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BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofitpublishers, academic institutions, research libraries, and research funders in the common goal of maximizing access tocritical research. Experimental Evidence That Scare Tactics and EffigiesReduce Corvid OccurrenceAuthor(s): Sara A Peterson and Mark A ColwellSource: Northwestern Naturalist, 95(2):103-112. 2014.Published By: Society for Northwestern Vertebrate BiologyDOI: http://dx.doi.org/10.1898/NWN13-18.1 URL: http://www.bioone.org/doi/full/10.1898/NWN13-18.1 BioOne (www.bioone.org ) is a nonprofit, online aggregation of core research in thebiological, ecological, and environmental sciences. 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EXPERIMENTALEVIDENCETHATSCARETACTICSANDEFFIGIES REDUCECORVIDOCCURRENCE S ARA AP ETERSONAND M ARK AC OLWELL WildlifeDepartment,HumboldtStateUniversity,Arcata,CA95521USA A BSTRACT —CommonRavens( Corvuscorax )andAmericanCrows( C.brachyrhynchos )are importantpredatorsofeggsandchicksoftheSnowyPlover( Charadriusnivosus ),which compromisespopulationrecoveryofthisfederallylistedspecies.Weusedabefore-after, control-impactexperimentovera4-dintervaltoexaminechangesincorvidoccurrencewithin1, 10,and50mofafeedingareainresponsetoscaretacticsandcorvideffigies,anon-lethalpredator controlmethod.WeconductedourstudyduringSeptember–FebruaryatClamBeach,California, wherecorvidsareabundantandploversexperiencehighreproductivefailurecomparedwith othersitesinnorthernCalifornia.OnDay1,foodandtrashattractedcorvidswithin1–2hafter sunrise,suggestingthatsomeindividualsfrequentedbeachestoscavengeforfoodleftbyhumans. OnDays2through4,effigiessignificantlyreducedaveragecorvidabundanceandincidence (percentageofobservationswithatleast1corvidpresent),buttheeffectwasonlysignificant withinthe50-mzone.Inallcases,however,some,albeitfewer,corvidscontinuedtooccuronplots witheffigies,suggestingthattheireffectivenessasadeterrentofcorvidsnearplovernestsduring thebreedingseasonislimited. Keywords:AmericanCrow,CommonRaven,corvids,effigy,non-lethal,predatormanage- ment,SnowyPlover Worldwide,corvidshavehadalongassoci- ationwithhumans;theyarebothreveredand persecutedinmanycultures(Moore2002; MarzluffandAngell2005;Londei2010).In NorthAmerica,corvidpopulationshavein- creasedandrangeshaveexpandedintoformer- lyunoccupiedhabitats(Restaniandothers2001; Kellyandothers2002;KristanandBoarman 2003;MarzluffandNeatherlin2006),primarily aidedbyincreasedfoodsupplementsmade availabletotheseintelligentomnivoresby humans(MarzluffandAngell2005).Increases incorvidabundanceareoftencorrelatedwith declinesintheirprey(LauroandTanacredi 2002;Kellyandothers2005;WebbandMarzluff 2007;Klausenandothers2010),especially threatenedandendangeredtaxasuchasthe DesertTortoise( Gopherusagassizii ;Kristanand Boarman2003),MarbledMurrelet( Brachyram- phusmarmoratus ;PeeryandHenry2010),Greater Sage-Grouse( Centrocercusurophasianus ;Coates andothers2008),andtheCaliforniaLeastTern ( Sternaantillarumbrowni ;Caffrey1995).Howev- er,causalrelationshipsbetweenpredatorabun- danceandpreydeclinearedifficulttoestablish becausethereareoftenmultiplepredatorsat workandthepredatorassemblagechangesover timeandspace(Luginbuhlandothers2001). Corvidsareadeptpredatorsofeggsandyoung ofmanybirdspecies,andthispredationis widelyrecognizedasanimportantecological factorthatmaylimitpopulationsizesinsome species(Ricklefs1969;Martin1993).Therefore, understandingcorvidbehaviorinresponseto potentialdeterrentsmaybeusefulforreducing predationandincreasingproductivityofthreat- enedandendangeredspecies. Avarietyofnon-lethalcontrolmethodshave beenusedbywildlifemanagerstodeteravian predators,includingscaretactics,repellents, andnestexclosures.Eachofthesemethods hasshortcomingsandmayonlyprovideshort termbenefits(Schmelzeisenandothers2004; HardyandColwell2008;Paulinyandothers 2008).Effigies,suchascarcassesandtaxadermic preparations,areamethodofnon-lethalpred- atorcontrolthatmimicsadeadmodelofthe predatorinanattempttoscareindividualsand detertheiruseofanarea.Effigieshavebeen usedtoscaregulls( Larus spp.),TurkeyVultures ( Cathartesaura ),ravens,andcrowsandtodeter themfromroostinginundesirablelocations NORTHWESTERNNATURALIST95:103–112A UTUMN 2014 103 suchasairportsandurbanareas(Caffrey1995; Seamans2004;Averyandothers2008;Ball 2009).Anecdotalreportssuggestthateffigies aresuccessfulatdeterringcorvidsnearCalifor- niaLeastTernbreedingcolonies(Caffrey1995). Lethalpredatorcontrolisoftencontroversial andlackspublicsupport(Messmerandothers 1999).Moreover,lethalcontrolmaynotbe effectiveatminimizingnegativeimpactson reproductivesuccessifcontroldoesnotreduce predatorpopulationsizeorfailstoremove individualsthatcauselossesofeggsandchicks. Removalofindividualsmaybeatemporary solutionifconspecificsfillvacantterritories (Webbandothers2012).Ineithercase,lethal controlalonemaynotbesufficienttosignifi- cantlyincreasereproductivesuccess(Done- howerandothers2007).Additionally,residual effectsofremovingpredatorsareoftenun- knownand,aswithotherpredatorcontrol methods,killingmayprovideonlyashortterm solutionwithoutthedesiredeffectofultimately increasingbreedingbirdpopulationsizes(Co ˆ te andSutherland1997).Beforeadecisiontokill predatorsismade,othermanagementoptions shouldbeexplored(Boarman2003;Marzluff andAngell2005). In1993,theUnitedStatesFishandWildlife Service(USFWS)listedthePacificcoastpopu- lationoftheSnowyPlover( Charadriusnivosus ) asthreatenedundertheEndangeredSpecies Act(USFWS1993).Oneoftheprincipalfactors limitingploverrecoveryiseggandchick predation(USFWS2007),especiallybyCom- monRavens( Corvuscorax )andtoalesser degreeAmericanCrows( C.brachyrhynchos ).To addressthenegativeimpactsofcorvidson plovers,avarietyoflethalandnon-lethal methodshavebeenusedwithvaryingdegrees ofsuccess.Althoughlethalmethodshavebeen usedinanefforttoincreasereproductive successofSnowyPloverselsewherealongthe Pacificcoast(forexample,Robinson-Nilsenand others2009),resultsaremixedwithregardto theireffectiveness(Neumanandothers2004). Toreducenestpredationrates,managershave: (1)exclosednestsfromlargemammalandavian predators(HardyandColwell2008);(2)re- storednestinghabitatbyremovinginvasive EuropeanBeachGrass( Ammophilaarenaria ) (MuirandColwell2010);(3)spreadoyster shellstoincreaseeggandchickcrypsis(USBLM 2010);and(4)killedpredators(Neumanand others2004).Despitetheseefforts,Snowy Ploverpopulationsremainwellbelowthe region-widerecoverygoalof3000breeding adults(USFWS2007). Overthepast12yandacrossapproximately 20locationsinNorthernCalifornia,theleading causeofnestfailurehasbeeneggpredationby corvids.Evidenceforcorvidpredationinthis regionincludesanegativecorrelationbetween ravenactivityandpercapitareproductive successofplovers(BurrellandColwell2012), andin2008and2009videocamerasshowed thatravensdepredated70%of20failedplover nestsatClamBeach,California,oneofthe region’smostimportantnestingsites(Burrell andColwell2012).Here,wepresentexperi- mentalevidenceevaluatingtheeffectivenessof ravencarcassesor‘‘effigies’’inreducingCom- monRavenandAmericanCrowpresenceand abundanceatClamBeach,asitewhereSnowy Plovereggandchickpredationbycorvidswas especiallyhigh(BurrellandColwell2012; HardyandColwell2012). M ETHODS StudyArea ClamBeach,HumboldtCounty,California (Fig.1)frontsthePacificOceanandextends approximately7kmbetweenthemouthsof LittleRiverandMadRiver.InvasiveEuropean BeachGrass,andnativeplantssuchasSea- rocket( Cakilemaritima ),SandVerbena( Ambro- niaumbellata ),andDuneGrass( Leymusmollis ) dominatedunevegetation.Unvegetatedsub- stratesconsistmostlyofsand,litteredwith shells,woodydebris,eelgrassbundles,and brownalgae(Colwellandothers2010;Hardy andColwell2012).Ofallploverbreedingsites innorthernCalifornia,ClamBeachhasthe highestlevelsofhumanuse(Burrelland Colwell2012),suchasjogging,clamming,dog walking,andhorsebackriding.Asaresult,the sitehasacomparativelylargeamountof anthropogenicgarbage(MAColwell,unpubl. data). Effigies WeacquiredravencarcassesfromtheHum- boldtWildlifeCareCenterwherebirdshad eitherdiedorwereeuthanizedbecausesevere 104NORTHWESTERNNATURALIST95(2) FIGURE1.LocationofexperimentalstudyoftheuseofeffigiesoncorvidoccurrenceonClamBeach,Humboldt County,CA,September2011throughFebruary2012.Pairedcontrol(C)andtreatment(T)plotsseparatedfromeach otherby25mwerelocatedatthemidpointof500-mbeachsecti ons(horizontaldashes),withanobserverlocatedatthe midpointofplots(X).Horizontalbarsassociatedwithth emeasurementscaleatthetopofthefigureshowsaverage numberofCommonRaven( & )andAmericanCrow( )alongfourteen500-msectionsofbeachduringMarch– August,2005–2011,coincidentwiththeSnowyPloversbreedingseason(seeColwellandothers2010formethods). A UTUMN 2014P ETERSONAND C OLWELL :C ORVID D ETERRENTS 105 traumaprecludedrehabilitation.Weprepared effigieswithwingsandtailfeathersspreadto portrayanunnaturalpositionofadeadbird. ExperimentalDesignandDataCollection Toavoidpossiblenegativeimpactstobreed- ingplovers,weconductedourexperimentfrom September2011throughFebruary2012,when ploversdonotbreed(Colwellandothers2010). WeusedArcGISv.9.3(ESRI,Redlands,CA)to subdividethenorth-southlengthofthebeach intofourteen500-msegments(Fig.1),and randomlyassignedeachsegmenttoatrialdate. Atthestartofeachtrial,weestablishedpaired 50-mradiuscontrolandtreatmentplotsonthe foredunesnorthandsouthofthemidpointof each500-msection,withplotedgesseparated by25m.Weplacedsmallpiecesofwoody debriswithinplotsat1,10,and50mfromthe centerofeachplotasplotmarkerstofacilitate observations.Observationsoccurredfroma blindatthemidpointbetweentheplots(Fig.1). Weusedabefore-after,control-impactdesign consistingofpairedtreatment(effigyandbait) andcontrol(baitonly)plots,whichwedeter- minedrandomlypriortoeachtrial.Duringa4- dinterval(ortrial),weobservedcorvidson pairedplotsfor4hbeginningatsunrise.Each trialconsistedofDay1(before)whenwebaited bothplotswithfood(alargecontainerofFrench fries,alargesoftdrinkcup,andapaperbag); Day2(after)whenweusedscaretacticsandan effigy;andDays3and4(after)whenwehung aneffigyinthetreatmentplot.Theeffigyhung froma0.5-mplasticpipeattachedhorizontally toa2.5-mmetalpoleplacedattheplot’scenter; weplacedanidenticalpole(withouttheeffigy) atthecenterofthecontrolplot.Bothplotshad baitforDays2,3and4.Toavoidvandalization andremovalofourequipment,weremovedall itemsfromplotsaftereachmorningobserva- tion.Initially,weconducted14trialsoneachof the14beachsections;werepeatedtrialslatein thestudyon4sectionsofbeach,whichresulted inatotalof18trials. Becausebirdsmaylearnthatanobjectis harmlessandsimplyignoreit(Godin1994; Schmelzeisenandothers2004),itisimportantto testifcorvidscontinuetoutilizeanattractive area(suchasanareawithbait)whenfacedwith ariskfactor(J.Marzluff,pers.comm.).Effec- tivenessofscaringdevicescanbeenhanced withloudnoises,whichreducethepotentialfor habituation(Godin1994).Accordingly,we supplementedeffigieswithscaretactics.On the1sttreatmentday(Day2)beforewehung theeffigy,anassistantwearingamaskto eliminatefacialrecognitionbycorvids(Cornell andothers2011)actedoutatheatricaldeath sceneatthecenterofthetreatmentplotas corvidspassednearby.Duringthisscene,the assistantyelledwhileshakingtheeffigy,threw theeffigyontheground,andpretendedto shootitwhileaspeakerbroadcastedloud gunshots.Immediatelyafterthescene,the assistanthungtheeffigyandplayedarecording ofravendistresscalls(CornellLabofOrnithol- ogy,MacaulayLibrary),whichplayedthe durationofthe4-hobservation.Weplaced informativesignsrequestingpubliccooperation innotdisturbingtheplotsataccesspointstothe beachandatplotboundariesduringeach observation.Werefertothecollectiveeffectof effigyandscaretactics(appliedonDay2only) andtheeffigy(Days3and4)aseffigy throughoutthepapertosimplifyreporting results.However,weacknowledgethatthe scaretacticsareanintegralfactorinfluencing ourresults. Oneofus(SAP)conductedallobservations fromablindhiddenamidstdunegrassata vantagepointthatmaximizedobservationof bothplotsandminimizedthelikelihoodthat corvidswereawareoftheobserver’spresence. Weusedaninstantaneoussamplingmethod (seeMartinandBateson2007)torecordthetotal numberofcorvidspresentwithin1,10,and50m ofthecenterofthepairedplotsatthebeginning ofeachobservation.Wechosethesedistancesto facilitateeaseofdatacollectionundersome- timesfast-pacedchangesinthepositionand behaviorofcorvids.Wealternatedobservations betweencontrol(baitonly)andtreatment(bait andeffigy)plotseveryminute,whichyieldeda totalof120observationsforeachplotduringa 4-hobservationperiod. DataSummaryandAnalysis Wesummarizedthenumberofcorvids presentwithin1,10and50mofeachplot centerbasedon120observations.Thispro- duced2responsevariables:abundance(average numberofcorvids)andincidence(percentage of120observationswithatleast1corvid).To 106NORTHWESTERNNATURALIST95(2) evaluatetheeffectivenessofbaitasanattrac- tant,wecalculatedtheaveragetime(min) requiredforthe1stcorvidtoapproachwithin 50,10,and1moftheplotcenteronDay1(bait butnoeffigy).Wefoundnosignificantdiffer- ence(ateachspatialscale;all P -values . 0.38; Peterson2013)inthetimeittookcorvidsto entertreatmentandcontrolplotsonDay1; therefore,wepooleddatatoincreasesample size( n 5 36). FollowingmethodsofTarrandothers(2010), wegaugedtheeffectivenessofeffigiesin reducingcorvidabundanceandincidenceusing theformula D i 5 (X IAi 2 X IBi ) 2 ( X CAi 2 X CBi ), where D i denotesthetreatmenteffect, X IA and X IB aremeanresponsesafterandbefore, respectively,ontreatmentplots,and X CA and X CB aremeanresponsesafterandbefore, respectively,oncontrolplots.Wecalculated relativedifferencesandeffectsizesbycompar- ingaveragecorvidabundanceandincidenceon plotswitheffigiesandcontrolplots.Inthis comparison,weusedmeansaveragedacross Days2,3,and4ofeachtrialbecausetherewere nosignificantdifferencesin D i acrossthesedays (all F , 0.93,all P . 0.40). Weusedpaired t -teststoexaminedifferences inresponsevariablesbetweenplots.Weused Spearman’srankcorrelationtesttoexamine whetherornottheeffectivenessoftheeffigy diminishedovertime(18trialsconducted September–February).Wecombineddatafrom the3daftertheuseofeffigies(seeabove)and analyzedeffigyeffectswithin1,10,and50mof theplotcenterinasimplebefore(Day1)and after(Days2,3and4)comparison.Insummary statistics,wepresentmean ± standarderror. R ESULTS Althoughbothravensandcrowswerepres- entonClamBeach,inmosttrials(83%)only ravensfrequentedplots;bothravensandcrows occurredin17%oftrials.Ravensoccurredin similarnumbers(similarincidenceandabun- danceacrossthe18trials)acrossthe7kmof beach,whereascrowswererecordedonplotsat themostnorthernstretchofbeachandnearthe mainpublicaccesspointsadjacenttoparking lotsandpicnicareas.Thesedistributionsare similartotheobservationsofravensandcrows duringtheploverbreedingseason(seeinset Fig.1). OnDay1,priortoscaretacticstheatrical displayandeffigyplacement,corvidsrespond- edreadilytobait,withthehighestnumbers within50moftheplotcenter(Fig.2).On average,the1stcorvidapproachedtowithin 50mofbaitapproximatelyanhour(53 ± 2min) afterthestartofobservations;however,ittook longerforcorvidstoapproachwithin10m(111 ± 3min)and1m(136 ± 4min)ofbait.OnDay 1,significantlymorecorvids(Paired t -test 5 2.91, P 5 0.01)frequented50mtreatment(0.29 ± 0.07)thancontrolplots(0.19 ± 0.05). Afterweintroducedscaretacticsandeffigy (Day2)andeffigies(Days2,3and4combined), corvidabundancedecreasedonbothplots (Fig.3),althoughplotswitheffigies(Paired t 17 5 4.2; P 5 0.001)decreasedtoagreaterextent thancontrolplots(Paired t 17 5 2.1; P 5 0.06). Overall,relativedifferencesbetweentreatment andcontrolplots(Table1)indicatedthateffigies reducedcorvidabundance27to70%andcorvid incidence55to100%.Thetreatments,which includedeffigieswithscaretactics,appearedto FIGURE2.Average( ± SE)abundanceandincidenceofcorvidswithin50,10,and1mofbaitonDay1. A UTUMN 2014P ETERSONAND C OLWELL :C ORVID D ETERRENTS 107 havetheirgreatesteffectwithin1and10mofthe effigy,althoughthesignificanceofresultswas minimizedbythefewcorvidsthatapproached baitthisclosely. Corvidresponsetoeffigiesappearedstron- gestintrialsconductedearlyinthestudy (Fig.4).However,averagenumber( r s 5 0.27, P 5 0.87)andincidence( r s 5 0.27, P 5 0.86)of corvidswasnotcorrelatedwithtrialsequence (trial1to18).InJanuaryandFebruarytherewas anoticeabletreatmenteffectwhentheexperi- mentoccurredinthenorthernmostsectionof thebeach,alocationwithlargenumbersof crows.Withcrowactivityremovedfromtrials, therewasstillnosignificanttreatmenteffectin averagenumber( r s 5 0.27, P 5 0.87)or incidence( r s 5 0.27, P 5 0.86)ofravenswith respecttotrialsequence. Finally,speciesotherthancorvidswere attractedtothebaitdespitetheearlymorning observations;non-corvidsdisturbedorcon- sumedbaitduringmost(97%)trials.Control andtreatmentplotswerevisitedbydogs(78% and78%,respectively),humans(78%and83%), andgulls(83%and67%).Insomecases,bait wascompletelyconsumedorremovedin controlandtreatmentplotsbydogs(44%and 17%,respectively),humans(6%and6%),and gulls(72%and67%).Inallinstances,however, trashremainedasanattractantforcorvids. D ISCUSSION Ourexperimentofferspreliminaryevidence thateffigiesmaybeeffectiveatreducingcorvid activity.Thisfindingappliedonlytothe50-m zonewithinplots;resultsforthe10-and1-m zonesweresimilarbutnotstatisticallysignifi- cant,probablyowingtothelargenumberof observationswithnocorvids.Interestingly, duringthe3doftreatment(effigies)corvid activityalsodecreasedoncontrolplots.We cautiouslyinterpretthisresultasatreatment ‘‘spillovereffect’’ontoadjacentcontrolplots separatedby25m.Overall,ourfindings corroboratereportsthateffigiesdetercorvid useofspecificareassuchasroosts(Averyand others2008),aswellasanecdotalevidenceof theireffectivenessnearcoloniesofbreeding birds(Caffrey1995).Weacknowledgethatour findingsarespecifictothepresenceofaneffigy coupledwithatheatricaldeathsceneand playingofdistresscalls,thereforeourinterpre- tationofcorvidresponsetothepresenceof effigiesincludesallofthesefactorspresent together,notseparately. Althougheffigiesalonemayholdpromiseas anon-lethalmethodforreducingcorvidim- pactsonSnowyPlovers,severalunanswered questionsremainregardingtheirutilityin boostingproductivityandaidingintherecov- eryofthislistedspecies.First,wedesignedour studyasaseriesofshorttrialstoprovide sufficientreplicationforstatisticalanalyses.How- ever,the4-dtrialrepresentedasmallpercentage (approximately12%)ofthe32d(encompassing egg-layingandincubationperiods;Pageand FIGURE3.Comparisonofaverage( ± SE)corvid abundancewithin50mofbaitonplotswitheffigies comparedwithcontrolplots. TABLE1.SummaryoftheeffectivenessofscaretacticsandaCommonRaveneffigyatreducingcorvid abundanceandincidencewithinvaryingdistancesofbait. Abundancetreatmenteffect:Incidencetreatmenteffect: Distance Average D i a SEts b df P Average D i SEtsdf P 50m 2 0.1340.042 2 3.16170.006 2 0.1120.043 2 2.58170.019 10m 2 0.0310.028 2 1.10170.287 2 0.0730.038 2 1.90170.074 1m 2 0.0160.021 2 0.75170.461 2 0.0290.018 2 1.66170.115 a D i 5 (X IAi 2 X IBi ) 2 ( X CAi 2 X CBi ),thedifferencebetweenthechangeincorvidactivityatthecontrolplotandthetreatmentplot. b ts 5 teststatistic,thevalueobtainedfromtheperformedstatisticaltestandusedtorejectoracceptthenullhypothesis. 108NORTHWESTERNNATURALIST95(2) others2009)duringwhichplovereggsare exposedtopredation.Ifeffigiesareusedto enhancenest(andchick)survival,thentheir efficacyshouldbeevaluatedforlongerintervals, ideallyduringtheploverbreedingseason.The effectivenessofeffigieswilllikelybesite- dependentandvarywiththeabundanceand behaviorsofcorvidsaspredatorsofplovernests andchicks. Second,effigiescouldpotentiallynegatively affectshorebirdnestsiteselection(Colwell 2010),whichmaydependonwhenandwhere effigiesaredeployed.Forexample,astrategyof erectingmultipleeffigiesinsuitablehabitat priortotheploverbreedingseasonmay adverselyaffectplovernestsiteselectionbe- haviorsifindividualsperceiveeffigiesasa danger(Lima2009)andavoidthesenesting areas.Alternatively,effigieserectedafterthe startofbreedinginareasofhighnestdensity (forexample,colonialseabirds;Caffrey1995) maybesuccessful,assumingthateffigiesdonot causenestabandonment.Ineithercase,plover behavioralresponsestoeffigiesremainun- knownandrequirefurtherstudy. Athirdissueconcerningtheeffectivenessof effigiesinboostingploverreproductivesuccess concernstheirutilityinprotectingnidifugous chicks.Specifically,whileeffigiesnearnests maybeusefulinprotectingeggs,precocial chicksoftenwanderwidelyunderthecareof adults(WilsonandColwell2010).Consequent- ly,effigyeffectivenesswilldiminishasbroods roamintoareasthatlackthem.Evenifeffigies proveeffectiveinprotectingeggs,theywillnot boostlocalproductivityunlesstheydiminish predationofchicks.Thisobservationistrueof anothercommonlyusednon-lethalmethodof predatorcontrol,nestexclosures,inwhichnests areabletohatchduetoprotection;however, mostchicksaredepredatedoncetheyleavethis protectedarea(HardyandColwell2008). Resolvingtheseissuesrequiresfurtherstudy ofeffigies. Theinclusionofbaitwasacriticalelementof ourstudydesignbecauseitshowedthatcorvids avoidedanarea,suchaswithaneffigythatwas otherwiseattractivetothem.Althoughwe informedthepublicofourstudy,weobserved humansremovingbait,andtheirdogsaswellas gullsconsumedbait.Ourresultswithstood disruptionofmosttrialsbytheseevents.Still, thepresenceoftrashaloneasanattractant resultedinacomparativelyrapidresponseof FIGURE4.Seasonalvariation(trialsequence1–18)intheeffect(D i )ofeffigiesonaveragenumberand incidenceofcorvidswithin50mofbait. A UTUMN 2014P ETERSONAND C OLWELL :C ORVID D ETERRENTS 109 corvidstobaitonDay1.Thisobservation suggeststhatsomecorvidsregularlypatrolled thebeachtoscavengefood.Corvidabundance hasbeenshowntobepositivelycorrelatedwith humansettlementsandcampgrounds,presum- ablybecausefoodisreadilyavailable(Kristan andBoarman2003;MarzluffandNeatherlin 2006;WitheyandMarzluff2009).Twoaddi- tionallinesofevidencesuggestanassociation betweencorvidsandhumansinourstudyarea: apositivecorrelationbetweentracksofcorvids andhumans(Colwell,unpubl.data),andthe concentrationofAmericanCrowsnearparking lotsandpicnicareas(asterisks,Fig.1).Collec- tively,theseobservationssuggestthateffortsto managecorvidsmustincludereducinghuman useofplovernestingbeacheswhenploversare nesting,andremovalofanthropogenicsources offood. Corvidsareintelligent(Emery2006),highly socialomnivoresthattransmitknowledge culturallyviainteractionswithconspecifics (Cornellandothers2011).Severalunforeseen problemsmayhaveinfluencedourresultsby alteringcorvidbehaviors.First,theeffectiveness ofthebaitmayhavebeencompromisedifitwas consumed,leadingtolowercorvidactivity. Therewas,however,nodetectabledifference inbaitlossfromcontrolandexperimentalplots (Peterson2013).Ideally,itwouldbebesttotest effigiesatsiteswherepublicinterferencewas minimal.Second,usingFrenchfriesasbait facilitatedrapidconsumptionbygullsanddogs onbothcontrol(54%)andtreatmentplots(43%), leavingonlytrashasanattractant.Corvidsmay nothaveremainedonplotsorreturnedon subsequentdaysiftherewasnoinitialreward. Becausesingleorpairedravensoftenrecruit groupsofravenstofoodsources(Heinrich1988; Wrightandothers2003),theinformationofno rewardmayhavebeencommunicatedto conspecifics.Abetteroptionmayhavebeento usealargecarcass,whichwouldhavepersisted longer.Athirdlimitationofthisstudywasthat wecollecteddatafromSeptemberthrough February.Corvidpopulations,distributions, andforagingbehaviorprobablyvaryseasonally (Heinrich1988;KristanandBoarman2003;Roth andothers2004;Preston2005);therefore, responsesmaynotrepresentthoseofcorvids duringtheploverbreedingseason.Finally, althoughwedetectednosignificantchangein theresponseofcorvidstoeffigiesover6mon, wesuspectthatlearningandhabituationby corvidsispossible.Ifso,theneffortstoincrease andsustaineffigyeffectivenesswillrequire additionalmeasures. 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