95: 23±33 No items of marine foods, such as remains of ®sh, Young EC 1963) Feeding habits of the South Polar skua Catha- cephalopod or krill, were found in skua pellets. Skuas racta maccormicki. Ibis 105: 301±318 were never observed ®shing, so it is likelythat marine Young EC 1990) Diet of the South Polar skua Catharacta organisms were onlyconsumed indirectlyfrom feeding maccormicki determined from regurgitated pellets: limitations of a technique. Polar Rec 26: 124±125 on the stomach contents of their prey. Moreover, marine Zipan W, Norman FI 1993) Foods of the South Polar skua preyare likelyto be nearlycompletelydigested byskuas, Catharacta maccormicki in the eastern Larsemann Hills, Prin- so that remains would be rare Young 1990). cess Elizabeth Land, East Antarctica. Polar Biol 13: 255±262