THE AUK A QUARTERLY JOURNAL OF ORNITHOLOGY VoL. 79 APRIL, 1962 No. 2 THE BIOLOGY OF THE GIANT PETREL MACRONECTES GIGANTEUS JOHN WARHAM THis accountof the Giant Petrel (Macronectesgiganteus) is based on investigationson MacquarieIsland from 29 December1959 to 12 March 1961 while the author was biologistto the 1960-1961 AustralianNational Antarctic ResearchExpedition (ANARE). MacquarieIsland (latitude 54030' S; longitude159 ø E) is about 1,300 km (800 miles) south of Tasmania and just north of the Antarctic Con- vergence. It lies approximately north and south and is about 34 km (21 miles) long by 4.8 km (3 miles) wide. A narrowbeach terrace borders much of the coastline,but inland the groundrises steeply in screesto a plateau averagingabout 260 meters (800 feet) abovesea level. There are no trees,and the vegetationis composedmostly of mosses,Maori Cabbage (Stilbocarpa), and tussockgrasses (Poa spp.); there is very little growth on the exposedparts of the plateau. The climate is wet with much mist on the high ground,and, as the island lies in the west wind belt, galesare frequent at all seasons. The mean monthly temperatures range from 3 ø C (37 ø F) in June to 7ø C (44 ø F) in January, and even in winter snowseldom remains for long on the groundat sealevel. A generalaccount of the island and of ANARE activities there has been given by Law and Burstall (1956). Macronectes giganteusis a large petrel of the sub-Antarctic zone. It is a surfacenester; too large for natural enemiesto attack it when ashore, it is active on land by day. Most recent studiesof the speciesare con- cernedwith migrationof juvenilesas revealedby the recoveriesof birds bandedas nestlings.On Macquarie Island 7,366 chicksand 1,157 adults have been banded up to March 1961, including 2,946 chicks during the courseof the presentwork. Recoverieshave been discussedby Howard (1956) and Ingham (1959). Sladenand Tickell (1958) and Tickell and Scotland(1961) have dealt with recapturesof birds marked by the Falk- land IslandsDependencies Surveys. Murphy (1936: 584-596) summarized 139 The Auk, 79-' 139-160. April, 1962 140 W^R•r^•x, Biology of Giant Petrel [ Auk [ Vol. 79 what was known of the speciesat that time. Various voyagers,e.g., Biermannand Voous( 1950: 35-39), recordedthe distributionof the Giant Petrel at sea. Falla (1937: 137-145) describedobservations made at Macquarie Island, Kerguelen, at sea, and on the coast of Antarctica. Downeset al. (1959: 69-78) have publishedobservations made at Heard Island. Severalmembers of the ANARE parties contributedto the present study. Messrs.K. Watsonand A. Evans helpedwith the bandingof the chicksin 1961,a projectthat wouldhave been impracticable single handed, and Messrs.J. McNally and A. Thomasalso assisted. Dr. R. Carrick, Dr. D. L. Serventy,Mr. S. Davies, and Miss S. A. Ingham kindly read the roughdraft, and the latter's summariesof the biologicallogs of previous partieshave been of help for comparativepurposes. Mr. W. B. Hitchcock providedthe bandingdata. The paper has alsobeen improvedas a result of a helpful discussionwith Dr. R. A. Falla. THE ANNUAL CYCLE Most Giant Petrels at Macquarie Island laid their eggs from about 5 to 11 October. A minority laid earlier--mostly from 21 August to 10 September.These did not nest coloniallylike the later breeders,but rather as solitary pairs or in small, very scatteredgroups. Such early nesters hatchedtheir eggsfrom about 19 Octoberto 6 November,and their chicks flew from about 1 February to 5 March. Most of the colonialpairs hatched their eggsbetween 5 to 20 December,and their chicksfirst flew between26 March to 30 April. Long beforethe last chicksleft there was an outburstof nest building, either at the edgesof the coloniesor in entirely new places. This was seenduring the first week of February and was attributed to submature birds. Thus on 6 February 1961 a small party of Giant Petrels was dis- turbed from a group of four freshly made nests. The birds were in dark gray-brownplumage, had dark heads, and were away from any estab- lished rookery. During the next three weeksa revival of nest building was noted at many places, but especially around the edges of existing colonies. This activity may have begun as early as 16 December. On 28 February six new nestswere found whoseoccupants were mostly rather dark-plumagedbirds. They were in pairs, braying and displaying mutually. Dark or freckle-headedpetrels predominatedamong the other autumnalnesting parties, and the absenceof light-headedbirds strongly suggestedthat such autumn nest makers belongedto a subadult class of the Giant Petrel community.They couldhardly have beenbreeding birds becausesuccessful pairs were still busy feeding chicks. The only other Vol.^uk 79 ] W^R•.•M, Biology of Giant Petrel 141 bird that frequentlymakes nests in the autumn at MacquarieIsland is the GentooPenguin (Pygoscelispapua), which is presentthroughout the year. Somebreeding stations of the Giant Petrel, suchas thoseon islandsoff Antarctica, are desertedduring the southernwinter, but at Macquarie Island the rookerieswere occupiedat all seasons.Successful breeders may have goneto sea after the departureof the chicks,but the colonieswere soonreoccupied by adults and continuedto be used during the winter. At that seasonmany Giant Petrels sat in pairs in the coloniesby day and engagedin energeticmutual display. Such activitieswere seeneven when the groundwas under snow,and pale-headedbirds predominated.This indicatesthat they were all mature individuals,a hypothesissupported by the captureon 2 June of a bird that had been bandedon its nest at the samecolony in November 1954 and by three similar recoverieson 25 July 1959 by S. Csordas. Four Giant Petrels banded as chicks have now been recovered on the island, three during the course of the present study: 130-03555, a white-phase bird banded 13 March 1955 and shot on 5 August 1958, three years and five months after banding. 130-o0160,banded 24 February 1957 north of Aurora Point and shot when a dark- eyed, dark-brcwn bird at Bauer Bay on 30 August 196o, three years and six months later. 13o-o8175, banded 22 February 1956 at Major Lake, was caught and released at Hasselboro Bay on 4 October 196o, four years and seven months after banding. It was describedby its captors as a "uniformly grey bird." 130-o2806,banded 24 February 1957 north of Aurora Point, was caught and released on lO March 1961, four years later on a rookery near Douglas Point. It was then a dark-eyed, dark gray-brown bird with a dark head and may have been one of several autumn-nesting birds on this rookery. The last of these recaptures,and the absenceof other birds banded as chickson the colonies,suggests that the Giant Petrel doesnot breed until at least five years old; this is in agreementwith Downes et al. (1959: 75), who came to the same conclusion from the lack of recoveries of chicks at Heard Island. Tickell and Scotland (1961: 261) report a banded bird nestingat eight yearsold but give no detailsof its age at first breedingor of its sex or plumage. POPULATION AND NEST SITES A censusof breederswas attempted during January and February 1961 when all the rookerieswere inspectedand the chicksbanded. A total of 2,846 weremarked with bandsissued under the AustralianBanding Scheme and inscribed "Write Wildlife CSIRO Canberra Australia." Twenty-one 142 W^Ra^•r,Biology of GiantPetrel [ Vol.^uk 79 other chicks too small to band were also counted. There were more than these2,867 chickson the island, as a few isolatednests on both west and east coastswere known to have been overlooked; a yard-by-yard search of the ruggedtussock-clad rocks along the indented coastlinewas im- practicable. It is estimatedthat not more than 150 such nestlingswere missed. Per contra, someof the countedchicks must have died through various natural causesafter banding. In all, the chick production for the 1960-1961 seasonis believedto have been about 3,000 birds. Assumingthat the breedingsuccess of Giant Petrels was 57 per cent that year, then about 5,300 eggswere laid during the season,the product of 10,600 breedingbirds. However, from the data available, the breeding successcould have been nearer 70 per cent, which givesonly 8,600 breed- ing birds. The total population,allowing for all classesof nonbreeders, was certainly much greater than this. The petrels bred in 70 colonies. All but one of these colonieswere on the west side of the island where they were exposedto the strongprevail- ing winds. Most were quite small, producingan average of 41 chicks. The three largestheld 167, 160, and 126 young birds, the smallestonly four to six at the time of banding. The greatest concentrationsof rookeries in both the 1959-1960 and 1960-1961 seasonswere in the area of Caroline Valley in the southwest corner,where 606 chickswere counted, and alongthe coastalrocks between Mawsonand Aurorapoints, where there were 940 chicksduring the census. Previousrecords suggest that these have always been the best breeding areas. Complete details of the 1960-1961 colonies are deposited at ANARE headquartersin Mdbourne. From previousplots showingthe sitesof someof the breedingcolonies it is clear that the locationsof many of the larger and more accessible rookeriesare relatively permanent. Severalwere evidently on the same sitesin 1952 and 1954 as in 1958-1961. A morecomplete mapping by K. Keith in 1956 reveals that 12 of the larger colonieswere in the same placesthen as they were during my stay on the island. Someshifting occurs;large coloniesmay split into two, and small onesmay coalesce. In the region of Major Lake the sites of the rookerieshave changed severaltimes between1952 and 1960. One colony near Mawson Point in which about 20 chicks were reared in the 1959-1960 season was not re- occupiedthe next year; insteada new site was usedabout 400 metersto the south, apparentlyby the samebirds. The faithfulnessof Giant Petrels to a particular rookery or breeding area was shownby six recapturesof breedingbirds during 1959-1961 at the samecolony or in the samegeneral area wherethey were bandedas Vol.Auk 79 ] W^R•^•, Biology of Giant Petrel 143 adultsseveral years before.