Nils D. Warnock, Point Reyes Bird Observatory, 4990 Shoreline Highway,
Stinson. Beach, CA 94970, USA (Present Address: Wildlife and Fisheries Biology
,.
Apparentage- segregation of DunlinwithinBolinasLagoon- a preliminarystudy Nils D. Wamock Warnock, N.D.1990. Apparent age-segregation ofDunlin withinBolinas Lagoona preliminary study.WaderStudyGroupBull.,60:27- 30
NilsD. Warnock, PointReyesBirdObservatory, 4990Shoreline Highway, Stinson Beach,CA94970,USA(Present Address: WildlifeandFisheries Biology, University of California, Davis,CA95616-5270,USA).
INTRODUCTION
STUDY AREA AND METHODS
Lomnicki (1988)suggests thatinorderto predictpopulation dynamics it isvitaltoidentify groups withina population which differin mortality andreproductive rates.Oneimportant category forgrouping organisms withina population isage (Lomnicki 1988,Marchetti & Price1989).If substantial agespecific differences existwithinspecies, particularly inspecies inwhichdifferent agegroupsaredifficult to discriminate physically, biaseddatamaybecollected regarding population dynamics (Catterall et aL1989).Nevertheless, ageoforganismisoftenignoredinthestudyof habitatdistribution. The localdistribution of severalspeciesof shorebirds has beenshownto be, in part,affectedbytheageof the individual (Groves1978,Puttick 1978,1984,Burger1980, Ens & Goss-Custard1984, van der Have et al. 1984, Barnard
The BolinasLagoonis a small(570 ha) saltwaterlagoon located24 kmnorthof San Francisco, USA(seePageet al. 1979formoredetail).FromAugustthroughMaylarge numbers of shorebirds, including 1,000-3,000Dunlin,utilize thelagoonas a wintering area.Since1979approximately 900 Dunlinhavebeenindividually color-banded at thissiteby researchers of PointReyesBirdObservatory.
Dunlinwerecaptured innylonmistnetssetupintheevening at thebirds'roostsites.Birdscaptured werebrought backto a laboratory andbandedwitha uniquecombination of UVresistant color-bands. Afterbeingheldovernight, the Dunlin werereleasedat dawnon the lagoonwheretheywere captured.
& Thompson 1985,Goss-Custard & Durell1987).Dissimilar Eachbirdwasmeasured andaged.Measurements included segments oflocalhabitats mayexposebirdstodifferent bodymass,culmenlength,headto thetopofthe bill,flatwing levelsof competition andpredation. Forsomespeciesof chord,naturalwingchordandtarsuslength.Dunlinwere shorebirds, including Turnstones Arenaria interpres (Evans or adultbirdsaccording to 1981),Oystercatchers Haematopus ostralegus (Goss-Custard agedeitherasjuvenile(first-year) & Durell1984,Swennen1984)andDunlinCalidrisalpina
(Page& Whitacre 1975,vanderHaveet aL1984,Kuset aL 1984),juvenile birdssuffersubstantially highermortality rates thando adultbirds.Differences in howbirdsutilizeareasmay accountforthesedifferences in mortality, yet littleworkhas beendoneonwithin-site distributions of differentaged shorebirds.
Theaimof thispaperisto investigate whethertheNorth AmericanDunlinCalidrisa. pacificaexhibitsdifferential age distribution withinoneestuary.
27
the presenceor absenceof buff-colored innerwingcovert tips:adultDunlinwingcovertsare usuallytippedinwhite (Page1974,Prateret aL 1977,Gromadzka & Przystupa 1984).
Observations forthisstudywereconducted at twositeswithin thelagoon(A & B in Figure1). Bothareasunderstudyare tidalmudflats whichbecomeexposedwhenthetidefalls belowthe 1.2 - 1.5 m level.AreaA is approximately 250 m x 50 m. It is borderedononesidebyGlasswort Salicomia and hasa fewsmallSalicomiaislandswithinit.Area B is apprimately100 m x 50 m andit is surrounded by mudflatsand
NEW
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Figure1. TheBolinas Lagoon - feedingsites:A, B, E; roostsites:C,
D.
Thenumberofjuvenileandadultcolor-banded Dunlin feeding inAreaA vs.AreaB werecompared. Theproportion of juvenilecolor-bandedDunlin(number ofjuvenilebanded Dunlinseenin an areadividedbythetotalnumberof banded Dunlinseenin thatareaduringonecensus)wascalculated for each area on each census date.
Datafromallofthecensuses werepooledbythemonth. The proportions ofjuvenile color-banded Dunlinpresentpermonth for each area were calculated from these data. The mean and
standard deviation of theproportion ofjuvenilecolor-banded Dunlinwascalculated foreachareausingdatafromallthe censuses, andusingdatafromonlythesimultaneous censuses.
RESULTS
channelsonall sidesandhasnovegetation. Thetwoareas areapproximately 300 m apartandseparated bya deep channel of water.
Eitheroneor bothareaswerecensused on33 different days between2 January1987and22 April1987.Censuses were conducted as birdslefttheirday-time, high-tide roostsites andresumed feeding.Dataincluding bandcombination, date, time,area,activityandtideheightwererecorded inthefield. Themajority of censuses wereconducted byoneperson. Resighting generallybeganin areaA (unless2 people participated, inwhichcaseresighting wasconducted simultaneously), andthenweatherpermitting, areaBwas censused. Totalcensustimeforbothareasgenerally took
Movements oftheDunlins afterthebreakupof theday-time roostflockwereas follows (seeFigure1 forarea).Depending ontheheightoftheday-time hightideDunlinroosted either ona largesandbar nearthemouthofthelagoon(areaC),or directly at themouthofthelagoon(areaD).Otherday-time roostsiteswereusedlessinfrequently, andwillnotbe discussed inthispaper.Approximately an hourafterthehigh tide,Dunlinbegandeparting fromtheroosting areasand frequently beganfeedingontheSE cornerof KentIsland (areaE). Mudflats firstbecameexposed at areaA andquickly thereafter at areaB.Typically, at thispoint,theroosting flock splitin twodirections. Onegroupwouldflyoutto areaB and thesurrounding mudflats whileanothergroupmovedina northerly direction alongtheeastedgeof KentIslandthrough area A.
less than two hours.
Figure2. Comparison oftheproportion ofcolor-banded juvenile Dunlinin AreaA vs.AreaB fordayswhenbothareaswerecensused simultaneously.
0.8
0.7
0_6 0.5
0.4 0_3
Themeannumber ofcolor-banded juvenileDunlinfound foraging in thetwodifferent areaswerevariedbutareaA usuallyhada higherproportion ofjuvenilecolor-banded DunlinthanAreaB (Table1). Whenusingthedatafor simultaneous censuses onlytheaverageproportion of juvenileDunlinforaging inareaA, forallscorescombined, was0.56.Theproportion ofjuvenilecolor-banded Dunlin foundforaging inareaBwas0.24.Similarproportions occurred whenallthecensuses werecombined regardless of whetherareasA andBweresearched simultaneously. Colour-banded juvenile Dunlin represented 56%oftheDunlin foraging in areaA butonly23% in area B.
0.:•
0.10
I'
I
I
I
I
15
25
28
40
56
60
S2
166
Census [Days since I January]
28
Theproportion ofcolor-banded juvenileDunlinforareaA, usingonlythesimultaneous censuses, rangedfrom0.30to 1.00forthemonths ofJanuary through April,whileinareaB
NEW
AHY
AREA A HY
HY/Total
Jan2 4
1 1
9 8
0.90 0.89
AHY
•
AREA B HY HY/Total
6 7 8 10 13
9 12 2
15
0
5
1.00
18 19
4
2 -
0.33 -
25 28 31 Feb7 9
16 12 10 0 14
0.41 0.56 0.38 1.00 0.30
11 13 17 21 25 Marl 4 8 11 16 23
2 4 2 1 14 5 3 14
11 15 6 3 6 9 5 7 3 9 6 5 7
2
1
0.33
Apr1
1
1
0.50
3 6 10 13 20 22
1 16 10 5
8 9 3 -
2 3 7 4
0.47 0.43 0.60 -
0.82 0.56 0.78 0.75 0.39 0.55 0.63 0.33
0.67 0.16 0.41
17 20 32 42 3
4 1 2 3 5 1
1.00
N=26
censuses
• = 0.56 •' = 0.23
pooled
s = 0.23 s = 0.26
6.6I
o I)
0.06 0.09 0.09 0.09 0.25
o
6.4
t
6.3
i
o
6.2
fl
6.1 H
1
0
0.00
11 21
3 3
0.27 0.13
2
3
0.60
8 4
1 0
0.11 0.00
10 7 6
7 3 1
0.41 0.30 0.14
3 8 12
4 1 0
0.57 0.11 0.00
I
I
I
FEB
HAR
APR
DISCUSSION
Non-random age distributions of shorebirds maybe common withinestuaries.Adultshorebirds aftendisplacejuvenilebirds fromareasof abundantfoodsupplies.In a studyof European Oystercatchers Haematopus ostralegus Goss-Custard & Durell(1987)demonstrated thatmoreprofitable musselbeds weredominated by adultbirdsduringperiodsof highdensities.Likewise, adultRuddyTurnstones Arenariainterpres alwayswonaggressive interactions overfoodwithjuvenile Turnstones forcingthejuvenilesto feedelsewhere(Groves 1978).Similarly juvenileLapwingsVanellusvanelluswere lesssuccessful in competing forfood,moreopento kleptoparasitism bygulls,andmaybe forcedintolowerquality areasbyadultLapwings (Barnard& Thompson 1985).
VanderHaveet aL(1984)foundthatdifferent locations within the DutchWaddenSea haddifferentpercentages ofjuvenile Dunlin.Dunlindistribution waslikenedto an overflowsystem inwhichat periodsof highdensities juvenileDunlinwere forcedintolesspreferredhabitats.In California, Ruizet al. (1989)haveshownthatDunlinsegregatebyageandsex withinroostflocks,yet reasonsforthisphenomenon are
0.44
All
A•AA
• AJqE•B P
0
SECTION
Figure 3. Meanproportion ofcolor-banded juvenile DunlininAreaA vs.Area B:alldayspooledbymonth.
Table1. Censuses foradult(AHY)andjuvenile (HY)color-banded Dunlin foraging onBalinas Lagoon, during1987.Simultaneous censuses ofareasA & B areindicated bydatesin bold.
Date
WORLD
N=18
unclear.
Only
N= 11 N= 11
simultaneous
• = 0.56 • = 0.24
censuses
s = 0.75 s = 0.30
AttheBolinas site,it appearsthatdifferences intheage distribution of Dunlinexistoververyshortdistances. Furthermore,I suspect thatsuchagedistribution in Dunlinmaybe common andperhapsdensitydependent. Duringperiodsof highDunlindensities,flocksof Dunlinoftenfeedin locations withinthelagoonwheretheyare notnormally seenfeeding. Banded Dunlin resighted intheseareasarealmostalways juvenilebirds (Warnock unpublished data).Thismayresult fromjuvenilesbeingforcedintolessdesirablehabitatas suggested byvanderHaveet al. (1984).
therangeforproportion ofjuveniles was0.00 to 1.00 (Figure2). Usingthecombined censuses, areaA hada rangeof 0.34to 0.56color-banded juvenileDunlinforthe months January- AprilwhileareaB hada rangeof 0.18to 0.29juvenileDunlin(Figure3).
Otherreasons fora differential distribution maybeequally valid.Patchiness of foodmayinfluence distribution. Juvenile Dunlinmayhavea "learning period" inwhichtheyneedto
29
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learnthelocationof the mostprofitable foodpatches(vander Haveet aL 1984).On the BolinasLagoon,preliminary data suggestthatinvertebrates suchas clamsandwormsare morepatchily distributed inareaA thaninareaB (Warhock, unpub.data).AdultDunlinmaybe flyingdirectly outto areas wheretheyknowtheywillfindfood.
REFERENCES
Barnard, C.J.& Thompson, D.B.A.1985.GullsandPlovers: TheEcology andBehavior of Mixed-species FeedingGroups. ColumbiaUniversity Press,NewYork.
Burger, J. 1980.Agedifferences inforaging Black-necked In addition,raptorpredation mayinfluence distribution. Onthe BollhasLagoonraptorstakesignificantly morejuvenileDunlin thanadults(Page8, Whitacre1975,Kus1982;Kuset aL 1984).AdultDunlinmaylearn(cf.vander Haveet aL 1984) whereareasof higherriskof predation are andavoidthese areaswhereasjuvenilesare stillunawareof thesehigh-risk
Stilts in Texas. Auk 97: 633-636.
Catterall, C.P.,Kikkawa, J. & Gray,C. 1989.Inter-related age-dependent patternsof ecologyandbehaviour in a population of Silvereyes (Aves:Zosteropidae). J. Anirn.Eco/. 58: 557-570.
areas.
Ens,B.J.& Goss-Custard, J.D. 1984.Interference among oystercatchers Haernatopus ostralegus feedingonmussels MytilusedulisontheExeEstuary.J. Anita.Ecol.53:217-231.
Preliminary resultsof thisstudysuggestthattheagecomposition of DunlindiffersbetweenareasA andB within BolinasLagoon.Sinceonlytwoareaswerecompared, and samplesizeswereoftensmall,caremustbetakenin interpretingthesedatawithregardto otherDunlinpopulations. Furtherinvestigation of thisphenomena is in progress.
Evans,P.R.1976.Energybalanceandoptimal foraging strategies inshorebirds: someimplications fortheirdistributionsandmovements inthenon-breeding season.Ardea64: 117-139.
Goss-Custard, J.D.& le V. ditDurell,S.E.A.1987.Agerelatedeffectsinoystercatchers Haernatopus ostralegus feedingonmussels Mytilusedulis.I. Foraging efficiency and
ACKNOWLEDGEMENTS
Thisstudycouldnothavebeencompleted withoutS. Griffin's helpwiththedatacollection. Invaluable logistical support was provided by M. Greene,SanDiegoStateUniversity andthe
University ofCalifornia, Davis.Thispaperbenefitted greatly throughout thecourseof itsevolution fromcomments byG. Ruiz.Additional comments byB. Collier,G. Cox,S. England, S. Griffin,B. KusandG. Pageimproved earlierdraftsofthis manuscript.
interference. J. Anita. Ecol. 56:521-536.
Gromadzka, J. & Przystupa, B. 1984.Problems withthe ageingofdunlins intheautumn.WaderStudyGroupBull41: 19-20.
Groves,S. 1978.Age-related differences in RuddyTurnstone foragingandaggressive behavior.Auk95:95-103.
Thisiscontribution number469 ofthe PointReyesBird Observatory.
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vander Have,T.M., Nieboer,E. & Boere,G.C. 1984.Agerelateddistribution of Dunlininthe DutchWaddenSea. Pp. 160-176In: CoastalWadersand Wildfowlin Winter(ed. P.R. Evans,J.D.Goss-Custard & W.G. Hale).Cambridge UniversityPress,Cambridge. Kus,B.E.1982.Dunlinsandmerlinsonthe BolinasLagoon. PointReyesNewsletter 58,,Reprint:1-4. Kus,B.E.,Ashman,P., Page,G.W.& Stenzel,L.E. 1984.Age-related mortality ina wintering population of dunlin. Auk 101: 69-73.
Lomnicki, A. 1988.Population Ecologyof Individuals. PrincetonUniversity Press,Princeton, NJ. Marchetti, K. & Price,T. 1989.Differences in theforaging of juvenileandadultbirds:theimportance of developmental constraints. BioL Rev. 64: 51-70.
Page,G.W.& Whitacre,D.F.1975.Raptorpredation on wintering shorebirds. Condor77:73-83. Page,G.W.,Stenzel,L.E.& Wolfe,C.M.1979.Aspects of the occurrence of shorebirds ona centralCalifornia estuary. Studiesin AvianBiologyNo.2:15-39. Prater,A.J., Marchant,J.H. & Vourinen,J. 1977. Guide to the
Identification andAgeingof HolarcticWadersBritishTrustfor Ornithology, Tring.
Puttick, G.M.1979.Foraging behavior andactivity budgets of CurlewSandpipers. Ardea67:111-122. Puttick, G.M.1979.Foraging andactivity patterns inwintering shorebirds. Pp.203-231In:Behavior of MarineAnimals,Vol6 (ed.J. BurgerandB.L.Olla).PlenumPress,NewYork. Ruiz, G., Connors,P.G., Griffin,S.E. & Pitelka,F.A. 1989.
Structure of a wintering dunlinpopulation. Condor91: 562570.
Whitfield, D.P. 1985.Raptorpredation onwintering wadersin southeast Scotland. Ibis 127: 544-558.
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