NEST-SITE COMPETITION BETWEEN ADELIE AND CHINSTRAP : AN ECOLOGICAL INTERPRETATION

WAYNE TRIVELPIECEAND NICHOLASj. VOLKMAN1 The Mount Desert Island Biological Laboratory, Salsbury Cove, Maine 04672 USA

ABSTRACT.--A study of nest-sitecompetition between Adelie (Pygoscelisadeliae) and Chinstrap (P. ) penguinswas conductedduring the 1977-78 austral summer at Point Thomas, King George Island, Antarctica. Data were collected on the timing of the reproductive cycle, the reproductivesuccess, and the behavioral interactionsbetween Adelie and Chinstrap penguinsin areas of competition. Adelie Penguinsin mixed sites had their nestsusurped by arriving Chinstrap males, and reproductivesuccess was significantlydepressed for Adelie pairs nestingin theseareas. We suggestthat nest-sitecompetition is a relatively recent phenomenoncorrelated with dramatic populationincreases among pygoscelidsin responseto the increasedavailability of in areas of past whaling activities. Received2 February 1979, acceptedI May 1979.

OBSERVATIONShave been recorded of Chinstrap ?enguins (Pygoscelisantarctica) usurping nest sites from congenericGentoo (P. papua) (Downes et al. 1959) and Adelie penguins (P. adeliae) (Crawford 1974). These relatively recent observations of interspecific nest-site competition are especially interesting in light of reports of dramatic increases in pygoscelid populations over the last century (Sladen 1964, Conroy 1975, Croxall and Kirkwood 1979, this study). During the 1976-77 austral summer, we observedChinstrap Penguinsinvading Adelie coloniesand aggressively displacing Adelies from occupiednest sites at Point Thomas, King George Island, Antarctica. In the 1977-78 field season, we returned to Point Thomas to investigate nest-sitecompetition between Chinstraps and Adeliesover the courseof the breeding season.The specificobjectives of the presentstudy were (1) to describethe aggressive interactions between Chinstraps and Adelies in areas of nest-site competition, (2) to document the effect of this competition on the reproductive successof both , and (3) to interpret thesedata in light of recent literature pointing to apparent increases in pygoscelidnumbers in the Antarctic.

STUDY SITE AND METHODS

Our study was conductedat Point Thomas, King George Island, (62ø10'S, 58ø30'W) from 1 November 1977 to 21 February 1978. The Point Thomas site containedbreeding colonies of all three pygoscelidspecies in both an east and a west rookery separatedby a glacial tongue 2-km wide (seedefinitions). A censusof the pygoscelidpopulation at Point Thomas was conducted1 2 weeks followingthe peak of egglaying for eachspecies. Chinstrap and Gentoodata are from individual counts of nest sites in each rookery. Adelies were counted in small coloniesand estimatedin large coloniesby determiningthe colony'sarea and using a figure of 1.13 pairs/m2, the mean of 5 small (n < 150 pair) Adelie coloniesgraphed to scale. The populationestimates for 1977 are shown in Table 1. Data were collectedon the dates of arrival, egg laying, chick hatching, crecheformation, and reoc- cupation for Adelie and Chinstrap penguins(definitions after Sladen 1958). These observationswere made daily from 1-11 November 1977 and at 4-day intervals thereafter. To facilitate data collection, penguinswere bandedwith flipper bandsthat were color codedto indicatesex. A 's sex was deter- mined by cloacal examination (Sladen and LeResche1970, Ainley 1975a), by the bird's positionduring

Presentaddress: Box 1507, Sedona,Arizona 86336 USA. 675 The Auk 96: 675-681. October 1979 676 TRIVELPIECEAND VOLKMAN [Auk, Vol. 96

TABLE 1. A 12-yr comparisonof pygoscelidpopulation growth in the east and west rookeriesat Point Thomas, King GeorgeIsland, Antarctica.

Number of breeding pairs 1965a 1977b Yearly increase Adelie 10,400 18,000 14% Gentoo 2,152 2,613 10% Chinstrap 240 1,015 35%

a Data from Conroy 1975. b This study.

an observedcopulation, or by tread markson the back of one memberof a breedingpair made by the male'sfeet during copulation.Nest sitesunder study were mapped(with referenceto codedbamboo poles driven into the ground in the colonies),and inter-nest distances(from nest rim to nearestnest rim) were measuredin single-and mixed-speciessites (see definitions). Eggs were marked with permanentink for identification.Nest checksfor reproductive-successdata were accomplishedby slowlyapproaching the penguins until they assumed an upright posture, allowing the investigators a clear view of the nest contents.Intrusion into the sites under study was kept to a minimum. The measuresof reproductive successused were: the number of eggs laid per pair, the number of chickshatched per pair that laid eggs,the number of chickssurviving to crecheper pair that laid eggs, and the number of chickscreched per chickshatched. Inter- and intraspecificstatistical comparisons for mixed- and single-speciessites were performed with the Kruskal-Wallis test for ranked data. Whenever a significant"H" value was obtained,Dunn's test for multiple comparisonsof unequalsample sizes was performed to differentiate significant groups. Aggressiveencounters between Adelie and Chinstrappenguins were recordedopportunistically during observationsat one of nine mixed-speciessites in early November. An observationperiod beganwhen an arriving ChinstrapPenguin made physical contact with an incubatingAdelie .Data collection continued for 1 h, or until one of the participants left the area.

DEFINITIONS

Rookery.---A geographicalarea, usuallya portion of an island or peninsula,that containsone or more colonies(groups) of breedingbirds and a landing beach or beachesthat the usedto reachthe nesting areas from the sea (Penney 1968). Colony.--A geographically continuousgroup of breeding birds whose territorial boundaries are con- tiguous (Penney 1968). Mixed-speciessites.--Discrete areas within AdeliePenguin colonies that werelater invadedby arriving Chinstrap Penguins.These areas were not all coloniesper se (accordingto Penney'sdefinition); some were separatedfrom other such sitesby homogeneousgroups of Adelies or Chinstraps. Single-speciessites.--Colonies of one speciesonly, or siteswithin large coloniescontaining both species that were separated from any mixed areas by at least 50 m.

RESULTS

Reproductive cycle.--Adelies arrived at the west rookery on Point Thomas in large numbers beginning20 October 1977, the day after ice left Admiralty Bay (P. Presler, pers. comm.). The peak of Adelie egg laying was 3 November, with 97% of all clutchescompleted by 15 November 1977. Chinstraps began arriving at the rookeryon 2 November, and 95% of the breedingpairs were presentby 18 Novem- ber 1977. The Chinstrap egg-laying peak was 27 November, and the Adelie and Chinstrapegg-laying and chick-hatchingpeaks were 24-25 days apart during the 1977-78 breedingseason (Fig. 1). The Chinstrap arrival and subsequentnest-site competitionoccurred shortly after the peak of Adelie egg laying, at a time when the male of the Adelie pair was incubating the completedclutch, the female having October1979] PenguinNest-Site Competition 677

CHINSTRAP

ADELIE

1 • 30

Ch instrap Takeovers- Tota• nests

i Egg Laying Period ChickChinstrapHatchingTok ...... PeriodNests wJeggs I

15 23 31 7 15 23 I 9 17 25 2

OCT NOV DEC JAN

Fig. 1. A comparisonof the timing of the nestsite competitionbetween Adelie and Chinstrappenguins with respectto their breedingcycles. The peak of Adelie nestlosses to Chinstrapsoccurred between 7- 11 November, shortly after the peak of egg laying. The relative numbersof eggslaid and/or chicks hatchedper nest-checkdates are indicatedby the widths of the bars.

returned to the sea to feed. The timing of the Adelie nest-sitetakeovers by Chinstraps is summarized in Fig. 1. In nine mixed-speciessites, 60% (89/148) of the Adelie nestswith eggswere lost to arriving Chinstrap Penguins. An additional 66 nests occupied by Adelie pairs were later claimed by Chinstraps,although the Adelieswere not known to have laid eggs before they disappeared.A total of 66% (141/214) of all Adelie nests under study in the mixed sites were used by Chinstrap pairs for their own reproduction during the breeding season.The sites under study in the west rookery represented approximately one-half of all such areas of interaction in that rookery; thus, we estimated that 300 nestsused for breeding by Chinstraps were usurped from Adelies. Chinstraps in the east rookery also engaged in nest-site competition with Adelie pairs. It was determined that approximately 40% (110/270) of the Chinstrap pairs nesting in this rookery used nests previously occupied by Adelie pmrs. The data from this rookery were collected during weekly visits, and, therefore, an accurate estimate of the number of eggs laid in Adelie nests lost to Chinstraps was not possible. Conflict description.--In all competitive encountersobserved between the two species,a male Chinstrap aggressivelycontested the incubating Adelie for its nest site (n = 18). Further evidencethat males played a major role in such interactions was obtained by banding lone unsexedChinstraps on newly acquired Adelie nests where the actual conflict was not witnessed. The birds were sexedfollowing pairing, and in 24 casesall of the Chinstraps on newly acquired nests proved to be males. The duration and intensity of the conflict between the male Chinstrap and incu- bating Adelie proved to be extremely variable. In 8 of the 18 aggressiveencounters, the Adelie lost its nest within 10 min, while 6 other encounterswere unsettled at the conclusionof a 1-h observationperiod. In the encountersthat were rapidly decided, the Chinstrap approachedthe incubating Adelie and began pecking at its head while 678 TRIVELPIECE AND VOLKMAN [Auk, VoL 96

Fig.2. Progressivestages of anaggressive interaction between an incubating Adelie Penguin and an arrivingChinstrap Penguin. (a) Adelie on nest incubating egg is attackedby a maleChinstrap. (b) and (c)Chinstrap aggression continues, and Adelie moves off itsformer nest until it is outsidethe nest cup, nowdefended by the Chinstrap.Note the position of the eggin thecenter of the nest (d) Chinstrap, havingusurped nest, is shown advertising for a matewith the ecstatic vocalization. Note that the Adelie's eggis still intactfollowing the nestloss to the Chinstrap.

repeatedlystriking it withits flippers. The incubatingAdelie gradually relinquished its nest under the continuedaggression of the Chinstrap, until the Chinstrappos- sessedthe nest.Upon acquiring the nest,the Chinstrapbegan advertising with the ecstaticvocalization first noted by Wilson(1907) in the AdeliePenguin. This vocal- ization has been describedas a display that is primarily a strong advertisement of self, mostlygiven by lone unpairedbut territorialmales; it tellsother males to stayaway and unpairedfemales to approachif readyto pair (Ainley1975b). Photographsdepicting the nest-site competition between the two speciesare shown in Fig. 2. The defeatedAdelie made several unsuccessful attempts to regainits nest,approaching its egg(s)in a distincthunched-back posture, while calling at the nestwith the 1ocomotoryhesitance vocalization observed between Adelie pairs duringnest relief (Ainley 1975b). Each approach elicited an attackfrom the Chin- strapnow in controlof the nest,and after severalexchanges of bill grabbingand flipperstriking, the Adelieleft the colonyin the directionof the beach. The Adelie'seggs were intact in nineof the 18recorded nest-site confficts following the Chinstrap'sacquisition of the nest.The eggswere normally lost or discarded from the nestduring the Chinstrap'scopulatory and nest-buildingactivities; how- ever, we recordedthe hatchingand successfulrearing of an Adeliechick to creche age by a Chinstrappair. DisplacedAdelie Penguins left for the seato feed and returned2-3 weekslater (with otherunsuccessful breeders and younginexperienced birds)during the reoccupationphase of the breedingcycle. Reoccupying Adelies were not observedto attack incubating Chinstraps. Of 12 nestswith eggsbeing incubatedby Chinstrapson one nest-checkdate, however,6 were found to be occupiedby a singleAdelie and 6 by an Adeliepair on the followingnest-check October1979] PenguinNest-Site Competition 6 79

TABLE 2. Comparisonof the mean (-D and standard error (SE) for the number of eggslaid/pair, chicks hatched/pair,chicks creched/pair, and chickscreched/chicks hatched for Adelie and Chinstrappenguins breeding in single- and mixed-speciessites.

Chicks Number of Chicks Chicks creched/ eggs/ hatched/ creched/ chicks pair pair a pair a hatched Sites of Adelie only (n = 13) œ 1.85 1.28 1.07 0.83 SE 0.02 0.09 0.10 0.04 Sites of Adelie mixed (n = 9) .• 1.77 0.37 b 0.30 b 0.76 SE 0.04 0.04 0.06 0.11 Sites of Chinstrap only (n = 3) • 1.91 1.09 1.00 0.91 SE 0.05 0.10 0.08 0.02 Sites of Chinstrap mixed (n = 9) .• 1.85 1.10 1.06 0.95 SE 0.04 0.05 0.06 0.02

a Dunn's multiple comparisontest (Hollander and Wolfe 1973: 124-126) was used to determine significantdifferences between pairs within the categorieschicks hatched/pair and chickscreched/pair. b p < 0.001; Kruskai-Wailistest for ranked data.

date. In two of these 12 cases,the eggsof the Chinstrap were still intact in the nest. One Adelie continuedincubating the eggsfor 2 weeks,the other for 1 week; neither acquired a partner during this time, and no eggswere known to have hatched. Inter-nestdistances between Chinstrap nests in mixed-speciessites were signifi- cantly greaterthan the inter-nestdistances of the originalAdelie nestsin thesesame sites (t = 8.98, df = 63, P < 0.001). The mean (-+SE) inter-nest distanceswere 370 -+ 8 mm for Adelies and 516 -+ 15 mm for Chinstrapsin thesemixed sites. Chin- straps having acquired a nest from an Adelie would shift the positionof the new nestsite to complywith spacingpatterns typical for their speciesin single-species sites (501 -+ 11 mm). Reproductivesuccess.--Adelies in mixed sitessuffered greater egg lossesthan Chinstrapsin mixed sites or either speciesin homogeneoussites (Table 2). The numberof chickssurviving to crecheper pair that laid eggswas significantlylower for Adelie pairs nestingin mixed siteswhen comparedto Chinstrapsin mixed sites or either penguin in single-speciessites (X2 = 18.84, P < 0.001). The number of chicks hatched per pair was also significantlydepressed in Adelie mixed sites (X2 = 20.28,P < 0.001). There were no differencesin eggproduction or the number of chickscreched per chickshatched between Adelies and Chinstrapsin mixed- or single-speciessites. No significantdifferences were found for any of the reproductive variablesmeasured among homogeneous Adelie sites, homogeneous Chinstrap sites, and Chinstrapsnesting in mixed-speciessites (Table 2).

DISCUSSION

The Adelie Penguin begins its reproductive cycle in the latitudes of the Antarctic Peninsulain early October, with clutchesinitiated betweenlate October and early November (Sladen 1958: 84), while the Chinstrap Penguin has a reproductivecycle that begins3-4 weekslater (Conroy et al. 1975a, b). Perhapsthe asynchronyin the pygoscelidreproductive cyclesbetter explains the Chinstrap's advantage in nest-site competitionthan do any inherent differencesin aggressionbetween the two species. The peak of nest-siteconflicts occurred shortly after the peak of Adelie egg laying (Fig. 1). At this time, Chinstrapsarrived fresh from the sea and were aggressively 680 TRIVELPIECEAND VOLKMAN [Auk, Vol. 96 competingwith broodymale Adeliesthat had beenfasting for 3-4 weeks.Although the relative weightsfor Adelies(4.5 kg) and Chinstraps(4.3 kg) are similar(Volkman et al. in press),the newly arrivingChinstrap may have a slightadvantage in bodymass towards the end of the male Adelie fasting period. Because there were no differences in the number of chicks that hatched and survived to crecheage in mixed- or single-speciessites, and significantlyhigher egg losses were found only for Adelies in mixed sites, the Adelie-Chinstrap conflict appearsto be the major sourceof the Adelie egg losses.Furthermore, Adelie chicks reared in mixed sites did not differ from Adelie chicks in single-speciessites for the growth parameters:body weight gain and culmen, flipper and foot growth (Volkman and Trivelpiece MS). This indicateslittle effect of the interspecificconflict on the chick phase of the reproductive cycle. Although quantitative behavioral studies of the Chinstrap have not been con- ducted to date, there are numerous anecdotal referencesto its aggressivenature summarized by Murphy (1936). Downes et al. (1959) reported Chinstraps on Heard Island taking over Gentoonests and incubatingtheir eggs.Crawford (1974) reported a transientChinstrap at Cape Hallett that was known to have acquirednine different Adelie nestsduring its brief 15-day visit. It is interestingto note that Sladen(1955) makes no mention of conflict between the two speciesat , though both are presentin large numbersand breed in closeproximity. White and Conroy (1975) suggestedthat Chinstraps may be forced to rocky slopeswhere they breed near Adelies. A quantitative analysisof habitat selectionamong pygoscelidsin a three- speciesrookery is under way ( Volkman and Trivelpiece MS), though the nestswe studiedfor the analysisreported here were on relatively flat, unbrokenslopes, hab- itat typicallydescribed for Adeliesby earlier authors(Sladen 1955, Conroy et al. 1975a, White and Conroy 1975). Recent censusdata indicate that pygoscelidpenguin populations are on the in- creasein Antarctic fringe areas such as the Scotia Arc (Sladen 1964, Conroy 1975, Croxall and Kirkwood 1979). More than any other species,the Chinstrap appears to have extendedits range and increasedmarkedly in numbers during the past few dec- ades(Conroy 1975, Croxall and Kirkwood 1979).Although all threepygoscelid species have increasedin number over the past 12 yr at Point Thomas, the 35% average annual increaseexhibited by Chinstrap Penguinsat this location is by far the most dra- matic change(Table 1). It has been generally acceptedthat the major causeof the recentpenguin popoulation increases is the decimationof the whale populationsof the southern oceansduring the latter half of the 19th and the early 20th centuries. Krill (prey of whalesand penguins)freed by the decreasein whale numbershas beenesti- mated to be enoughto rear 2-3 x 108penguins/yr (Sladen 1964, Emison 1968). Al- thoughpenguins are not the only benefactorsof the krill surplus,the rapid increasein their numbers at locationsof past whale concentrationsin the Antarctic supportsthe contentionthat penguinshave respondedto the increasedfood supply. It is interesting that actual possessionof a nesting territory does not outweigh, in defenseterms, lossof conditionby the incubator. We hypothesizethat the nest-site competitiondescribed in this study is a direct result of the recent expansionof pygoscelidpopulations. Penguins are long-livedbirds, and conditionsat theserook- eries have been different in the recent past, as evidencedby the substantialincreases in Adelie and Chinstrap numbers (Table 1). It is possiblethat Chinstrap pairs orig- inally selectednesting sites on what was then the Adelie colony'speriphery. As populationsexpand, Adelies,establishing breeding territories before the Chinstraps' October1979] PenguinNest-Site Competition 681 arrival, might be selectingnest sitesviewed by the later arriving Chinstraps as their own. The ensuing conflict would clearly give the arriving Chinstrap an advantage over the fasting Adelie and account for the results observed here.

ACKNOWLEDGMENTS

Field support for the austral summer 1977-78 was provided by the Polish Academy of Sciences, Arctowski Station, where the authors were visiting scientists.We are deeply indebted to J. Jersak, station leader, and many other personnelat Arctowski for their assistance,allowing us to completethis research. Further logistic support was provided by the U.S. Antarctic Research Program (USARP) and the RV Hero under the command of Captain P. Lenie. We thank R. G. Butler for his helpful criticisms of earlier versionsof the manuscript. H. Church provided photographicassistance, and D. S. Miller provided encouragementand financial assistanceduring the manuscript's preparation. This study was financed by a grant to D. Muller-Schwarze from the Division of Polar Programs, National Science Foundation, Washington.

LITERATURE CITED

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