Calibration ofvisuallyestimateddistancestomigrating seabirds with radarmeasurements
Mar´ıaMateos,1,2,4GonzaloM.Arroyo,1,2AlejandroRodr´ıguez,3DavidCuenca,1
andAndre´sdelaCruz2
1BiologyDepartment, Facultyof MarineandEnvironmentalSciences,University ofCa´diz,Av.RepublicaSaharaui,s/n,
11510PuertoReal,Ca´diz,Spain
2Fundacio´nMigres,ComplejoHuertaGrande,Ctra.N-340Km96.7,11390Pelayo,Algeciras,Ca´diz,Spain
3Department ofConservationBiology,Estacio´nBiolo´gicadeDon˜ana,CSIC,Am´ericoVespucios/n,IsladelaCartuja,
41092 Sevilla,Spain
ABSTRACT. Censusingseabirdsfromcoastalareasrequiresreliableestimatesofbirdnumbersandthedistances ofthebirdsfromthecoastline.Logisticalconstraintsmakevisualestimationofdistancestheonlyfeasiblemethod inmanystudies.Wetestedtheaccuracyofvisuallyestimatedoffshoredistancesofsix migratory seabirdspeciesin theStraitofGibraltarusingsimultaneousmeasurementsobtainedbyradar.Mostbirds(91%)weredetectedwithin
3kmofthecoastandwetruncatedourcalibrationatthisdistance.Wefoundastrongcorrelation betweenradar
andvisualestimates(R2=0.83,P0.0001).Themagnitudeoferrorsinvisualestimateswasmoderateandranged
from0.08to0.20fordifferentdistancesandobservers.Amongthefactorspotentiallyaffectingtheaccuracyof
visualestimatesofdistancetoseabirdinourstudywereobserveridentity,birdspecies,birdbehavior,andweather;
themostparsimoniousmodelinourstudyincludedobserver identityastheonlypredictor,andnomodelwithmore
thanonepredictorhadasmallerAkaike’sinformationcriterionvalue.Radarcanbeusedtohelptrainobserversand
toreducebiasesinvisualestimatesofdistancesbymeansofcalibration.Whennoothermethodsareavailableto
accuratelymeasuredistancestoseabirds,visualestimatesofdistances,recordedbyexperiencedobserversand once
calibratedwithradar(orotherground-truthingmethods),maybeacceptablefordifferentspeciesunderawide
rangeofenvironmentalconditions.
RESUMEN. Utilizacio´ndemedidasderadarparalacalibracio´ndedistanciasaavesmarinas migratorias estimadasvisualmente
Censaraves marinasdesdea´reascosterasrequiereestimacionesfiablesdelnu´merodeavesydeladistanciade estasacosta.Restriccioneslog´ısticashacenqueaveceslasestimaciones visualesdelasdistanciasseanelu´nico me´todofactiblepara llevaracaboestosestudios.Pusimosaprueba, enelEstrechodeGibraltar,laexactitudde lasestimaciones visualesdesdecosta,utilizandoseisespeciesdeavesmarinas,concurrentemente,conmedidas obtenidasporradar.Lamayor´ıadelasaves(91%)fuerondetectadasdentrodeuna´reade3kmdelacostay ajustamosnuestracalibracio´naestadistancia.Encontramosunafuertecorrelacio´nentrelasestimaciones visualesy lasdelradar(R2 =0.83,P0.0001).Lamagnituddeloserroresvisualesalestimarlasdistanciasfuemoderada yvariaronde 0.08a 0.20paradiferentesdistanciasyobservadores.Entrelosfactoresqueafectarona laexactitud delasestimacionesvisuales,seencuentranelobservador,la especiedeave,la conductaexhibidaporelaveylas condicionesmeteorolo´gicas.Elmodeloconmayorparsimoniaincluyo´laidentidaddelobservadorcomoelu´nico vaticinadoryningu´notromodelo,conma´sdeunvaticinador,tuvounvalordeAICmenor.Elradarpuedeser utilizadoparaentrenaralpersonaly reduceelsesgoenlasestimacionesvisualesmediantelacalibracio´n.Cuando nohaydisponibleotrosme´todosparamedirconexactitudladistanciaaquesemueven lasavesmarinas, elusode observadoresexperimentadosyunacalibracio´nconradar(uotrome´todo),puedenseraceptablesparatrabajarcon diferentesespeciesbajounaampliagamadecondicionesambientales.
Keywords: coastalmigratoryroutes,distancecalibration,distanceunderestimation,seabirdabundance,sea- clutter,StraitofGibraltar
Distancesamplingisawidespread method for estimatinganimalabundance(Buckland et al.2001, 2004), and maybe usefulfor estimatingdensitiesof marinebirds(Komdeur
4Corresponding author. Email: maria.mateos@
uca.es
etal.1992,Camphuysen etal.2004).Along seabirdmigratorypathwaysorfrequentlyused flightpaths,suchasbetweenforagingareas andbreedingcolonies,surveysfromstationary observationplatformsarerequiredto census birds(Camphuysenetal.2004).Thesecounts mayyieldabundanceestimateswithhighac- curacyprovidedthatdistancestobirdscanbe
measuredwithoutbiases(Bucklandetal.2004). Differentprocedureshavebeenusedtoobtain thosedistances,includingvisualestimates,opti- calrangefinders(Heinemann1981), binocular reticules(Yenetal.2004),radars(Petersenetal.
2006),andlaserrangefinders(Ransomand Pinchak2003).Characteristicsofthesurveyed populations,environmentalconditions,andthe accuracyrequiredwilldictatewhichmethods shouldbeused(e.g.,Bibbyetal.1992).Estimat- ingdistancebyeyehasbeencriticized,butcould betheonlyoptionundersome circumstances (Bibbyetal.1992),andnearlyallseabirdcensus work,whetherusingshore-basedorshipboard observers,reliesonvisuallyestimateddistances. However,noassessmentoftheaccuracywith whichobserversestimatethesedistancesexists. Previousattemptstomeasureerrorindistance estimatestoanimalsatsea,andthepossible effectsoffactorssuchasweatherontheaccuracy ofvisualestimatesofdistance,havefocusedon cetaceans(Schweder1997,BairdandBurkhart
2000,Williamsetal.2007).Becausedetection conditionsandmethodsforestimatingdistances tocetaceansdifferfromthoseusedforseabirds, calibratingdistanceestimatesto birdsatsea mayrequire specificstudiesaswell.However, toourknowledge,nostudyhasassessederrors indistanceestimatestoseabirdsorhasexamined thefactorsthatmayinfluencetheseerrors.
Differenttypesoferrormayarisefromvisual estimationofdistances,withbiaseddistance estimatesposingthemainproblembecausethey aredifficulttodetectandwilltransfererrorsto theestimatesofbirdabundance.Sucherrors canbecorrectedeitherinthefieldthrough testingprocedures and calibration(Marques
2004,Williamsetal.2007)orthroughstatis- ticaltechniquesemployedduringdataanalysis (Chen1998,Marques2004).Posthocanalytical approachesarelessrobustthan field-testing programs(Marques2004,Marquesetal.2006, Williamsetal.2007).Calibrationofdistance estimatestoseabirdsistherefore neededtoim- proveestimatesofbirdabundance.Calibration willbefurtherenhancediferrorsindistance estimatescouldbepredictedfrom factorssuch asbirdsize,birdbehavior,weatherconditions, orobserverability.
Werecordedandcomparedvisualandradar estimatesofdistancestoflyingseabirdsalonga migratorypathway.Ourspecificaimswereto: (1)testtheaccuracyofvisuallyestimatedoff-
shoredistancestoseabirdsbymeans ofradar measurements,and(2)determinetheeffectof birdsize,birdbehavior,weather,andobserver skillon theaccuracyofvisualestimates.
METHODS
Ourstudywasconductedfrom5Marchto
20April and16Octoberto18November2006
attheStraitofGibraltar(Fig.1).Thischannelis
14kmwideatitsnarrowestpointandistheonly
connectionbetweentheAtlantic Oceanandthe
MediterraneanSea.
FieldworkwascarriedoutatTarifaIsland
(southwestSpain),thesouthernmostpointof
thenorthcoastoftheStraitofGibraltar(Fig.1).
Fromthispoint,simultaneousobservationsof
migratingseabirdswerecollectedbyanobserver
andaradaroperator.Theradarantennaand
observerswereatthesamevantagepoint.Ob-
serversscannedtheseawithandwithouttheaid
ofbinoculars(VictoryFL10×42T;Zeiss,
Oberkochen,Germany)andtelescopes (Zeiss
20–60×85).Eachseabirdflockdetectedwas
identifiedtospecieslevelanditsdistancefrom
thecoastwasestimated bytheobserverwho
thencommunicatedthepositionoftheflock
anditsflightdirectiontotheradaroperator.
Ifthetargetwasfoundonthescreen,distance
wasalsomeasuredsimultaneously byan S-
bandsurveillanceradar(FR-2137SBB,Furuno,
Nishinomiya,Japan;peakpoweroutput30kW,
variablepulse0.07- 1.0 jis, transmittingat
3050MHz)locatedona platform10mabove
sealevel.Distanceestimatesweremadewhen
targetscrossedanimaginarylineperpendicular
tothecoastattheobservationpoint.
Atthebeginningofeachobservationperiod,
theradaroperator adjustedtheradarparameters
(range,gain,andseaandrainfilter)to optimize
the radarpicture.Wecombinedboththe range
functionandtheoff-centerfunction,placing
theorigin(antennaposition)inthelowerpart
ofthescreen, toreachascanningradiusof
eitherupto4.5km(shortrange) or9.1km
(longrange),withscanningradiusalternated
with eachobservationperiod.Gain,thereceptor
sensitivitytotheenergyreflected,wasadjusted
from45to100,where100correspondedtothe
maximumsensitivity.Sensitivitywasbetween
51and7075%ofthetime,between45and
5010%ofthetime,andbetween71and100
15%ofthetime.Theseaclutterfilterreduced
Fig.1. DetailsoftheAfricanandSpanishcoastlineintheareaofTarifa.Notethecharacteristicnarrow funnel-shapeoftheStraitofGibraltar.TarifaIslandisthemostefficientlocationforsurveyingseabirdsthe StraitofGibraltarfromthecoast (ProgramaMigres2009).
theclutterproducedbywaves(0=off;100= maximum).However,usingthisfiltermayresult inthe lossofweakechoes.Thebestsettingwas achievedwhenthesizeofthespotsproducedby seaclutterontheradarscreenwasminimized whileweakechoesunequivocallyproducedby flyingbirdsremainedvisible.Duringourstudy, theseaclutterfilter wassetatvaluesranging from0to40,withthefiltersetbetween11and
3075%ofthetime.Therainfilter(range=0–
100)reducedtheclutterproducedbytherain,
andwasalwayssetatvalues 30(andwassetat
zero90% oftime).
Datawerecollectedunderawiderangeof
meteorologicalconditions,andvisualobserva-
tionsweremadebythreeexperiencedobservers.
Allobservers hadcensusedseabirdsformore
than 4yearsatthestudysite,werefamiliarwith
thebirdspeciesandenvironmentalconditions
occurringinthestudyarea,andhadworked
withthe radarfor4monthsbeforethe onsetof
thisstudy.
Werestricted ouranalyses tothesixmost
abundantseabirds,includingNorthernGannets
(Morusbassanus),Cory’sShearwaters(Calonec-
trisdiomedea), BalearicShearwaters (Puffinus mauretanicus),AtlanticPuffins(Fraterculaarc- tica),Razorbills (Alcatorda),andGreatSkuas (Catharactaskua).Due to thedifficultyofiden- tificationand the frequentoccurrenceofmixed flocks,AtlanticPuffinsandRazorbillswerecon- sideredtogetherandreferredtoasauks.Foreach sighting,observersrecordedspecies,flocksize, flightdirection,andestimateddistance.
Eachhour,werecordedthespeedanddi- rectionofwindattheobservationsite.After examiningthefrequencydistributionofwind directionsduring2006attheTarifameteo- rologicalstation,winddirectionswerecatego- rizedaseithereastorwestbecausewindscame fromthesedirections94% oftime.Easterly winds(51.2%ofrecords)hadameandirection (±1SD)of79.8±22.2◦,andwesterlywinds (42.8%ofrecords)ameandirectionof271.0±
39.8◦.Allflyingbirdswere observedheading eithereastorwest.Flightdirectionswerecat- egorizedaseitherwithaheadwindortailwind (Spear andAinley1997).Visibilitywasassigned tooneofthreecategoriesbasedonhowclouds, haze, andlightlevelsaffected visibility ofthe
Africancoastlocated14kmaway:(1)good,with theAfricancoastclearlyvisible,(2)moderate, withonlythesilhouetteofthecoastvisible,and (3)bad,withtheAfricancoastnotvisible,but theseasurfacevisibleatadistanceofabout7km. Noobservationsweremadewhenboats,which typicallyfollowaroutealongthecenterof the strait,couldnotbeseen.
Althoughradarrecordswereusedtocalibrate visual estimates, radarmeasurementsarenot freefromerror.Thetechnicalspecificationsof ourradarestablishaninherentmeasurement errorof1%,withamaximumerrorof30m. Furthermore,birds appearonaradarscreen asatwo-dimensionalechoinsteadofaone- dimensionalpoint,especiallywheninflocks.We measuredthedistancetothecenteroftheecho. Thus,theremightbeasystematicerrorinthese radardistancesbecausethesizeofanechoonthe screencouldvarywiththedistancetothetarget duetothelevelofenergyreflected.Totestthe magnitudeofthiserror,werandomlyselecteda sampleof100targets.Wemeasuredthe echosize onthenorth–southaxisontheradarscreenwith theaidofageographical informationsystem (ArcView3.2.;EnvironmentalSystemsResearch Institute1999).Wefinallyexaminedwhether echosizevariedwithdistancetothetargetas measuredbyradar.
Statistical analyses. Theerrorofthe visualestimateofdistancewascalculated as
Error=|radarestimate–visualestimate|/radar estimate.Onlybirdsorflocksthatweredetected visuallyandhaddistancesestimatedwithradar wereincludedintheanalysis.Wedidnotassume thattherelationshipbetweenvisualestimatesof distance,ortheirerror,andthecorresponding radarestimateswerelinear.Therefore,weana- lyzedthisrelationshipwithgeneralizedadditive modelsthatmayfitnonlinear,smoothfunctions (HastieandTibshirani1990).
Second,weexaminedwhetherobserveriden-
tity,birdspecies,birdbehavior (flocksizeand
flightdirectionrelativetowinddirection),and
meteorologicalconditions(windspeedandvisi-
bility)influencedtheaccuracyofvisualestimates
ofdistance.Weusedvisuallyestimateddistances
asthe dependentvariableand includedthe
distanceestimatedbyradarasacovariatesothat
theeffectofpredictorswasassumedtoexplain
measurementerrors.Webuiltonemodelfor
eachpredictorandcomparedtheirfittothedata
withtheAkaike’sinformationcriterion(AIC).
Wethenexaminedwhetherthefitimproved whentwoormorepredictorswerecombined inthesamemodel. Weselectedthemostparsi- moniousmodelasthathavingthelowestAIC value(BurnhamandAnderson2002).
Analyses wereperformedwiththepackage
“gam”inR2.6.2(RDevelopmentCoreTeam
2008).Webuiltmodels assuming agamma dis-
tribution,usingthelog-linkfunction andthin
plateregression splinesassmoothsfunctions.
Weallowedforamaximumofthreedegreesof
freedomforthesmoothfunctions.Valuesare
reportedasmeans±1SD.
RESULTS
Observersmade1173 sightingsthat were subsequentlylocatedbyradar,including402
NorthernGannets,375Cory’sShearwaters,265
BalearicShearwaters, 117auks,and14Great
Skuas.Overall,42.4%ofsightingsweresingle
birdsandtherestwereflocks.
Wemade3.4%ofobservationsduringcalm
conditions(windspeed0.5ms−1),44.9%
duringperiodswitheasterlywinds(range=0.8–
15.8ms−1,mean=3.9±2.8ms−1),and51.7%
duringperiodswithwesterly winds(range=
0.6–9.4m s−1, mean=3.6 ±2.4 m s−1).
Visibilitywasgoodfor28.1%oftheobser-
vations,moderatefor 45.5%, and bad for
26.5%.
Radarestimatesofdistanceto birdsight-
ingsrangedfrom183to6913m(mean=
1559± 1009m),andvisuallyestimateddis-
tancesrangedfrom100 to 7000 m (mean
1438±978m).Mostcontacts(91%)were
within3000mofthecoast,as estimated by
radar,and,forfurtheranalysis,wetruncated our
dataatthatdistance.
Theestimatedsizeofechoesvariedfrom9to
33m(mean=17.9±7.8m,N=100flocks)
fordistancestothetargetsfrom105to2950m.
Echosizeontheradarscreendidnotvarywith
distancetothetarget(R2 =0.004,P=0.65).
Theerror indistanceestimatesattributedtothe
sizeof theechowassmall,always0.04,and,
becausemosttargetspassedwithin500m,this
errorwasoften0.02.
Visualestimatesofdistancewerecorrelated
withthecorrespondingradarestimates(R2 =
0.83,F =1135,P0.0001,N =1068;
Figs.2AandB).Theabsolutemeandifference
betweenvisualandradarestimatesofdistance
Fig.2. Relationshipbetweenradarestimatesofdistancetoflyingseabirds andcorresponding visualestimates intherange0–3km.Thebrokenlineindicatesanunbiasedrelationship.(A)Datarecordedbyobserver1, themostaccurateobserver.(B)Datarecordedbyobserver3,theleastaccurateobserver.
increased withincreasingdistance,withdif- ferencesof141m, 200m, and327m for distances0–1,1–2,and2–3km,respectively. The error,however,decreasedwithdistance, witherrorvaluesof0.20,0.15,and0.13for thethreedistanceintervals,respectively(overall meanerror=0.17±0.15).
Amongthefactorspotentiallyaffectingtheac-
curacyofvisualestimatesofdistanceto seabird,
themostparsimoniousmodelincludedobserver
identityas theonlypredictor(Table1),and
no modelwithtwoormorepredictorshad
asmaller AICvalue.Allobservers tendedto
underestimatedistances,andunderestimation
washigheratgreaterdistances(2km;Figs. 2A andB). Errorsinestimatingdistancewereless forobserver1(meanerror=0.13)thanfor observers2and3(meanerror =0.18and0.20, respectively)foralldistancesupto3km.Error differencesbetweenobserverswereexacerbated atgreaterdistances.Forthe2–3kmdistance interval,meanerrorswere0.08,0.19,and 0.20 forobservers1,2,and3, respectively.
DISCUSSION
Wefoundthatobserverscouldestimatedis- tanceswith reasonableaccuracy,asreflectedby
Table1. Generalizedadditivemodelsexamining theeffectoffourtypesofpredictors,namelyvisual observer,birdspecies,birdbehavior (flocksizeand flightdirection),andweatherconditions(windspeed andvisibility)onvisualestimatesofdistancesto seabirds.Distancesestimatedwithradarwerefittedas acovariateinallmodels,takingtheformofaspline with3df.
andBurkhart2000,Bucklandetal.2001),the threeobserversinourstudytendedtounderes- timatedistances.Underestimationincreasedat greaterdistances,whichwas alsoobservedin flaggingexperiments(Camp2007).Moreover, wedetectedanobserver-specificeffectofunder- estimationthatproduced asystematicerrorthat
shouldbecorrectedbeforeusingdatatoesti-
M odel Predictors AICa AAICb wic
aAkaike’sinformationcriterion.
bAkaikedifferences.
cAkaikeweights.
thesignificant correlation betweenradarand visualestimates. Furthermore,theerrorofvi- sualestimateswasrelativelylow(between0.08 and0.20,dependingontherangeof distances andobserver)andsimilar to errorsreported forvisualestimatesofdistancestocetaceans (Williamsetal.2007).Therequiredlevelof accuracyindistanceestimatesvariesamongspe- cificecologicalapplications.However,Marques (2007)suggestedthaterrorslessthan10%are neededbeforeapplyingestimatestodistance samplingmethodology.Errorsindistancees- timates produceerrorsofasimilarmagnitudein densityestimates,thatis,distanceconsistently overestimatedby10% underestimatesdensi- tiesby9%,anddistancesunderestimatedby
10%producedensitiesoverestimatedby11% (Bucklandetal.2001,2004).Errorsofvisual estimatesofdistancein our studywereon averagehigherthanrecommendedforareliable useofdistancesampling methods,andmay translateintounacceptablybiasedestimatesof populationsize.However,anumberofauthors havesuggestedthatsucherrorscanbecorrected throughcalibration,specificallybyusingregres- sionequationsobtainedfromthecomparisonof visualestimatesofdistancestoseabirdswitha ground-truthingmethod(Chen1998,Williams etal.2007).
Observeridentityaffectedthefitbetweenvi-
suallyestimateddistancesanddistancemeasured
byradar.Asalso reported inotherstudies(Baird
mateabundance(Bucklandetal.2001). Inour study,themostparsimoniousmodel(model2, Table1)maybeusedas acalibrationmodel tocorrecttheonlysourceofbiaswefound (Mateos2009).Differencesinaccuracybetween observersmaynotbeattributedtotheirdegreeof trainingorexperience,whichwassimilaramong thethreeobservers,buttovariationinvisual acuityorsharpness,perceptionofdistance,or otheraspectsof theirsensoryabilities.Lessex- periencedobserverswouldlikelyhaveproduced largererrors.Previousstudiesoftheeffectof trainingontheaccuracyofdistanceestimation onland(GibsonandBergman1954,Gibson etal.1955)andatsea(OienandSchweder1992, Schweder1996,1997)haveclearlyshownthat trainingimprovesestimationofdistance.Baird andBurkhart(2000)foundasimilareffectof experiencewhencensusingwhales.
Calibrationexperimentsinvolvingtheuseof
radarorotherground-truthingmethodscanbe
alsousefulfortrainingobserversandreducing
differencesindistanceestimates.Theaccuracy
ofvisualestimatesmightalsobeimprovedby
specifictrainingprograms, whereobserversare
toldthedistancesmeasuredwiththeradarafter
theyestimatethedistancetothetarget.Such
trainingwouldhelpobserverstobetterestimate
distances(Camp2007).
For investigatorswithout accessto radar-
estimateddistances,othermethodsareavailable
fortrainingobserverswhenmeasuringdistances
(e.g.,usingbuoysorfloatsplacedatknown
distancesfromtheobservationpoint)andwe
encouragetheimplementationoftrainingpro-
gramspriortofieldwork(eitherwithradar
orothermethods) forimprovingtheaccuracy
ofdistanceestimates.Furthermore,ourresults
suggestthat,whenpossible,selectingobservers
whoseinherentability toestimatedistanceis
bestamongallpotentialcandidatesisadvisable.
Ourresultsmay bealsousefulfordetermining
stripwidthwhendataaretakeninthefieldonly
bydistanceintervals(Bucklandetal.2001).For
example,inourcase,becausethehighesterror
valuewas0.2within3kmofthecoast,trying toestablishstripwidthsnarrowerthan600m wouldnotmakesense.
Distanceestimatesin our studywerenot
species-dependent,evenforspeciesthatvaried
considerablyinsize(e.g.,398gand549mm
wingspanforAtlanticPuffinsand3010gand
1850mmwingspanforNorthernGannets)and
flightbehavior(e.g.,fast-flappingauks,interme-
diateflap-glidingNorthernGannetsandglide-
flappingBalearic Shearwaters,and dynamic-
soaringCory’sShearwaters). Similarly,theac-
curacyofdistancesestimatedvisuallywasnot
affectedbyweatherconditions,flightdirection,
orwinddirection.Consequently,wesuggest
thatwhennoothermethodispractical,visual
estimatescouldbereliablyusedforawiderange
ofseabirdspeciesandfieldconditions,provided
thatobserversaresufficientlytrained.
Measuringdistancewith devicessuch as
telemeters andradarinthefieldisnotfreefrom
errorandthepossibleconsequencesofthiserror
ontheestimationof birdabundancearerarely
considered.Particularlywhenusingradar,the
signalontheradarscreenappearsas aspotthat
occupiesaspacegenerallylargerthanabirdor
flockinthefield.Nevertheless,themagnitudeof
thiserrorinourstudywaslow(generally2%),
anddidnotsubstantiallyaffectthevalidityof
distancemeasurements.
Tostatisticallyaddressmeasurementerrors,
manyobservationsare needed(Williamsetal.
2007).Ontheotherhand,collectingsufficient
ground-truthingdatatocalibratevisually es-
timateddistancescouldbecostly(Chenand
Cowling2001).However,usingradar,wefound
itpossibletoobtainaconsiderableamountof
independentdata,suitableforcalibration,with
relativelylittleeffort,andlargesamplespermit
morereliablecalibrationthansmallersamples.
Weconcludethat,whennoothermethods
areavailabletoaccuratelymeasuredistancesin
seabirdstudies,surveysbyexperiencedobservers
canprovidereasonably accuratedistanceesti-
matesunderawiderangeofenvironmental
conditionsandfordifferentspecies.Ourcon-
clusionscouldbeapplicabletoat-seasurveysfor
seabirdswhenobserversareonamovingship,as
longaslandmarkslocatedatknowndistances,
measuredwithradar,areavailable.Thismight
bethecaseforstudiesfromboatsclosetothe
coastorwhereshiptrafficisrelativelycommon.
Finally,theabilityofradartodetectlow-flying
seabirdsis limitedwhenwavescausesea-clutter, butradarcanbeusedtocalibratevisualob- servationsthat,inturn,canbeobtainedunder conditionswhenradarcannotbeoperated.
ACKNOWLEDGMENTS
Thisstudywasconductedwithinacollaborationagree- mentbetweentheMigresFoundationandtheUniversity ofCa´diz.TheradarfacilitywassuppliedbyCeowind CapitalEnergyOffShoreCompany.M.Mateoswassup- portedbytheJuntadeAndaluc´ıawithaFPUfellowship. WethanktheMigresFoundationtechnicalstafffortheir helpwiththefieldworkandB.Brudererforextensive commentsonearlydrafts.Themanuscriptwasgreatly improvedthankstothesuggestionsofA.E. Burger,one anonymousreviewer,andtheeditor.
LITERATURECITED
BAIRD,R.W.,AND S.M.BURKHART.2000.Biasand variabilityindistanceestimationonthewater:impli- cationsforthemanagementofwhalewatching.Paper SC/52/WW1,IWCScientificCommittee,Adelaide, Australia.
BIBBY,C.J.,N.D.BURGESS,ANDD.A.HILL.1992.Bird censustechniques.AcademicPress,London,UK.
BUCKLAND,S.T., D. R.ANDERSON,K.P.BURNHAM, J.L.LAAKE,D. L.BORCHERS,AND L.THOMAS.2001. Introductiontodistancesampling,estimatingabun- danceofbiologicalpopulations.OxfordUniversity Press,Oxford,UK.
———.2004.Advanceddistancesampling.OxfordUni- versityPress,Oxford,UK.
BURNHAM, K. P., AND D. R. ANDERSON. 2002.
Modelselectionandmultimodelinference:apracti-
cal information-theoreticapproach.Springer-Verlag,
NewYork,NY.
CAMP,R.J.2007.MeasurementerrorsinHawaiianforest
birdsurveysandtheireffectondensityestimation.
Hawai‘iCooperativeStudiesUnitTechnicalReport
HCSU-005,UniversityofHawai‘iatHilo,Hilo,HI.
CAMPHUYSEN,C.J.,A.D.FOX,M.F.LEOPOLD,AND
I.K.PETERSEN[online].2004.Towardsstandardised
seabirdsat seacensustechniques in connection
with environmental impact assessments for
offshorewindfarms intheUK.RoyalNetherlands
InstituteforSeaResearchandtheDanishNational
Environmental ResearchInstitute, Crown Estate
Commissioners,London,UK.Availableat:
phase1_final_04_05_06.pdf(10March2010).
CHEN, S.X.1998.Measurementerrorsinlinetransect
surveys.Biometrics54:899–908.
———,ANDA.COWLING.2001.Measurementerrorsin
linetransectsurveyswhere detectabilityvarieswith
distanceandsize.Biometrics57:732–742.
ENVIRONMENTAL SYSTEMSRESEARCHINSTITUTE. 1999.
ArcViewGIS3.2.EnvironmentalSystemsResearch
Institute,Inc.,Redlands,CA.
GIBSON,E.J.,AND R.BERGMAN.1954.Theeffectof
training on absoluteestimation of distanceover
theground.JournalofExperimentalPsychology48:
473–482.
———, AND J. PURDY. 1955. The effectof prior
trainingwithascaleofdistanceonabsoluteand
relativejudgmentsofdistanceoverground.Journal
ofExperimentalPsychology50:97–105.
HASTIE,T., AND R.J.TIBSHIRANI.1990. Generalized
additivemodels.ChapmanandHall,London,UK.
HEINEMANN, D.1981.Arangefinderforpelagicbird
censusing.JournalofWildlifeManagement45:489–
493.
KOMDEUR,J.,J.BERTELSEN,ANDG.CRACKNELL.1992.
Manualfor aeroplaneandship surveysofwaterfowl
andseabirds.IWRBSpecialPublicationNo.19, Na-
tionalEnvironmentalResearchInstitute,Roskilde,
Denmark.
MARQUES,T. A.2004. Predictingand correctingbias
causedbymeasurementerrorinlinetransectsam-
plingusingmultiplicativeerrormodels.Biometrics
60:757–763.
———. 2007. Incorporating measurementerror and
density gradients in Distance Sampling surveys.
Ph.D. dissertation,UniversityofStAndrews,St.
Andrews,Scotland,UK.
———, M. ANDERSEN,S.CHRISTENSEN-DALSGAARD,
S.BELIKOV,A.BOLTUNOV,O.WIIG,S.T.BUCK-
LAND,ANDJ.AARS.2006.TheuseofGPStorecord
distancesinahelicopterline-transectsurvey.Wildlife
SocietyBulletin34:759–763.
MATEOS,M. 2009. Radartechnologyappliedtothestudy
ofseabirdmigrationacrosstheStraitofGibraltar.
Ph.D.dissertation,UniversityofCadiz,Cadiz,Spain.
OIEN,N.,ANDT.SCHWEDER.1992.Estimatesofbiasand
variabilityinvisualdistancemeasurementsmadeby
observersduringshipboardsurveysofnortheastern
Atlanticminkewhales.Reports oftheInternational
WhalingCommission42:407–412.
PETERSEN,I.K.,T.K.CHRISTENSEN, J.KAHLERT,M.
DESHOLM, AND A.D.FOX.2006.Finalresultsof
birdsstudiesattheoffshore windfarmsatNysted
andHornsReef,Denmark.NationalEnvironmental
ResearchInstitute, Ministryofthe Environment,
Roskilde,Denmark.
PROGRAMAMIGRES.2009.Seguimientodelamigracio´n delasavesenelEstrechodeGibraltar:resultadosdel ProgramaMigres2008.Migres,RevistadeEcolog´ıa
1:83–101.
RDEVELOPMENT CORETEAM.2008.R:alanguageand
environmentforstatisticalcomputing.RFoundation
forStatisticalComputing,Vienna,Austria.
RANSOM,D. J.,AND W.E.PINCHAK.2003.Assessing
accuracy ofalaserrangefinderinestimatinggrassland
bird density.WildlifeSocietyBulletin 31: 460–
463.
SCHWEDER,T.1996.Anoteonabuoy-sightingexper-
iment intheNorth Seain1990. Reportsofthe
InternationalWhalingCommission46:383–385.
———.1997.MeasurementerrormodelsfortheNor-
wegianminke whalesurveyin 1995. Reportsof
the International WhalingCommission47:485–
488.
SPEAR,L.B.,ANDD.G.AINLEY.1997.Flightspeedof
seabirdsinrelationtowindspeedanddirection.Ibis
139:234–251.
WILLIAMS,R., R. LEAPER,A.N. ZERBINI, AND P. S.
HAMMOND. 2007.Methodsforinvestigatingmea-
surement error in cetaceanline-transectsurveys.
JournaloftheMarineBiologicalAssociationofthe
UnitedKingdom87:313–320.
YEN,P.P.W.,W.J.SYDEMAN,ANDK.D.HYRENBACH.
2004.Marinebirdcetaceanassociationswithbathy-
metrichabitatsandshallow-watertopographies:im-
plicationsfortropictransferandconservation.Jour-
nalofMarineSystems50:79–99.