Competition for Nest Sites Between Kelp Gulls (Larus Dominicanus)And Terns (Sterna Maxima and S

1068 ShortCommunications [Auk, Vol. 115

The Auk 115(4):1068-1071, 1998

Competition for Nest Sites Between Kelp Gulls (Larus dominicanus)and Terns (Sterna maxima and S. eurygnatha)in Patagonia

FLAVIO QUINTANA 1 AND PABLO YORIO 2CentroNacional Patag6nico, CONICET, Boulevard Brown s/n 9120, PuertoMadryn, Chubut,Argentina; and WildlifeConservation Society, New York, New York10460, USA

Seabirdsfrequently nest in mixed-speciescolonies quantifytheir temporaland spatialpatterns of set- (Burger 1985). Severalstudies have showndiffer- tlement, describeand quantify gull aggressivebe- encesin habitat and nest-siteuse amongseabirds havior toward terns, and determine the allocation of (Buckleyand Buckley1980, Croxall and Prince1980, nesting space as a result of spatial interactions Fasola and Canova 1992). However, some degree of amongthese species. overlapin habitat and nest-siterequirements may Study area and methods.--PuntaLe6n (43ø04'S, existbetween species, resulting in interspecificcom- 64ø02'W)is located10 km southof the mouthof Gol- petitionthat oftenleads to nest-sitedisplacement or fo Nuevo,Chubut, Argentina. The coastin this area even the replacementof one speciesby another is characterizedby gravel beacheswith extensive (Buckleyand Buckley1980, Burger 1985). Spatial in- cliffs30 to 100m high.A siltplatform seaward of the teractions between seabirds can be more intense in cliffs and coveredby vegetation(mainly Suaedadi- areaswhere habitatis limited or when the popula- varicata,Atriplex lampa, and Lyciumchilense) is used as tion of oneof the speciesis expanding(Burger and a nestingsite by severalseabird species, including Shisler1978, Trivelpiece and Volkman1979, Duffy Kelp Gulls (6,500 pairs), Royal Terns (650 pairs), 1983, Ainley 1990). Cayenne Terns (1,150 pairs), Imperial Cormorants At manymixed colonies of larids,larger species of- (Phalacrocoraxatriceps; 2,650 pairs), Neotropic Cor- tendisplace smaller species from their territories, oc- morants(P. olivaceus; 100 pairs),Rock Shags (P. ma- casionallyforcing them to abandonthe breeding gellanicus;1 pair), and Guanay Cormorants(P. bou- area (Burgerand Shisler1978, Burger 1985).In ad- gainvillii;2 pairs;numbers from Yorioet al. 1994). dition, larger larids generallyarrive at breedingsites During 1990to 1992,we obtainedinformation on earlier(Morris and Hunter 1976, Burger 1985), which temporaland spatial settlementpatterns of Royal gives them a competitiveadvantage over smaller Terns,Cayenne Terns, and Kelp Gulls.To determine species.Gulls (Larusspp.) frequently have been in- settlementpatterns by terns,we countedthe number volved in the displacementof other speciesfrom of nestsevery two to threedays from mid-September breedinghabitats (Thomas 1972, Burger 1979, Brad- to early November,using spotting scopes (20 to 45X ) ley and Monaghan1986, Furnessand Monaghan from an observationpoint on the cliffs 50 to 70 m 1987).Terns appear to be one of the mostaffected fromthe colony.We monitored the growthof thecol- seabirds,and sometern populationshave decreased onyby mappingits sizeand shapeat intervalsof two asa resultof populationexpansion by gulls(Thomas to four days.We obtained settlement patterns of Kelp 1972,Nisbet 1973,Greenhalgh 1974, Courtney and Gulls from late August to mid-Novemberthrough Blokpoel1983, Parnell et al. 1988,Ainley and Hunt weekly countsof breeding individuals at a study 1991,Blokpoel and Scharf1991). area closeto the tern colonyand during 1990 and At Punta Le6n, Argentina, Royal Terns (Sterna 1991by monitoring53 and 40 markednests, respec- maxima)and CayenneTerns (S. eurygnatha) breed in- tively,every two to threedays. The 20 x 20 m study termingledin a densecluster of nestswithin a Kelp area used to monitor gull settlementwas locatedin Gull (Larusdominicanus) colony. The Kelp Gull col- the southernpart of the Kelp Gull colony. ony increasedin size from 3,200 pairs in 1982 to To studythe effectsof Kelp Gull territorialbehav- 6,500 in 1995, and gulls currently occupyalmost all of the availablenesting space (Yorio et al. 1994).Giv- ior on tern settlement,during 1990we conducted23 en the overlapin nest-siteuse and the currentex- one-hourobservations during whichwe scannedthe pansionof KelpGulls, we expectedthat competition colonyfor settlingterns (n 73 groups).When a for nesting spacecould result in negativeeffects on group of settling terns was detected,we recorded both speciesof terns.In this paper,we document whetherthe terns were attackedand whetherthey spatial interactionsbetween Kelp Gulls and Royal were totally or partially displaced by territorial and Cayenne terns at Punta Le6n, Patagonia.We gulls. We defined "settling terns" as a variablenum- ber of birdsof bothtern speciesthat tried to settleat the colonyperiphery. Most pairs of ternsbuild nests E-mail: [email protected] and lay theireggs on the sameday that theysettle in Addressfor correspondence. the colonysite (Quintanaand Yorio 1997).We de- October1998] ShortCommunications 1069 fined"settled tern" as any bird sittingon a nest,with 20 • or without eggs.We defineda gull "attack"as any behaviorconsisting of a short chaseby a peripheral territorialgull (and on occasiona peck withoutes- tablishingphysical contact) that resulted in theflight of the group of settlingterns or of an incubatingtern adjacentto that gull territory.We defineda peripheral territorial gull as any bird holding a territory, with or without a nest, located at a distance of less than 3 m from the tern colony. To quantifyKelp Gull territorialbehavior (gull ato tacks)toward terns, we madeobservations with spotting scopesand binoculars from the observation point on the cliff throughoutthe daylight hours (0700 to 2100EST) during most of thetern breeding cycle. Dur- ing eachhour (n = 307),we quantifiedthe rate of gull attacksdirected at settlingor incubatingterns. During

1991and 1992,we analyzedthe differencein territo- 0 I I I I I I I I rial behaviorbetween gulls nesting near settling terns 1 2 3 4 5 6 7 8 versusthose nesting near settled terns. Week of the breeding cycle To documentthe displacementof nestingpairs of Kelp Gulls by settling terns, we monitoredat two- FIc. 1. Seasonalvariation in rate of Kelp Gull at- dayintervals 38 and40 gull pairsbreeding at thetern tacks (• + SD) on Royal Terns and Cayenne Terns colonyperiphery during 1991and 1992,respectively. during 1990(week 1 = 18 to 24 October),Punta Le6n, During eachnest check, we recordedthe presenceof Patagonia. gulls at the nest,the stateof the breedingcycle, and the estimateddistance of the nestto the tern colony periphery.If the gull nestdisappeared (loss of nest- in the direction of vegetatedground and in areas ing materialand eggs),we recordedwhether the site withoutgull territories.We did notquantify gull nest- was occupiedby a nestingtern. ing densitiesat the time of tern settlementto avoid Results.--KelpGulls settled at the colonyover a disturbanceto gulls and terns.However, qualitative two-monthperiod (Yorioet al. 1994).Gulls arrived observationsshowed that during all years,sections of at the colonyand selectedterritories that were dis- the gull colonyhad lowernest densities than the sec- persedthroughout the area.Late arrivals settled be- tionswhere ternsbred. Moreover,during 1991,terns tweenexisting territories. Kelp Gull nestingdensity changedthe location of theircolony to anarea of high- was variable,ranging from 0.002 to 0.75 nests/m2 er gull densitythan that used in theprevious seasons. (Yorio et al. 1994). Colony formationby Royal and Thisnew area had oneof thehighest nesting densities Cayenneterns was initiatedby a group of about100 of KelpGulls on theentire colony (0.50 nests/m2; Yor- to 150 individuals that settled on available bare io et al. 1994).Terns continued to breedin thispart of groundamong Kelp Gull nests,and the colonygrew the colonyfor the nextthree years. throughthe settlement of new groupsof bothspecies The settlementof tern groups(g = 25.7 individu- at the colonyperiphery (Quintana and Yorio1997). als,range 5 to 120,n = 73) at the colonywas affected The resultingnesting density for both tern species by the presenceof territorialKelp Gulls, with and combinedranged between 9 and 11 nests/m2 (Yorio withoutnests, at the peripheryof the colony.During et al. 1994).During all studyyears, Kelp Gulls settled 1990,79.4% (n = 73) of the tern groupsthat we fol- more than a month earlier than the terns. Gulls start- lowedduring settlement were attacked by peripheral ed claimingterritories during late Augustand early gulls. Attacks by Kelp Gulls frequently forced September,but bothtern speciesdid not settleat the groupsof settlingterns to take flight, on occasion colonysite until the secondor third weekof October. causingthe group to leavethe colony.Of 58 groups However,given the asynchronyin timing of breed- that were attacked, 31% were forced to leave the site ing,some Kelp Gull pairssettled adjacent to thetern and53.4% were partially driven away by gullsby the colonyduring and after the tern layingperiod. Both end of the observationperiod. In 15.6%of the cases, gulls and ternsstarted laying in mid-October(Yorio we could not determine the results of the interaction. et al. 1994, Quintana and Yorio 1997). The mean numberof Kelp Gull territorial attacks AlthoughKelp Gulls settled in the nestingarea be- per hourtoward tern groupswas 13.8 + SD of 8.8 (n forethe terns,both tern species were able to establish = 121)in 1990.The numberof attacksper hour de- andbreed among gull territoriesin all threeseasons. creased significantly as the season progressed The growthof the tern colonythrough the settlement (Spearmancorrelation, rs = -0.98, P < 0.01;Fig. 1). of groupsof birdsat the colonyperiphery was mainly Mostspatial interactions between gulls and terns oc- 1070 ShortCommunications [Auk,Vol. 115

Le6n exhibit overlapin habitatuse (Quintanaand []Settling terns Yorio 1997), and the larger Kelp Gulls arrive at the [] Setted terns colonyearlier, Royal Ternsand CayenneTerns were able to form breeding coloniesby displacingthe gulls. Aggressivebehavior by Kelp Gulls toward terns seems to be related to defense of breeding space,as suggestedby the significantlyhigher rate of attackstoward settling versussettled terns. The settlementand nesting strategiesof Royal Ternsand CayenneTerns appear to be important de- terminantsof competitiveinteractions with gulls. Althoughlarge body size and early timing of settlement canbe importantdeterminants of the outcome of spatialinteractions (Burger and Shisler1978, Duf- fy 1983, Burger 1985), settlementin dense groups may allow smallerbirds to displacelarger and earlier settlingbirds that nestat lowerdensities (Burger 1985). At Punta Le6n, Royal Terns and Cayenne Terns settle at the colony in relatively large and densegroups that are ableto displacenesting Kelp Gulls. Similar observationshave been reportedfor other tern species,such as SandwichTerns (Sterna 1991 1992 Year sandvicensis)nesting with Black-headedGulls (Larus ridibundus;Taverner 1970, Smith 1975, Veen 1977), El- FIG. 2. Rate of Kelp Gull attacks(• -+ SD) toward egant Terns(Sterna elegans) nesting with Heerman's settling and settled Royal Ternsand CayenneTerns Gulls (Larusheermanni; Barrie 1975), and Arctic Terns during 1991 and 1992, Punta Le6n, Patagonia. (Sternaparadisaea) nesting with Herring Gulls (Larus argentatus;Bianki 1977).In this respect,the nesting traitsof the crestedtern groupgive theman advan- curredduring the periodof tern settlement,and the tageover other tern species,many of whichare or highestrate of gull attackswas during the first two havebeen affected as a resultof expandinggull pop- weeks of the terns' breedingcycle (Fig. 1). The av- ulations(Thomas 1972, Nisbet 1973,Courtney and eragenumber of gull pecksand chasesper hourdur- Blokpoel1983, Ainley and Hunt 1991,Blokpoel and ing 1991 and 1992 was significantlyhigher toward Scharf1991). settlingthan settledterns (Wilcoxon test; 1991,Z = Even though we found no evidencethat terns at 3.43, P • 0.001; 1992, Z = 4.56, P • 0.001;Fig. 2). Punta Le6n currently are affected by spatial com- Eventhough some Kelp Gull pairsalso settled after petitionwith Kelp Gulls,an increasein gull nesting ternshad selected the colony site, gulls were never ob- density due to population expansionmay interfere serveddisplacing terns that had alreadylaid eggs.In with ternsettlement in the future.In addition,Royal only a few occasions,gull territorial behaviorforced Ternsand CayenneTerns may pay a costby nesting an incubatingtern to leavethe egg exposed for several among Kelp Gulls owing to delayedbreeding, ener- minutes.In contrast,gull territoriesthat had nests, gy lossfrom territorialbehavior, and/or prolonged someof themwith eggs,were taken over by settling exposureof eggsduring territorialinteractions. Any ternsduring all threeyears of the study.In 1991and such costsprobably will increasewith increasing 1992,28.9% (n = 38) and 20% (n = 40), respectively, densitiesof nestinggulls. During 1991,for example, of theKelp Gull pairs nesting in thevicinity of thetern the locationof the tern colonychanged to an areaof colonylost their territories and nests to settlingterns. highergull density.During that season,the growth Gull nestdisplacement occurred as the tern colony of the tern colonywas slowerthan in previoussea- grew and groupsof settling terns intruded into the sons (Quintana and Yorio 1997). Unfortunately,it territoriesof gulls,taking overthe nestingspace. On cannotbe determinedwhether the delay in settle- occasion,Kelp Gull nestswere graduallysurrounded ment was a direct consequenceof territorial inter- by ternpairs, and incubating gulls retained their nests actions between gulls and terns, a result of lower for sometime before abandoning them. Displacement food availability,or a combinationof thesefactors. of nestinggulls by ternsoccurred even after gulls had Giventhe potential negative effects on ternsof anin- started laying eggs.During 1991 and 1992,54.5% (n creasein nestingdensities of Kelp Gulls, we recom- = 11) and 87.5%(n = 8), respectively,of the displaced mendthe establishmentof a monitoringprogram to gullshad already started laying before they were dis- allow the early detectionof conflictsbetween these placed. species. Discussion.--Eventhough gulls and ternsat Punta Acknowledgments.--Wethank the Wildlife Conser- October1998] ShortCommunications 1071 vationSociety and WilliamConway for the support and competitive interactionsof Herring and that allowed us to conduct this study. The project Laughinggulls in New Jersey.Auk 95:252-266. was sponsoredby Fundaci6nPatagonia Natural. We COURTNEY, P.g., AND H. BLOKPOEL.1983. Distribu- thank Gabriela Aguilar, Marcelo Bertellotti,Laura tion and numbers of Common Terns on the low- Biscayart,Luciana Chiesa, Mariana Lanfiutti, Cyn- er Great Lakes during 1900-1980: A review. Co- thia Padula, Fabian P•rez, and Cecilia Ramirez for lonial Waterbirds 6:107-120. help with field work.Drs. BernieTershy and Burney CROXALL,J.P., ANDP.g. PRINCE.1980. Food, feeding Le Boeufmade helpful commentson earlierversions ecologyand ecological segregation of seabirdsat of the manuscript.We thank the OrganismoProvin- SouthGeorgia. Biological Journal of the Linnean cial de Turismo,Chubut, for permitsto work at the Society14:103-131. Punta Le6n Reserve,and Centro Nacional Patag6n- DUFFY,D.C. 1983. Competition for nesting space among Peruvian guanobirds. Auk 100:680-688. ico (CONICET) for institutionalsupport. FASOLA,g., AND L. CANOvA. 1992. Nest habitat se- lectionby eight syntopicspecies of Mediterra- LITERATURE CITED nean gulls and terns. Colonial Waterbirds15: 169-178. AINLEY, D. 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