Download and Terns

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[Auk, Vol. 115
The Auk 115(4):1068-1071, 1998
Competition for Nest SitesBetween Kelp Gulls (Larusdominicanus)and
Terns (Sterna maxima and S. eurygnatha)in Patagonia
FLAVIO QUINTANA 1 AND PABLO YORIO
2CentroNacionalPatag6nico,
CONICET,Boulevard
Browns/n 9120,PuertoMadryn,Chubut,Argentina;and
WildlifeConservation
Society,
NewYork,NewYork10460,USA
Seabirdsfrequentlynestin mixed-species
colonies quantifytheir temporaland spatialpatternsof set(Burger 1985).Severalstudieshave showndiffer- tlement, describeand quantify gull aggressivebeencesin 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 amongthesespecies.
overlapin habitatand nest-siterequirementsmay
Study area and methods.--PuntaLe6n (43ø04'S,
existbetweenspecies,resultingin interspecific
com- 64ø02'W)is located10km southof the mouthof Golpetitionthat oftenleadsto nest-sitedisplacement
or fo Nuevo,Chubut,Argentina.The coastin this area
even the replacementof one speciesby another is characterizedby gravel beacheswith extensive
(Buckleyand Buckley1980,Burger1985).Spatialin- cliffs30to 100m high.A siltplatformseawardof the
teractions
between
seabirds
can be more intense
in
cliffs and coveredby vegetation(mainly Suaedadi-
areaswherehabitatis limited or when the popula- varicata,Atriplexlampa,and Lyciumchilense)
is usedas
tion of oneof the species
is expanding(Burgerand a nestingsite by severalseabirdspecies,including
Shisler1978,Trivelpieceand Volkman1979,Duffy Kelp Gulls (6,500 pairs), Royal Terns (650 pairs),
1983, Ainley 1990).
At manymixedcoloniesof larids,largerspecies
of-
Cayenne Terns (1,150 pairs), Imperial Cormorants
(Phalacrocorax
atriceps;2,650 pairs), NeotropicCor-
tendisplacesmallerspecies
fromtheirterritories,
oc- morants(P.olivaceus;
100pairs),RockShags(P.macasionallyforcing them to abandonthe breeding gellanicus;
1 pair), and Guanay Cormorants(P.bouarea (Burgerand Shisler1978,Burger 1985).In addition, larger larids generallyarrive at breedingsites
earlier(MorrisandHunter1976,Burger1985),which
gives them a competitiveadvantageover smaller
species.Gulls (Larusspp.)frequentlyhavebeeninvolved in the displacement
of other speciesfrom
breedinghabitats(Thomas1972,Burger1979,Bradley and Monaghan1986, Furnessand Monaghan
1987).Ternsappearto be one of the mostaffected
seabirds,andsometernpopulations
havedecreased
asa resultof populationexpansion
by gulls(Thomas
1972,Nisbet 1973,Greenhalgh1974,Courtneyand
Blokpoel1983,Parnellet al. 1988,Ainley and Hunt
1991,Blokpoeland Scharf1991).
At Punta Le6n, Argentina, Royal Terns (Sterna
maxima)and CayenneTerns(S.eurygnatha)
breedintermingledin a denseclusterof nestswithin a Kelp
Gull (Larusdominicanus)
colony.The Kelp Gull colony increasedin size from 3,200 pairs in 1982 to
6,500 in 1995, and gulls currently occupyalmostall
of the availablenestingspace(Yorioet al. 1994).Given the overlapin nest-siteuse and the currentex-
pansionof KelpGulls,we expected
thatcompetition
for nestingspacecould result in negativeeffectson
both speciesof terns.In this paper,we document
spatial interactionsbetweenKelp Gulls and Royal
and Cayenne terns at Punta Le6n, Patagonia.We
E-mail:[email protected]
Addressfor correspondence.
gainvillii;2 pairs;numbersfrom Yorioet al. 1994).
During 1990to 1992,we obtainedinformationon
temporaland spatialsettlementpatternsof Royal
Terns,CayenneTerns,and Kelp Gulls.Todetermine
settlementpatternsby terns,we countedthenumber
of nestseverytwo to threedaysfrommid-September
to earlyNovember,usingspottingscopes(20to 45X )
from an observation
point on the cliffs 50 to 70 m
fromthecolony.Wemonitoredthe growthof thecolonyby mappingits sizeandshapeat intervalsof two
to fourdays.Weobtainedsettlementpatternsof Kelp
Gulls from late August to mid-Novemberthrough
weekly countsof breeding individuals at a study
area closeto the tern colonyand during 1990 and
1991by monitoring53 and 40 markednests,respectively,everytwo to threedays.The 20 x 20 m study
area used to monitor gull settlementwas locatedin
the southernpart of the KelpGull colony.
To studythe effectsof Kelp Gull territorialbehavior on tern settlement,during 1990we conducted23
one-hourobservations
during whichwe scannedthe
colonyfor settlingterns (n 73 groups).When a
group of settling terns was detected,we recorded
whetherthe ternswere attackedand whetherthey
were totally or partially displaced by territorial
gulls. We defined "settling terns" asa variablenumber of birdsof bothtern speciesthattried to settleat
the colonyperiphery.Mostpairsof ternsbuild nests
andlay theireggson the samedaythat theysettlein
the colonysite (Quintanaand Yorio1997).We de-
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ShortCommunications
fined"settledtern"asanybird sittingona nest,with
or without eggs.We defineda gull "attack"as any
behaviorconsistingof a shortchaseby a peripheral
territorialgull (and on occasiona peckwithoutestablishingphysicalcontact)thatresultedin theflight
of the groupof settlingternsor of an incubatingtern
adjacentto that gull territory.We defineda peripheral territorialgull as any bird holdinga territory,
1069
20 •
with or without a nest, located at a distance of less
than 3 m from the tern colony.
To quantifyKelpGull territorialbehavior(gull ato
tacks)towardterns,we madeobservations
with spottingscopes
andbinoculars
fromtheobservation
point
on the cliff throughoutthe daylighthours (0700 to
2100EST)duringmostoftheternbreedingcycle.Dur-
ingeachhour(n = 307),we quantified
therateofgull
attacksdirectedatsettlingor incubatingterns.During
1991and 1992,we analyzedthe differencein territorial behaviorbetweengullsnestingnearsettlingterns
versusthosenestingnearsettledterns.
To documentthe displacement
of nestingpairsof
Kelp Gulls by settlingterns,we monitoredat twodayintervals38and40gull pairsbreedingat thetern
colonyperipheryduring1991and1992,respectively.
Duringeachnestcheck,we recordedthepresence
of
gulls at the nest,the stateof the breedingcycle,and
the estimateddistanceof the nestto the tern colony
periphery.If the gull nestdisappeared(lossof nesting materialandeggs),we recordedwhetherthesite
was occupiedby a nestingtern.
Results.--KelpGulls settled at the colonyover a
two-monthperiod (Yorioet al. 1994).Gulls arrived
at the colonyand selectedterritoriesthat were dis-
0
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1
2
3
4
5
6
7
8
Week of the breeding cycle
FIc. 1. Seasonalvariationin rate of Kelp Gull attacks (• + SD) on Royal Terns and CayenneTerns
during1990(week1 = 18to 24October),PuntaLe6n,
Patagonia.
in the direction of vegetatedground and in areas
withoutgull territories.
Wedid notquantifygullnesting densitiesat the time of tern settlementto avoid
disturbanceto gulls and terns.However,qualitative
observations
showedthatduringall years,sections
of
thegull colonyhad lowernestdensities
thanthesec-
persedthroughoutthearea.Latearrivalssettledbe-
tionswhere ternsbred. Moreover,during 1991,terns
tweenexistingterritories.Kelp Gull nestingdensity
was variable,ranging from 0.002 to 0.75 nests/m2
(Yorioet al. 1994).Colonyformationby Royal and
Cayenneternswasinitiatedby a groupof about100
changed
thelocation
oftheircolonytoanareaofhigher gull densitythanthatusedin thepreviousseasons.
Thisnewareahadoneof thehighestnestingdensities
of KelpGullson theentirecolony(0.50nests/m2;
Yorio et al. 1994).Ternscontinuedto breedin thispartof
the colonyfor the nextthreeyears.
to 150 individuals
that settled on available
bare
groundamongKelp Gull nests,and the colonygrew
throughthesettlementof newgroupsofbothspecies
at the colonyperiphery(Quintanaand Yorio1997).
The resultingnestingdensityfor both tern species
combinedrangedbetween9 and 11 nests/m2 (Yorio
et al. 1994).Duringall studyyears,KelpGullssettled
The settlementof tern groups(g = 25.7 individuals,range5 to 120,n = 73) at thecolonywasaffected
by the presenceof territorialKelp Gulls,with and
withoutnests,at theperipheryof thecolony.During
1990,79.4%(n = 73) of the tern groupsthat we folmore than a month earlier than the terns. Gulls startlowedduringsettlement
wereattacked
by peripheral
ed claimingterritoriesduring late Augustand early gulls. Attacks by Kelp Gulls frequently forced
September,
but bothternspeciesdid notsettleat the groupsof settlingternsto take flight, on occasion
colonysiteuntil thesecondor 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,someKelpGull pairssettledadjacent
to thetern and53.4%werepartiallydrivenawaybygullsby the
period.In 15.6%of the cases,
colonyduringand afterthe tern layingperiod.Both end of the observation
gulls and ternsstartedlaying 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
AlthoughKelpGullssettledin the nestingareabe- per hourtowardterngroupswas13.8+ SD of 8.8 (n
foretheterns,bothternspecies
wereableto establish = 121)in 1990.The numberof attacksper hour deandbreedamonggull territoriesin all threeseasons. creased significantly as the season progressed
The growthof theterncolonythroughthesettlement (Spearmancorrelation,rs= -0.98, P < 0.01;Fig. 1).
betweengullsandternsocof groupsofbirdsat thecolonyperipherywasmainly Mostspatialinteractions
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ShortCommunications
[Auk,Vol. 115
Le6n exhibitoverlapin habitatuse (Quintanaand
Yorio
1997),and the larger Kelp Gulls arrive at the
[]Settling
terns
colonyearlier, Royal Ternsand CayenneTernswere
[] Settedterns
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 significantlyhigherrate
of attackstoward settlingversussettledterns.
The settlementand nestingstrategiesof Royal
Ternsand CayenneTernsappearto be important determinantsof competitiveinteractionswith gulls.
Althoughlargebodysizeand earlytimingof settlementcanbe importantdeterminantsof the outcome
of spatialinteractions(Burgerand Shisler1978,Duffy 1983, Burger 1985), settlementin dense groups
may allow smallerbirdsto displacelarger and earlier settlingbirdsthatnestat lowerdensities(Burger
1985). At Punta Le6n, Royal Terns and Cayenne
Terns settle at the colonyin relatively large and
densegroupsthat are ableto displacenestingKelp
Gulls. Similar observations
havebeenreportedfor
other tern species,suchas SandwichTerns(Sterna
1991
1992
sandvicensis)
nestingwith Black-headedGulls (Larus
Year
ridibundus;
Taverner1970,Smith1975,Veen1977),Elnestingwith Heerman's
FIG. 2. Rate of Kelp Gull attacks(• -+ SD) toward egantTerns(Sternaelegans)
settling and settled Royal Ternsand CayenneTerns Gulls (Larusheermanni;Barrie 1975),and Arctic Terns
(Sternaparadisaea)
nestingwith Herring Gulls (Larus
during 1991and 1992, Punta Le6n, Patagonia.
argentatus;
Bianki 1977).In this respect,the nesting
traitsof the crestedtern groupgivetheman advancurredduringthe periodof ternsettlement,and the tageoverothertern species,manyof whichare or
highestrate of gull attackswasduring the first two havebeenaffectedasa resultof expanding
gullpopweeksof the terns' breedingcycle(Fig. 1). The av- ulations(Thomas1972,Nisbet 1973,Courtneyand
eragenumberof gull pecksandchases
per hourdur- Blokpoel1983,Ainley and Hunt 1991,Blokpoeland
ing 1991 and 1992 was significantlyhigher toward Scharf1991).
settlingthan settledterns(Wilcoxontest;1991,Z =
Even thoughwe 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 comEventhoughsomeKelpGull pairsalsosettledafter petitionwith KelpGulls,an increasein gull nesting
ternshadselected
thecolonysite,gullswereneverob- density due to population expansionmay interfere
serveddisplacingternsthathad alreadylaid eggs.In with ternsettlement
in thefuture.In addition,Royal
only a few occasions,
gull territorial behaviorforced Ternsand CayenneTernsmay pay a costby nesting
an incubatingternto leavetheeggexposedforseveral amongKelp Gulls owing to delayedbreeding,enerminutes.In contrast,gull territoriesthat had nests, gy lossfrom territorialbehavior,and/or prolonged
someof themwith eggs,weretakenoverby settling exposureof eggsduring territorialinteractions.Any
ternsduringall threeyearsof thestudy.In 1991and such costsprobablywill increasewith increasing
1992,28.9%(n = 38) and 20% (n = 40), respectively, densitiesof nestinggulls.During 1991,for example,
oftheKelpGullpairsnestingin thevicinityofthetern the locationof the terncolonychangedto an areaof
colonylosttheirterritoriesandneststo settlingterns. highergull density.Duringthat season,
the growth
Gull nestdisplacement
occurredas the tern colony of the tern colonywasslowerthan in previousseagrew and groupsof settling terns intruded into the sons (Quintana and Yorio 1997). Unfortunately,it
territoriesof gulls,takingoverthe nestingspace.On cannotbe determinedwhether the delay in settleoccasion,Kelp Gull nestswere graduallysurrounded ment was a direct consequenceof territorial interby ternpairs,andincubatinggullsretainedtheirnests actions between gulls and terns, a result of lower
for sometimebeforeabandoning
them.Displacement food availability,or a combinationof thesefactors.
of nestinggullsby ternsoccurredevenaftergullshad Giventhepotentialnegativeeffectsonternsof aninstarted laying eggs.During 1991 and 1992,54.5% (n creasein nestingdensitiesof Kelp Gulls, we recom= 11)and87.5%(n = 8), respectively,
of thedisplaced mendtheestablishment
of a monitoringprogramto
gullshadalreadystartedlayingbeforetheyweredis- allow the early detectionof conflictsbetweenthese
placed.
species.
Discussion.--Even
thoughgullsand ternsat Punta
Acknowledgments.--We
thank the Wildlife Conser-
October1998]
ShortCommunications
vationSocietyand WilliamConwayfor the support
that allowed us to conductthis study. The project
wassponsored
by Fundaci6nPatagoniaNatural.We
thank Gabriela Aguilar, Marcelo Bertellotti,Laura
Biscayart,LucianaChiesa,Mariana Lanfiutti,Cynthia Padula, Fabian P•rez, and Cecilia Ramirez for
1071
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