(ISBN: 0-943610-39-7) THE MARINE ECOLOGY OF BIRDS IN THE ROSS SEA, ANTARCTICA BY DAVID G AINLEY, EDMUND F O'CONNOR AND ROBERT J BOEKELHEIDE Point Reyes Bird Observatory Stinson Beach, California ORNITHOLOGICAL MONOGRAPHS PUBLISHED THE AMERICAN BY ORNITHOLOGISTS' WASHINGTON, 1984 NO 32 D.C UNION THE MARINE ECOLOGY OF BIRDS IN THE ROSS SEA, ANTARCTIC ORNITHOLOGICAL MONOGRAPHS This series,publishedby the American Ornithologists'Union, has been established for major paperstoo long for inclusion in the Union's journal, The Auk Publicationhas been made possiblethroughthe generosityof the late Mrs Carll Tucker and the Marcia Brady Tucker Foundation, Inc Correspondenceconcerningmanuscriptsfor publication in the seriesshould be addressedto the Editor, Dr Mercedes S Foster, USFWS, National Museum of Natural History, Washington, D.C 20560 Copies of OrnithologicalMonographsmay be ordered from the Assistantto the Treasurerof the AOU, Frank R Moore, Departmentof Biology,University of SouthernMississippi,SouthernStation Box 5018, Hattiesburg,Mississippi 39406 (See price list on back and inside back covers.) OrnithologicalMonographs,No 32, x + 97 pp Editor of AOU Monographs,MercedesS Foster SpecialReviewersfor this issue,John P Croxall, British Antarctic Survey, Cambridge, England; Peter A Prince, British Antarctic Survey, Cam- bridge,England; GeorgeE Watson,Divisionof Birds,NationalMuseum of Natural History, Washington, D.C Authors, David G Ainley, Edmund F O'Connor, and Robert J Boekelheide, Point ReyesBird Observatory,StinsonBeach,California 94970 First received, 21 April 1982; accepted 27 October 1982; final revision completed, 10 August 1983 Issued February 23, 1984 Price $9.00 prepaid ($8.00 to AOU members) Library of CongressCatalogue Card Number 84-70267 Printed by the Allen Press,Inc., Lawrence, Kansas 66044 Copyright ¸ by the American Ornithologists'Union, 1984 ISBN: 0-943610-39-7 THE MARINE ECOLOGY OF BIRDS IN THE ROSS SEA, ANTARCTICA BY DAVID G AINLEY, EDMUND F O'CONNOR AND ROBERT J BOEKELHEIDE Point Reyes Bird Observatory Stinson Beach, California ORNITHOLOGICAL MONOGRAPHS PUBLISHED THE AMERICAN BY ORNITHOLOGISTS' WASHINGTON, 1984 NO D.C UNION 32 TABLE LIST OF CONTENTS OF FIGURES vi LIST OF TABLES viii LIST OF APPENDICES INTRODUCTION METHODS ix CENSUSES POPULATION ESTIMATES FEEDING STUDIES ENVIRONMENT SPECIES DISTRIBUTIONS EMPEROR PENGUIN KING PENGUIN AND NUMBERS 17 17 21 AD•LIE PENGUIN 23 ALBATROSS 29 SOUTHERN GIANT FULMAR 32 SOUTHERN FULMAR 32 ANTARCTIC PETREL 37 CAPE PETREL 38 SNOW PETREL 38 ANTARCTIC PRION 46 BLUE PETREL 46 WHITE-HEADED PETREL 46 MOTTLED PETREL 46 WHITE-CHINNED PETREL 49 SOOTY SHEARWATER 50 WILSON'S STORM-PETREL 53 BLACK-BELLIED STORM-PETREL 53 DIVING PETREL 56 BROWN SKUA 56 SOUTH POLAR SKUA 56 ARCTIC TERN 61 FEEDING BEHAVIOR 61 TROPHIC RELATIONS 63 DIET COMPOSITION 64 DIET OVERLAP 68 SYNTHESIS AND DISCUSSION 72 COMMUNITY COMPOSITION 72 FACTORS AFFECTING SEABIRD OCCURRENCE 76 COMMUNITY BIOMASS 78 TROPHIC INTERACTIONS 81 ACKNOWLEDGMENTS SUMMARY 86 87 LITERATURE CITED 88 APPENDICES 92 LIST OF FIGURES Figure Cruise tracks and collecting localities between 15 December and January Cruise tracks and collecting localities between 16 January and 21 February Ross Sea and Antarctic localities mentioned in the text 10 Temperature profile with depth along the 1979 USCGC North- wind track 11 The Antarctic Slope Front as indicated in the depth profiles of temperature,salinity, and turbidity alonga 1976 USCGC Northwind track 12 Sea surface isotherms, and positions of the Antarctic Convergence, Polar Front Zone, and Antarctic Slope Front, from 15 December to January 13 Sea surface isotherms from 16 January to 21 February 14 Ice cover during late December and early January 15 Ice cover during late January to late February 16 10 Sea surface isopleths from 15 December to January 18 11 Sea surface isopleths from 16 January to 21 February 19 12 Distributions of Emperor and King Penguins during early summer 20 13 Occurrenceof Emperor and King Penguinsalong cruise tracks during late summer 22 14 Distribution of Adfilie Penguins during early summer 25 15 Proportion of Adfilie Penguinsin adult plumage along December cruise tracks 16 26 Occurrence of Adfilie Penguins along late summer cruise tracks 27 17 Occurrenceof Light-mantled Sooty Albatross along December cruise tracks 30 18 Occurrence of Light-mantled Sooty Albatross and Wandering andRoyalAlbatrossalongFebruarycruisetracks 31 19 Occurrenceof Black-browedand Gray-headed Albatrossalong December 20 21 cruise tracks 33 Occurrenceof Black-browedand Gray-headed Albatross along late January and February cruise tracks 34 Occurrence of Southern Giant Fulmars and Southern Fulmars along December cruise tracks 35 vi 22 Occurrence of Southern Giant Fulmars and Southern Fulmars 23 24 along late January and February cruise tracks 36 Distribution of Antarctic Petrels during December 39 Occurrenceof Antarctic Petrelsalong late January and February cruise tracks 40 25 Occurrenceof Cape Petrelsand White-headed Petrelsalong December cruise tracks 42 26 27 28 Occurrence of Cape Petrels and White-headed Petrels along late January and early February cruise tracks 43 Distribution of Snow Petrels during December 44 Occurrence of Snow Petrels along late January and k'ebruary cruise tracks 45 29 Occurrenceof Antarctic Prions and Blue PetrelsalongDecember cruise tracks 47 30 31 32 Occurrence of Antarctic Prions along February cruise tracks 48 Occurrenceof Mottled Petrels along December cruise tracks 49 Occurrence of Mottled Petrels along late January and February cruise tracks 50 33 Occurrence of Sooty Shearwaters along December cruise tracks 34 35 51 Occurrence of Sooty Shearwaters and White-chinned Petrels along late January and February cruise tracks 52 Distribution of Wilson's and Black-bellied Storm-Petrelsduring December 36 37 54 Occurrence of Wilson's and Black-bellied Storm-Petrels along late January and February cruise tracks 55 Occurrenceof diving petrels and Arctic Terns along December cruise tracks 57 38 39 40 41 42 Occurrenceof diving petrelsand Arctic Terns along late January and February cruise tracks 58 Distribution of skuas during December 59 Occurrence of South Polar Skuas and a Brown Skua during late January and February 60 Daily patterns of feeding in Antarctic seabirds 62 Size distribution of squid beaks in the stomachs of Ross Sea seabirds 73 vii LIST Table OF TABLES Estimated numbers of penguinsin breeding populations of the Ross Sea 23 Occurrence of Ad61ie Penguins in relation to ice concentration over the Ross Sea continental slope 28 Association of birds with certain pack ice habitats 41 Feedingbehaviors of Antarctic seabirds 63 Average number of prey items and their frequencyof occurrence in stomachs of seabirds collected at deep ocean localities 65 Average number of prey items and their frequencyof occurrence in stomachs of seabirds collected at continental slope localities 66 Average number of prey items and their frequencyof occurrence in stomachs of seabirds collected at continental shelf localities 67 Estimated averageprey composition, by weight, of stomachcontents of seabirds from localities of the South Pacific Ocean and_ Ross Sea 69 Average lengthsand weightsof squid eaten by seabirds 70 10 Average lengthsand weights of the fish, Pleuragrarnrna antarcticurn, eaten by seabirds in the Ross Sea and South Pacific Ocean 70 11 Morisita's Index of overlap in the diets of seabird species in oceanic habitats 71 12 Morisita's Index of overlap in the diets of seabird species in continental slope habitats 71 13 Morisita's Index of overlap in the diets of seabird species in continental 14 shelf habitats 72 Bill sizes of seabirds from the South Pacific Ocean and Ross Sea 72 15 Distributions and habitat preferencesof seabirdsin the Antarctic South Pacific Ocean and Ross Sea, December to February 74 16 Numbers and biomass of seabirds in the Ross Sea during December and early January 79 17 Numbers and biomass of seabird chicks at Ross Sea breeding localitiesduring late December and early January 80 18 Frequencyof occurrenceof prey in the stomachsof five seabird speciescollected at sea: a comparison of three studies 83 19 Mean number and mean total weight of eachprey in the stomachs of petrels collected at sea:a comparison of two studies 84 20 Estimated wet weight of euphausiids, squid, or fish needed by Antarctic seabirds to meet daily energy requirements at 0øC ambient temperature 85 viii LIST I II Habitats OF APPENDICES and localities where seabirds were collected 92 Lengths of Euphausia superba eaten by seabirds at various deep ocean and continental slope localities 93 III Lengthsof Euphausia crystallorophiaseaten by seabirdsat various continental IV V shelf localities 94 Size of beaks of squid eaten by seabirds at various deep ocean and continental slope localities 95 Size of otoliths of the fish, Pleuragramma antarcticurn, eaten by seabirds at various localities 97 ix MARINE ECOLOGY OF ROSS SEA BIRDS 85 TABLE 20 ESTIMATEDWET WEIGHT OF EUPHAUSIIDS,SQUID, OR FISH NEEDED BY ANTARCTICSEABIRDS TO MEET DAILY ENERGYREQUIREMENTS AT 0øC AMBIENT TEMPERATURE Existence Wet wt (g) Total foodneeded/day • Max v.,t.(g) Body v.'L _+s.d (g)• energy (kcal/day)b energy (kcal/day) E S F stomach contentsẵ 346.1 484.5 495 505 289 378 Light-mantled Sooty Albatross 3876 ñ 100 (34) 2875 + 60 (2) 295.4 413.6 422 431 246 21 Southern 3975 + 46 350.4 490.6 501 5l 292 30 839 + 48 153.5 214.9 219 224 128 70 Cape Petrel (13) 454 ñ 54 (4) 111.0 155.4 159 162 93 Antarctic 735 ñ 100 143.6 201.0 205 209 120 35 84.6 118.4 120 123 70 14 61.7 86.4 88 90 52 101.9 142.7 146 149 85 33.0 46.2 47 48 28 196.0 274.4 280 286 163 60 Seabirdspecies Ad61iePenguin Giant Fulmar Southern Fulrnar Petrel (2) (3l) Snow Petrel 272 ñ 27 (39) Antarctic Prion 150 ñ (3) Mottled Petrel 385 ñ 38 (3) Wilson's Storm-Petrel 46 ñ (29) South Polar Skua 1326 ñ 139 (23) ßData for Ad61iesare from Ainley and Emison (1972) for December;,remainderfrom this study.Number of birds weighedis given in parentheses bBasedon the equation M = 4.337 Wt ø-s3(Kendeigh 1970); valuescomparableto thosesummarizedby Croxall (1982) • Wiensand Scott(1975) increasedexistrice energyby 40% to allow for flight,swimming,etc in seabirds;the figure,however,is a guess d E = euphausiids(4.9 kcal/g dry wt., 0.2 g dry to 1.0 g wet wt.), S = squid (4.8 kcal/g dry wt., 0.2 g dry to 1.0 g wet wt.), F = fish (5.6 kcal/gdry w•., 0.3 g dry to 1.0 g wet w•.; ratiosbasedon references in Wiensand Scott(1975), but the resultantestimatesof food requirements not take into accountdifferences in el•ciencyof digestionof differentprey(thus,amountsrequiredare underestimated, not necessarilyequally for each food type) • Data from this study; no stomach was entirely full; see Croxall and Prince 1980a: table and fish (Zusi 1975) Nevertheless,fish at times comprisea significantportion of the Ad61ie'sdiet (Emison 1968) At lower latitudes, diets of the Chinstrap Penguin and crestedpenguins(Eudyptesspp.) are similar to that of the Ad61ie The preponderanceof squid and fish in the diets of the larger Gentoo, Emperor, and King Penguinsis well known (Kooyman 1975; Zusi 1975) Quite different from the diets of seabirdsat high latitudes, it seems,are those of seabirdsbreeding at South Georgia Island, which is situated in the northern Scotia Sea, at the northern limits of the Antarctic In fact, the marine avifauna of SouthGeorgia(Croxall and Prince 1980a:table 3) and the northernScotiaSea (Kock and Reinsch 1978) is largely Subantarctic.There is little overlap in species between South Georgia and higher latitude areas, and becauseof this, the diets only of the Cape Petrel and Wilson's Storm-Petrel can be compared among the various studies(actually the data in Croxall and Prince for thesetwo speciesare 86 ORNITHOLOGICAL MONOGRAPHS NO 32 from breedingbirds at Signy Island, in the southernScotia Sea) Basedon weight, thesetwo speciesate euphausiidsalmostexclusively(!) while feedingchicks,which was also true of all other South Georgia speciesexceptthe six largestones:King Penguin, three albatrosses,and two giant fulmars These data, however, were based on food regurgitatedto chicks (no gizzard samples), and sampleswere preservedin formalin Therefore, techniquesand, perhaps,resultsare not closely comparable to those in the present study (nor to Falla 1937, nor Bierman and Voous 1950), but they indicate the importanceof euphausiidsas prey of many seabirds,and not just the smaller penguins,in the Scotia Sea The resultsof the South Georgia study are consistentwith the fact that euphausiids,especiallythe large Euphausia superba,are far more abundantin the vicinity of South Georgia and in the Scotia Sea, in general,than elsewherein the Antarctic (Marr 1962) At high latitudes the Ad61ie Penguin is the only avian speciesthat comes close to being a food "specialist." This is due to anatomical constraints(Zusi 1975) but ultimately is probably due to the Ad61ie'srelatively limited ability to search for prey Like all penguins,becauseof the high energeticcost of swimming and the time constraintsof supplyingfood to chicks,the Ad61ierequiresreliablefood sourcesin the areaswhere it occursand especiallynear where it nests(see also Boersma 1978; Crawford and Shelton 1978) Aerial speciescan cope with more patchily-occurring prey becausethey can search much more ocean, much more rapidly than can penguins(Ainley and Boekelheide 1984) Even the Emperor Penguin may be better off than the Ad61ie in searchingfor food, becauseit can dive to far greaterdepthsthan the Ad61ie(Kooyman 1975) Thus, much more of the water column and much more habitat is potentially available to it for exploitation Ad61iesmay feed primarily on euphausiidsbecausethey constitutethe most reliably available shallow-depth prey in range of nesting sites For less obviousreasons,nonbreedingCape Petrelsand Mottled Petrelsseemto rely more heavily on squid, and South Polar Skuas, regardlessof breeding status, more heavily on fish, than other birds of approximatelysimilar size Otherwise,within the same habitats and, more importantly, the same localities during summer, Antarctic seabirdshave extremelysimilar diets (Tables 5-7, 11-13) When species segregateecologically,it is by feeding method, the size of the largest prey, or microhabitat preference.Basedon the broad overlapin diets,we suggestthat food abundancemay not be a limiting factor to high latitude Antarctic bird populations during summer Further study is, of course,needed ACKNOWLEDGMENTS We wish to thank the officers and crews of the USCG icebreakers Northwind (1976 and 1979), Burton Island (1976 and 1977), and Glacier (1978) for their indispensable assistance.United States Antarctic Research Program personnel from the National ScienceFoundation and Holmes and Narver, Inc., particularly A F Betzeland M A Lanyon,helpedto coordinateour logisticneeds.A Amos, G Divoky, and S Jacobsprovided assistanceand guidance on various cruises; L Karl designedsoftware for our shipboard computer entry of censusdata and assistedin its analysisby computer.J Fitch identified all otoliths, and P Slattery identified crustaceanprey Squid beakswere identified by DGA at a squid identification workshop held at the Marine Biological Association Laboratory, Plymouth, England in June 1981; M R Clarke and M J Imber were particularly MARINE ECOLOGY OF ROSS SEA BIRDS 87 helpful in that effort H DeWitt provided unpublished regressionsof otolith size against fish length and weight S Jacobsprovided unpublished data on temperature/depth profilesalong various cruisetracks Through his detailed criticisms, and those of J.P Croxall and M S Foster, the manuscript was improved con- siderably.O'B Young and E Tuomi helpedimmenselyin the preparationof the manuscript This projectwas funded by the National ScienceFoundation, Division of Polar Programs,Grants DPP-7615358, DPP-7615358 A01, DPP-7820755, 7820755 A01, and DPP-7920796 DGA's attendance at the squid workshop was made possibleby a travel grant from the National Academy of Sciences,Polar Research Board This is Contribution No 202 of the Point Reyes Bird Observatory SUMMARY Seabirds were censused from icebreakers on six cruises in the Ross Sea and the adjacent South Pacific sectorsof the Antarctic Ocean south of 60øSbetween 1976 and 1980 Birds were countedwithin 300 m of one forequarterof the ship when ship's speedexceededsix knots (max 12 knots); censuseswere conductedfor oneø half hour out of everyhour whenthe shipwasunderwayduringdaylight.Daylight was largely continuous.During early summer, 15 December to January, we made 598 half-hour transects;during late summer, 16 Januaryto 21 February, we made 264 transects.Bird density and biomass were calculatedbased on these censusresults.The number of birds in the Ross Seawas calculatedfor each species basedon densitiesand the area of occurrence.Oceanographicmeasurementssuch as seasurfacetemperatureand salinity, the thermal structureof the upper 400 m of water, and water clarity were made at regular,frequentintervals;ice conditions were recorded for each transect Seabirds were collected at 14 localities to determine diet The most clearly defined bird community was associatedwith pack ice and adjacentopenseasinfluencedstronglyby the presence of the packice nearby.It wascomprised of ninespecies andwasdominatednumericallyby AntarcticPetrels (52%) and in biomassby Emperorand Ad61iePenguins(82%) A second,lesswell definedcommunityoverlappingwith the latterwasalsocomprisedof nine species and wasassociated with seasfree of packice but with abundanticebergs.It was dominatedby the AntarcticPetrel.The third communitywascomprisedof 13 specieshavingaffinitieswith the Subantarctic Thesebirds generallyoccurred whereseaswerecompletely freeof ice.It appeared thattheAntarcticConvergence is not an avifaunalbarrierto the extentpreviouslybelieved.The presenceor absence of icein the seainfluenced species' occurrences to a muchgreaterdegree Within the pack ice and icebergzone communities, birds were not evenly distributed.Exceptfor the SnowPetrel,entirepopulations, includingimmature nonbreeders, werecontracted towardbreedingsitesin December(theegg-laying/ hatchingperiod),but by Februarypopulations haddispersed to occupyall available preferred habitat Populationswere also concentratedin areaswhere ocean productivitywashigh,especiallyalongthe AntarcticSlopeFront Only the Snow Petrel appearedto preferpack ice of certainconcentrationover others,and this probablycausedits populationto be more dispersedthan otherspecies.For most packiceseabirds,the presence of oceanographic fronts,whichprobablyenhanced prey availability in the water column,was more important than ice concentration 88 ORNITHOLOGICAL MONOGRAPHS NO 32 in determining their occurrence.Within the ice, some species,such as the Snow Petrel, exhibited a marked preferencefor lead or floe edges,while others, such as the Antarctic Petrel, tended to frequent the centersof leadsand polynyas.Southern Fulmars and Mottled Petrels were closelyassociatedwith areas of icebergconcentration We calculatedan overall avian biomassof 44.47 kg/km in the Ross Sea during late summer Few birds occurred in a large central area over the Ross Sea continental shelfwhere an intensephytoplankton bloom existed.Thus, actual biomass where birds occurredwas double the above figure We hypothesizedthat in the fall, seabirds are even more concentrated because the ice pack then reaches its annual minimum, and numbers of birds at seaare augmentedby the addition of recently fledged individuals Diet overlappedextensivelyamong species.By weight, squid and fish were the dominant prey in all speciesexcept the Ad61ie Penguin which came closestto being an euphausiid specialist Squid dominated diets of other birds in oceanic habitats, and fish dominated diets in shelf waters 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seaswith scatteredice bergsat 66ø37'S,170ø32'E(25 December1977);Light-mantledSooty Albatross(2), SouthernFulmar (7), Cape Petrel (1), Antarctic Petrel (1) Open seaswith scatteredice bergsat 68ø4I'S, 171ø49'W (27 December 1979); SouthernFulrnar (6), Cape Petrel (3), AntarcticPetrel (6), Antarctic Prion (3), Mottled Petrel (3) Six okraspack ice at 69ø39'S,17l*09'E (26 December 1977); SnowPetrel (3) ContinentalSlope(depths600-3000 m) Open seasat 75ø22'S, 174ø52'W(30 December 1979); Antarctic Petrel (12), Snow Petrel (2), Wilson'sStorm-Petrel(7) Open seaswith scatteredice bergsat 73ø59'S, 179ø44'W (31 December 1979); Antarctic Petrel (7), Snow Petrel(8), Wilson'sStorm-Petrel(3) Four oktaspack ice at 71ø32'S,171ø19'E(26 December 1977); Adflie Penguin(2) Seven oktas pack ice near pack edgeat 73ø44'S, 172ø18'E(6 January 1980); Giant Fulmar (1), SnowPetrel (8), Wilson'sStorm-Petrel(17), Brown Skua (1), SouthPolar Skua(2) Three oktas pack ice at 76'01'S, 166ø17'W(30 December 1979); Giant Fulmar (1), Antarctic Petrel (13), Snow Petrel(12), Wilson'sStorm-Petrel(1), SouthPolar Skua(1) ContinentalShelf(depths