Sex Differences in the Stopover Ecology of Curlew Sandpipers Calidris Ferruginea at a Refuelling

SexdifferencesinthestopoverecologyofCurlewSandpipersCalidrisferrugineaatarefuellingareaduringautumnmigration

JORDIFIGUEROLA*andALBERTBERTOLERODepartmentofAnimalBiology(Vertebrate),FacultyofBiology,UniversityofBarcelona,Avda.Diagonal645,E-08028Barcelona,Spain

WeinvestigatedthestopoverpatternsofmaleandfemaleCurlewSandpipersatastopover areainnortheast Spain.CurlewSandpipers weretrapped andcolour-ringedduringautumnmigrationin1992and1993.Stopoverlengthwassimilartothosereportedpreviouslyforthisandotherwadersthatmigrateusingasmallnumberofwidelyseparatedstagingareas,butweregreaterthanstopovers reported forotherwadersthatmigrate usingalargenumber of stagingareasseparatedbyshortdistances.Thedifferencesinstopoverlengthbetweenthe birdsusingthesetwostrategiescouldbe relatedtothe fuelreservesthathavetobeaccumulatedtoreachthenextstagingarea.Malesstayedlongerin theareathanfemales.Seasonalchangesinpreyavailabilityorsexdifferencesinmoultingandmigratorypatternsdonotaccountforthesedifferencesinstopoverecology.Following atime-selectedmodelofoptimalmigration,sex differencesinstopoverecologycouldberelatedtoadominanceofthelargerfemalesoverthemalesortoahigherforagingefficiencyorashortersearchandsettlingtimeinfemales.Whetherthesedifferencesarerestrictedtothestudiedareaorarewidespreadinotherstagingareasusedbythespeciescouldbe important for assessing the possible differences in the migration speed ofCurlewSandpipers.Thefindingthatmalesleavethebreedinggrounds21—35daysbeforefemalesbutarriveatthestudyareawithonlya10-daydifferencesupportsthehypothesisthatfemalesmigratefasterthanmalesatleastinthefirsthalfoftheirmigration.

uring migration, long-distance migrantshavetoreplenishtheirenergeticreservesseveraltimesatanumberofstagingareas.1Theoveralltimeneededtocompletemigrationislimited by the time necessary to accumulatenewreserves2 (shortperiodsofflightalternatewithlonger onesoffataccumulation)and,toa

lesser extent, by flight speed. Two strategies ofmigrationhavebeendescribedanddocu-mented mainlyinwaders.3Thefirststrategyisusedby‘hoppers’whichmakeshortflightsandusealargenumberofstagingareasalongtheir

*Correspondenceauthor.

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migratory route. The second is used by

‘jumpers’whichflylargedistancesbetweenthe

smallnumberofstagingareastheyuseto com-

pletetheirmigration.

AlerstamLindström2 producedamodel

based on the optimization criteria of the

decisionsfacedbyabirdaccumulatingfatata

staging area.Thesemodelsassumedthatabird

assessedthemomenttoleaveastagingareain

terms of its present fat reserves, rate of fat

accumulationanditsexpectedfuturefatgain.2,4

These authors presented different models to

explainthedecisionsofabirdatastopover

area. The differences between these models

relyonthevariablesthebirdstrytooptimize.

‘Time-selected’ migrants are expected to

maximizemigrationspeedandsoareverysensitivetovariationsintheirfatdepositionrate.Accordingtothismodel,birdswithhigher intake rates or which select habitatswith greater food resources will migratefasterthanotherbirdsthatdonotforageasefficiently or use less favourable habitats.

‘Energy-selected’migrantstrytocompletetheirmigrationwithlowerenergeticcostsbymaximizingtherelationshipbetweenflightrangeandfatload.Theutilityofaccumulatingadditionalfatdecreaseswithincreasingfatreservesbecauseheavybodymassincreasesflightcosts.5 AlerstamLindström’smodelpredictsthatmigrationdecisionsofenergy-selectedmigrantsarelesssensitivetovari-ationsinfatdepositionrate.

Accordingtothetime-selectedmigrationmodel,dominantindividualsormoreefficientforagersshoulddepartearlierand/orwithgreaterfatreservesthansubdominantsbecausedominantswillbeabletoforageinmorefavourableareas.Inspecieswithenergy-selectedmigration,dominantsshoulddepartwithlowerfatloadsthansubdominantsand,accordingtothismodel,nodifferenceinstopoverlengthistobeexpectedbetweenthesetwogroups.Inthismodeltheprimarybenefitfordominantsisareducedcostforsearchandsettling.2Theeffectofsocialstatusondeparturemassandstopoverlengthintime-selectedmigrantscouldexplainthedifferencesinstopoverecologyofyoungandadultsofseveralspeciesduringautumnmigration6,7 andofmalesandfemalesinspring.8,9

Despitethefactthatmoststudiesofthestopoverecologyofwadershavefoundage-relateddifferencesintheresidencepatternsinautumn,7,1Onostudyhasfocusedonthestopoverpatternsofmalesandfemalesatastagingareaduringtheautumnmigration.Suchstudiescouldprovidebasicinformationtodeterminethepossibleintraspecificdiffer-encesinthecostsofmigration,informationnecessarytodeterminethefactorsresponsibleforthewinterdistributionofwaders.11 InthisstudyweanalysethestopoverpatternsofadultmaleandfemaleCurlewSandpipersCalidrisferrugineaatastagingareainautumn.ThestopoverecologyofthisspecieswasexaminedinrelationtotheoptimizationmodelsofAlerstamLindström2totestthehypothesisthatthesexesdifferintheirstopoverlength.

METHODS

Curlew Sandpiper migration was studied atLesSalinesdelaTrinitat(4O°37′NOO°35′E;northeastSpain).Thestudyareaconsistedofanareaofmudandsand-flatsinasaltpannearthesandycoastoftheMediterraneanSea.Littletidaloscillationoccursonthiscoastandthegreatestvariationinwaterlevelinthesaltpanwasproducedbychangesinwindforceanddirection,orwhensaltextractionwasinprogress. Birds were trapped at night bymist-netandbydayusingwalk-in-trapsfrom

27 July to 12 October 1992 and 25 July to

24September1993.Birdswerecolour-ringed

andbodymass,wing,billandtarsuslengthsof

mostbirdsweremeasured.12 Individualswere

aged12 andthesexofadultbirds(morethan

one-year-old)wasdeterminedfromwingand

billlengthusingthediscriminantformulaof

Wymengaetal.13Bodymassatringingwascor-

rectedtoremovetheeffectofwaterlossandthe

emptying of the gut occurring between

captureanddatarecording,assumingarateof

masslossofO.88%perhourofcaptivity.14 Data

from moulting birds were not included in

the analysis because moulting increases the

energeticrequirementsofbirds15,16 andtherate

ofreserveaccumulationatthestudyareawas

lower in moulting than in non-moulting

Curlew Sandpipers.17 Because the rate of

reserveaccumulationwasoneofthevariables

thataffectstopoverdecisions,2 theinclusionof

thesebirdswouldhaveaffectedtheresultsof

ouranalysis.

Estimationofstopoverlength

Approximatelyeveryotherdayatransectof2.2kmwassurveyedtolocateandidentifycolour-ringedbirdspresentinthearea.Fromthistransectwecensused44haofmudandsand-flats,coveringmostofthesuitablehabitatforCurlewSandpipersinthesaltpan.Additionalrecordsofcolour-markedbirdsobtainedfromcasualobservationswerealsoconsidered.Estimatingtruestopoverlengthisdifficultbecausebirdswereusuallynottrappedimme-diatelyafterarrivalnorjustbeforeleavingthestudy area. To estimate stopover length weusedthemethoddescribedinHolmgrenetal.7

Thismethodisbasedonamark—recapturemodelderivedfrom Jolly18 and Seber.19 The

assumptionsofthismodelarethateachindividualhasaconstantprobabilityofstayingintheareaandthattheprobabilityofresightingwasthesameforallbirds.Usingthismodelwecalculatedthemaximumlikelihoodestimateofthestopoverprobability(ϖ)andtheestimateofmeanstopoverlength(τ) separatelyfor eachsexoneachyear(seeHolmgrenetal.7 foradetaileddescriptionofthemodelandparame-terestimation).

Statisticalanalyses

Differencesinstopoverprobabilitiesweretest-edbycalculatingthestandardnormaldeviate(d)andtheassociatedPvalue.7Differencesinbodyconditionamonggroupsweretestedwithsquare-root-transformedbodymassusing a trifactorial ANCOVA. In this analysissex(male/female),control(resighted/notresighted)andyear(1992/1993)wereusedasfactors.Culmenlengthwasintroducedasacovariatetoremovetheeffectofbodysizeonbodymass.WhensignificantinteractionsbetweenthefactorsweredetectedweusedScheffèmultiplerangeteststoexaminethedifferencesbetweenparticularlevelsofthefactors.2O

RESULTS

Relationship ofbodymassandstopoverlength

Althoughfemaleswerelargerthanmales,21differencesinbodymasswerenotsignificantaftercontrollingfortheeffectofbirdsize(three-wayANCOVA,F1,9O1 =O.27,P=O.61)and

noneoftheinteractionsofsexwiththeothertwofactorswassignificant(sex—control:F1,9O1=

2.11,P=O.15;sex—year:F1,9O1 =1.27,P=O.26).Bodymassofmarkedbirdswasgreaterin1992thanin1993(three-wayANCOVA,F1,9O1=17.O7,P

O.OOO1). Bodymassof birds resighted didnotdifferfromthosethatwerenotresighted(three-wayANCOVA,factorcontrol,F1,9O1=1.27,P=O.26).

However,asignificantinteractionwasdetectedbetweentwoofthefactors(control—year,F1,9O1 =4.29,P=O.O4).Thissignificantinteractionappearedbecausebirdsresightedin

1993wereleanerthanbirdstrappedin1992(bothresightedandnotresighted,Scheffètests,PO.OO3forbothcomparisonsandPO.2Ofortherestofcontrast).Therelationshipsbetweenbody mass at ringing and stopover lengthwerealsoanalysedseparatelyforeachsexandyear.Noneofthelinearregressionsbetweenbodymassandstopoverlengthwassignificant(rrangingfrom—O.O5to—O.12andPfromO.3OtoO.71).

Frequencyandlengthofstopover

In1992wecolour-marked517individuals,ofwhich99werelaterresighted(19.1%).In1993,

89outof393markedindividualswereresight-edafterthedateofringing(22.6%).Numbersofmarkedandresightedindividualsdidnotdifferbetweenyears(χ2 =1.67,1df,P=O.2O),norbetweensexes(1992:χ2=1.43,1df,P=O.23;

1993:χ2=O.45,1df,P=O.5O).Theestimatedstopoverlengthofmaleswaslongerthanfemalesin1992(d = 1.84,P O.O5), in1993(d=1.87,PO.O5),andwhendataofbothyearswerecombined(d=2.64,PO.OO5),seeTable1.

Table1.Observedmeanstopoverlengthofmalesandfemalesresightedatleastonce(t)andestimatedmeanstopoverlengthoftheCurlewSandpiperpopulation(τ)bothexpressedindays.Dailystopoverprobability(ϖ)anditsstandarddeviation.

DISCUSSION

Stopoverlengthandmigrationstrategy

Comparingthestopoverlengthsreportedindifferentstudiesisdifficultbecauseauthorsusuallyusedifferentmethodstoestimatemeanstopoverlength.Accountingforthisshort-coming,thestopoverlengthspresentedinthisstudy were similar to the approximately 14daysreportedatastagingareaontheGermanWaddenseainautumn1992,22andfallwithintherangeof7—21daysforindividualsrefuellingonaBritishestuary.23 SimilarstopoverlengthswerealsoreportedfortheSemipalmatedSandpiperCalidrispusillainNorthAmerica.24,25Bothspecieswereconsid-eredtouseajumpingstrategy.24,26 However,alltheseestimateswerehigherthanthestopoverlengths reported for other waders of similarsizethatwereconsideredtouseahoppingstrategy.1O,27,28AllDunlinsCalidrisalpinamigrat-ingthroughOttenbystayedintheareaforlessthanninedays(ameanofO.55—1.44days7)andveryshortstopoverswerealsoreportedfortheWesternSandpiperCalidrismauriandLeastSandpiperCalidrisminutilladuringautumnmigrationalongthewesterncoastofNorthAmerica.27,29

Thedifferencesinflightdistancebetweenspeciesthatjumpandthosethathopsuggestthatjumpershavetodepartwithhigherlevelsofreservesandhavetostaylongerinthestagingareastoreplenishtheirfatreserves.AlthoughtheoreticalflightrangesofjumpersandhoppersreadytodepartinspringdidnotdifferatawinteringareainMauritania,14thecomparisonofstopoverlengthsofseveralspeciesindifferentareassuggeststhatjumperswillstopforlongerperiodsinthestagingareastoreplenishtheirfatreserves.Theapparentdiscrepancybetweenthesetworesultscouldoccurbecausebothjumpingandhoppingstrategiesmightoccurinthesamespeciesandthusamongtheindividualsusingthesamerefuellingarea.

ThispatternhasbeendemonstratedbySkagenandKnopf3O withtheuseofradio-trackingtodescribetheresidencypatternsofSemi-palmatedSandpipersandWhite-rumpedSandpipersCalidrisfuscicollisatastagingareainspring.Theirstudyshowedthatresidencypatternswereintraspecificallyvariableandalthoughsomebirdsdepartedwithlargefat

loadsandinapositiontocarryoutlongunin-terruptedflights,othersleavewithsmallreservesandwereonlycapableofshorthops.TheoccurrenceinonespeciesofbothmigrationstrategiescouldexplainwhydifferencesinflightrangewerenotfoundbyZwartsetal.14andthatreportedestimatesofstopoverlength(avariableprobablymoreinfluencedbybirdsstayinglongerinthearea)weregreaterinjumpersthaninhoppers.

Optimalmodelsofbirdmigrationpredictthatintime-selectedmigrants,leanerbirdswillmakelongerstopoversthanfatterbirds.Wedidnotfindthisrelationshipinourstudynorinother studies of several species of waderswithtime-selectedmigration7,25,29,31,32 butseeMascher.33 AsHolmgrenetal.7 pointedout,theseapparentlyrandomstopoverscouldappearifbirdstendtowaitforsuitableweatherconditionsinordertoresumemigra-tionandifweatherchangesrandomly.Inthissituationtime-selectedmigrantswouldcontin-uetodepositfat,butenergy-selectedmigrantswouldnot.

Sexdifferences instopoverecology

Ourresultsshowthat,inautumn,maleCurlewSandpipersstayedforlongerthanfemalesatthestudiedstopoverarea.Severalfactorscouldhavecausedthesedifferences.MaleCurlewSandpipersmigrateaheadofthefemaleswhichremaininthebreedinggroundstakingcareofthebrood.21,26 Thedensityofpreyavailableforbirdsatastagingareacouldchangeduringautumnmigrationandpreydepletionbyearlymigrantshasbeenreportedinseveralstudies(reviewedinSzèkelyBamberger34).Malesarrivedatourstudyareatendaysbeforefemalesin1993andpreydensityavailableforeachsexcouldhavechangedinthistime.However, in 1992 the breeding success ofarcticwaderswasverylow35andinthisyearprobablyledtomalesarrivingonlyfourdaysbeforefemales.36

Iftemporalvariationinpreyavailabilitywasthecauseofthedifferencesinstopoverecologyreportedinthisstudy,differencesinstopoverlengthofmalesandfemalesshouldappearin

1993butnotin1992(whenbothsexesmigratedapproximatelyatthesametimeandstayedinthestudyareaunderthesamebioticandabioticconditions).This,however,isnotthe

caseandsowerejectthehypothesisthattem-poralvariationinpreyavailabilitymayhaveproducedthedifferencesinstopoverlengthreportedinthisstudy.

Anotherpossiblesourceofbiasinouresti-matesofstopoverlengthcouldberelatedtotheinclusioninthedatasetofbirdsinactivewingmoult.Althoughwehaveexcludedfromouranalysisbirdstrappedinactivewingmoult,itispossiblethatsomebirdsstartedprimarymoultjustafterbeingringed,therebyincreas-ingourestimatesofstopoverlength.SomestudieshavereportedthatmaleCurlewSandpipersstarttomoultbeforefemales,26,37,38althoughothersfailedtofindanydifference.39,4OMoremalesstartingprimarymoultinourstudyareacouldhaveincreasedtheestimatedstopoverlengthofthissex.However,thenumberofbirdsthatstartedwingmoultinthestudyareawassmall(6.2%oftheadultbirdsringedinautumns1992and1993),andalthoughmoremalesstartedtomoultinthestudyareain1993,nodifferenceinthenumberofmalesandfemalesmoultingintheareawasdetectedin1992.17Longerstopoversofmaleswerereportedinbothyears,sowealsorejectthepossibilitythatdifferencesinmoultpatternswerethecauseofthereportedsexdifferencesinstopoverlength(atleastofthosefoundin1992).

Accordingtothemodelsofoptimalmigra-tion,2 avarietyoffactorscouldexplainthesexdifferencesinstopoverlength.Femalescouldstayforashorterperiodinthestudyareaiftheyleavewithlowerlevelsofaccumulatedreserves.Toexaminethisissue,FiguerolaBertolero41 comparedtheoreticalflightrangesofmalesandfemalesstayingattheEbroDelta.Arrivalbodymasswascalculatedastheaveragebodymassofthe1O%ofleanestbirdstrappedinthearea,anddeparturebodymasswascalculatedastheaveragebodymassofthe

1O%ofheaviestbirds.Theseapproximatecalculationssuggestedthatmalesaccumulatedapproximately3O.4gandfemales32.5gofbodyreserves.Ifthesefiguresreflecttherealsituation, the sexes did not differ greatly intheirestimatedflightrange(3171versus3288km)andthedifferenceswerenotconsistentwiththehypothesisthatfemalesleavewithsmallerfatloadsthanmales.

Thetime-selectedmodelofoptimalmigra-tion suggests that birds with higher fat

depositionratesoralowersearch-settlingtime(timebetweenarrivalattheareaandinitiationofefficientfatdeposition)coulddepartearlierfromthestagingareas.2Thesedifferencesinfatdepositionrateandsearchcostscouldappearifonesexdominatestheotherorifsexesuseecologicallydifferentresources.InafieldexperimentatastagingareainScandinavia,Lindströmetal.42 demonstratedthatdominantBluethroatsLusciniasvecicaaccumulatedfatatahigherratethansubdominantones.AlthoughinformationonthedominancestructureofCurlewSandpiperpopulationsisscarce(butseePuttick43),itiscommonamongbirdsthatthe larger sexdominatesthe smallerone.44,45

FemaleCurlewSandpipersarelargerthanmalessoitisfairtoexpectthattheydominatemales.Sexualdimorphisminbilllengthisalsoacuteandcouldallowfemalestocapturepreyburiedmoredeeplyinthemud.AccordingtothesedifferencesPuttick46 foundinawinteringareainSouthAfricathatthedietofthesexesdifferedandthatfemalesdominatedmales,foraged more successfully and needed lesstimeforagingtocovertheirenergeticrequire-ments.43

Alltheseobservationslendsupporttothehypothesisthatthesexdifferencesinstopoverlengthreportedinthisstudycouldbeproducedbydifferencesbetweensexesinthecapacitytoaccumulatefatinthestudyareaand/orbecausefemalescanstarttodepositfatearlierthanmales(i.e.haveashortersearch-settlingtime).Ourstudyareacouldbeespeciallysuitableforfemales,whereasotherareascouldbemorefavourableformales.

IfthedifferenceswefoundinthestopoverecologyofCurlewSandpiperalsooccurintheotherstagingareasusedbythespecies,thiswouldmeanthatthefemaleshadahigheroverallmigrationspeed,becausethissexwillstayatthestopoverareasforlesstimethanmales.Malesleavebreedinggroundsoverthreeweeksbeforefemales21 butarrivedatourstudyareain1993only1Odaysaheadoffemales.36Thisreductionofthedifferencesinthetimingofpassageforeachsexgivessupporttothehypothesisthatsexesdifferintheirmigrationspeed.Obviously,theresultspresentedheredonotdemonstratethatinCurlewSandpiperssexesdifferintheirmigrationefficiency.However,theyareconsis-tent with this hypothesis. We hope that this

papermightstimulatefurtherresearchonsexdifferencesinstopoverecologyandsearch-settlingpatternsinspecieswithsignificantnormalorreversesexualdimorphisminbodysize.

ACKNOWLEDGEMENTS

ThisworkwasfundedbytheEbroDeltaNaturalPark,DiputaciódeTarragonaandGrupCatalàd‘Anellament.TheGeneralitatdeCatalunya(CatalanAutonomousGovernment)provided the permits necessary for carryingoutthefieldwork.L.M.CopeteandL.Gustamantemadeanindispensablecontribu-tiontothefieldwork.WewouldalsoliketothankJ.M.Arcos,L.Brotons,L.Carrera,D.Escobar(FonotecadelMuseodeZoologiadeBarcelona),M.A.Franch,D.Froelich,S.Galan,R.Marine,R.Marti,A.Martinez,R.Mateo,A.Motis,D.Oró,X.Riera,A.Salmerón,J.Solans,F.VicentsandR.Vidalfortheirhelpandfriend-ship.M.LockwoodimprovedtheEnglishandJ.A.Amat,B.J.Ens,J.D.Goss-Custard,A.Lindström,T.Piersma,J.C.SenarandD.A.Stroudmadevaluablecommentsonearlierdraftsofthismanuscript.

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