(Anas penelope) in northwest Europe

14 Pages·2016·2.15 MB·English

OrnisFennica93:12–25.2016
Seeking explanations for recent changes in abundance
Anas penelope
of wintering Eurasian Wigeon ( )
in northwest Europe
Anthony D. Fox*, Lars Dalby, Thomas Kjær Christensen, Szabolcs Nagy,
Thorsten J.S. Balsby, Olivia Crowe, Preben Clausen, Bernard Deceuninck,
Koen Devos, Chas A. Holt, Menno Hornman, Verena Keller, Tom
Langendoen, Aleksi Lehikoinen, Svein-Håkon Lorentsen, Blas Molina,
Leif Nilsson, Antra St(cid:1)pniece, Jens-ChristianSvenning & Johannes Wahl
A.D.Fox,L.Dalby,T.K.Christensen,T.J.S.Balsby&P.Clausen,DepartmentofBiosci-
ence,AarhusUniversity,Kalø,Grenåvej14,DK-8410Rønde,Denmark.*Correspond-
ingauthor’se-mail:[email protected]
S.Nagy&T.Langendoen,WetlandsInternational,Horapark9,Ede6717LZ,TheNeth-
erlands
O. Crowe, BirdWatch Ireland, Unit 20 Block D Bullford Business Campus, Kilcoole,
CountyWicklowIreland
B.Deceuninck,LPO-BirdLifeFrance,FonderiesRoyales,8rueduDrPujos,BP90263,
RochefortCedexF-17305,France
K.Devos,ResearchInstituteforNatureandForest,Kliniekstraat25,1070Brussel,Bel-
gium
C.Holt,BritishTrustforOrnithology,TheNunnery,Thetford,NorfolkIP242PU,UK
M.Hornman,SovonDutchCentreforFieldOrnithology,P.O.Box6521,Nijmegen6503
GA,TheNetherlands
V.Keller,SwissOrnithologicalInstitute,Sempach,CH-6204,Switzerland
A.Lehikoinen,TheHelsinkiLabofOrnithology,FinnishMuseumofNaturalHistory,P.O.
Box17,UniversityofHelsinki,Helsinki,Finland
S.-H.Lorentsen,NorwegianInstituteforNatureResearch,7485Trondheim,Norway
B. Molina, Bird Monitoring Unit, SEO BirdLife, C/ Melquíades Biencinto,34-28053
Madrid,Spain
L.Nilsson,DepartmentofBiology,UniversityofLund,EcologyBuilding,Lund,S-223
62,Sweden
A.St(cid:1)pn(cid:2)ece,InstituteofBiology,UniversityofLatvia,MieraStreet3,LV-2169Salaspils,
Latvia
J.-C. Svenning, Section for Ecoinformatics &Biodiversity, Department of Bioscience,
AarhusUniversity,NyMunkegade114,DK-8000AarhusC,Denmark
J. Wahl, Dachverband Deutscher Avifaunisten e.V. (DDA), Federation of German
Avifaunists,AndenSpeichern6,D-48157Münster,Germany
Received6June2015,accepted3October2015
CommunicatedbyMarkusÖst
Foxetal.:ChangingwinteringWigeonabundanceinnorthwestEurope 13
WeanalysedannualchangesinabundanceofEurasianWigeon(Anaspenelope)derived
frommid-winterInternationalWaterbirdCensusdatathroughoutitsnorthwestEuropean
flywaysince1988usinglog-linearPoissonregressionmodelling.Increasesinabundance
inthenorthandeastofthewinteringrange(Norway,Sweden,Denmark,Germany,Swit-
zerland),stablenumbersinthecentralrange(Belgium,Netherlands,UKandFrance)and
decliningabundanceinthewestandsouthofthewinteringrange(SpainandIreland)sug-
gestashiftinwinteringdistributionconsistentwithmilderwintersthroughouttherange.
However,becauseover75%ofthepopulationofover1millionindividualswintersin
Belgium,theNetherlands,UKandFrance,therewasnoevidenceforamajormovement
inthecentreofgravityofthewinteringdistribution.Between-winterchangesinoverall
flywayabundancewerehighlysignificantlypositivelycorrelated(P=0.003)withrepro-
ductivesuccessmeasuredbyageratiosinDanishhunterwingsurveysandlessstrongly
andinverselycorrelated(P=0.05)withmeanJanuarytemperaturesinthecentreofthe
winteringrange,suggestingthatwinterseveritymayalsocontributetoinfluencesurvival.
However,addingwinterseveritytoamodelpredictingpopulationsizebasedonannual
reproductivesuccessalonedidnotcontributetomoreeffectivelymodellingtheobserved
changes in population size. Patterns in annual reproductive success seem therefore to
largelyexplaintherecentdynamicsinpopulationsizeofnorthwestEuropeanWigeon.
SummerNAOsignificantlyandpositivelyexplained27%ofvarianceinannualbreeding
success.Otherlocalfactorssuchaseutrophicationofbreedingsitesandchangesinpreda-
tionpressureundoubtedlycontributetochangesintheannualproductionofyoungand
differences in hunting pressure as well as winter severity affect annual survival rates.
However,itseemslikelythattheobservedflywaypopulationtrendsince1988hasbeen
mostlyinfluencedbyclimateeffectsonthebreedinggroundsaffectingreproductivesuc-
cessandmarginallyonthewinterquartersaffectingsurvival.Weurgeimproveddemo-
graphicmonitoringofthepopulationtobetterassessannualsurvivalandreproductive
success.Wealsorecommenddevelopmentofanadaptivemanagementframeworktore-
moveuncertaintiesinourknowledgeofWigeonpopulationdynamicsasinformationis
forthcomingtobetterinformmanagement,especiallytoattempttoharmonisetheharvest
withannualchangesindemographytoensuresustainableexploitationofthisimportant
quarryspeciesnowandinthefuture.
1.Introduction nationasRamsarwetlandsitesofinternationalim-
portance,butlatterlyascohesivenetworksofNa-
SincetheendoftheSecondWorldWar,manyof tura2000sitesestablishedalongtheirflyways.As
thehuntableduckspeciesofnorthwesternEurope viewed from the perspective of the late 1990s
haveshownsustainedincreasesinabundance(e.g. therefore, the favourable conservation status of
Eltringham&Atkinson-Willes1961,Owenetal. mosthuntabledabblingduckspeciesrepresenteda
1986,Nagyetal.2014).Thishaspresumablybeen majorconservationsuccessstoryinnorthwestEu-
atleastpartiallytheresultofmorerestrictivehunt- rope,balancingincreasinglyabundantpopulations
inglegislationthathasreduceduncontrolledhar- withaharvestableofftakethatwasacknowledged
vestofsuchpopulations(Berry1939,Owenetal. torepresentgenuinely“sustainableuse”withina
1986). In addition, the results of site protection timeframeofdecades.
havepresumablyalsoreducedtherateofhabitat Unfortunately,thereareincreasinglysignsof
loss and degradation for these duck species, ini- contemporarychangeinthissituation,asthepro-
tiallyintheformoflocalandnationalnaturere- ductionofyoungamongstEurasianWigeon(Anas
servesandinternationalprotection,suchasdesig- penelope, hereafter Wigeon), Northern Pintail
14 ORNISFENNICAVol.93,2016
(Anasacuta)andNorthernShoveler(Anasclype- wetlands in Finland and Russia, avoiding detec-
ata)inthisflywayhasfallenconsistentlyoverthe tionbyoccurringinareasnotcurrentlysubjectto
last30years(assampledbyhunters,Christensen mid-winterIWCcoverage(becausetheywerefor-
&Fox2014)andtheirmajorEUbreedingaggre- merly frozen in winter). The omission of such
gationswithinFinlandareshowingrecentdeclines birdsfromthecountnetworkthroughwinterfull-
in abundance (Pöysä et al. 2013). Between-year orpartialshort-stopping(i.e.birdsoverwintering
changesinabundanceofseveralspecieslistedon in areas closer to breeding areas as a result of
Annex II of the EU Birds Directive (i.e. those milderwinters,inthiscasetothenorthandeastof
whichmaybehunted)arenowshowingsignsof their former wintering range, see Elmberg et al.
stabilisationanddeclineattheflywaylevel(Nagy 2014 for precise definitions) could contribute to
et al. 2014, Ramão 2015). Although there is no the perception that there have been declines in
doubtthatthepopulationabundanceofthesespe- overallannualabundanceinrecentyears.Forthis
ciescontinuestobewellabovethoseofthe1950s, reason, we use IWC data to look for signs of
thecontemporarydeclinespresentachallengeto changesinthewinteringdistributionofWigeonto
their effective management and sustainable har- testthishypothesis.
vest.Attheveryleast,agreaterunderstandingof It is known from detailed studies at a single
thesedeclinesisrequiredifwearetobeinaposi- majorWigeonbreedingresort(MývatninIceland)
tiontobeabletoimplementmanagementactions thatthesizeofthebreedingWigeonpopulationin
to inhibit and reverse them. For this reason, it is year t is highly correlated to the production of
helpfultoinvestigatethenatureofrecentchanges youngatthesamesiteinyeart–1(Gardarsson&
inoverallabundanceofthesespeciesandtolook Einarsson1994).Hence,itmightbeexpectedthat,
forsupportforvarioushypothesesthatwecould iftherecruitmentoffirstwinterbirdstothesubse-
putforwardfortheirdeclines. quent breeding class makes such a difference to
Inthisanalysis,welookindetailattheabun- overallbreedingnumbers,thevariationsinrepro-
danceofwinteringWigeoninnorthwestEuropeto ductivesuccessattheflywaylevelmaycontribute
assessthedegreeofitsrecentdeclineandattempt tooverallabundanceinwinter.Wethereforetestto
toaccountforitsdownturn.Wedothisbyexamin- seeifthisisthecaseusingacombinationofwing
ing data from the mid-January International samplessubmittedbyDanishhunterstomeasure
WaterbirdCensus(IWC)gatheredbynationalor- annualreproductivesuccessandIWCdataonan-
ganisationsthroughouttheWesternPalearcticand nualchangesinoverallpopulationsize.Giventhe
centrallycollatedbyWetlandsInternational(WI). earlierfindingthatWigeonbreedingsuccessisin-
WeconsiderthatthenorthwestEuropeanWigeon fluenced bysummer temperature (Mitchell et al.
population is relatively closed in winter because 2008),wealsolookforcorrelationsbetweensum-
long term ringing recovery data has established merNorthAtlanticOscillation(NAO)indicesand
thatbirdsdonotcontinueintonorthandwestern reproductiveoutputtoaccountforrecentdeclines
Africa in winter (e.g. Donker 1959, Owen & inproductivityinthisflyway(Christensen&Fox
Mitchell1988,Saurolaetal.2013).Forthisrea- 2014).
son,weassumethatthecontemporarycountnet- Wealsocheckforsignsofdensitydependence
workinnorthwestEuropeiseffectiveatcovering in reproductive output to see if the increase in
theentirepopulationandthatchangesintheesti- Wigeonpopulationsizehasalsocontributedtothe
matedyear-on-yearabundancecanonlyarisefrom decline.Finally,becauserecoveriesofWigeonare
differences in count coverage, reproductive suc- greater during spells of hard winter weather
cessandsurvival.Recentanalysishasshownthat (Ridgill&Fox1990),wetesttofindsupportfor
several diving duck species have extended their thehypothesisthatcoldwintersinnorthwestEu-
northwest European wintering range north and ropecouldcontributetoexplainingdeclinesinthe
eastwithamelioratingwintertemperatures(Lehi- overallpopulationsizeinthefollowingyearsus-
koinen etal. 2013, Pavón-Jordan etal. 2015). If ingIWCdatabyregressingannualchangeinpo-
thisisthecasefortheWigeon,therecouldpoten- pulationsizeagainstmeanJanuarytemperaturein
tiallybelargenumbersofthisspeciesoccurringin thecentreofthewinteringrange.
Foxetal.:ChangingwinteringWigeonabundanceinnorthwestEurope 15
Fig.1.Mapshowing
thegradientofvalues
establishedfromSWto
NEusedaspredictor
totestthehypothesis
thattrendsinnumbers
amongEuropeandab-
blingduckstendtoin-
creasetowardsthe
NE.
2.Methods generatedinTRIM(actualcountsorinthecaseof
missingvalues,imputedcounts).Spatialcoverage
2.1.Short-stopping in IWC is biased towards certain countries with
very dense counting schemes (e.g. United King-
Weusedmid-JanuarycountdataforWigeonfrom domandtheNetherlands)andtocompensatefor
theIWC(Gilissenetal.2002,Nagyetal.2014) theunevendistributionofsitesweaggregatedthe
between1990and2009fromcountrieswithinthe sitebasedvaluestoa1°by1°gridtakingthemean
north-west European flyway (sensu Atkinson- Tauvalueforallsitesfallingwithinagridcellus-
Willes1976).Inallcountries,onlysitescountedat ingthe“raster”package(Hijmans2013)inR3.0.1
least twice entered the analysis. We fitted trends (R Core Team2013). This resulted in a mean of
foreachcountryusingthesoftwareTRIMversion 9.38 sites per grid square (range 1–77) based on
3.54 (Van Strien et al. 2004, Pannekoek & van 2,888siteswithWigeoncountdatathroughoutthe
Strien2005)imputingmissingcountsandestimat- region.
ing trends by log-linear Poisson regression. To Totestiftherewasatendencyforgridcellslo-
standardisecomparisonsoftrendsandtherelative cated further to thenortheast(closer to thebree-
magnitude of trends, we indexed the counts to a ding grounds) to show increasing numbers (i.e.
valueofunitystartingin1990forallnationalanal- positiveTauvalues),wefittedageneralisedaddi-
yses. tive model (GAM) using the “mgcv” package
ToestimatesitetrendswecalculatedKendall’s (Wood2006)withtheTauvaluesasresponsevari-
Tauonlog-transformedcountsusingthedataset ableandagradientfromsouthwesttonortheastas
16 ORNISFENNICAVol.93,2016
explanatoryvariable.WedefinedtheSW–NEgra- 2.4.Effectsofwinterweather
dientasthelatitudeandlongitudeofthestudyre- onbetweenyearchangesinabundance
gionscaledfromzerotooneandsummed,givinga
vectorfromzero(SW)totwo(NE,seeFig.1and To testfor supportfor the hypothesis thatsevere
Dalby2013). weatheronthewinterquartersaffectssurvival,we
Toassesswhetherchangesinthedistribution also tested for a correlation between the propor-
ofWigeonduring1990–2009hadresultedinsys- tionalchangeinwinterpopulationbetweenyeart
tematicmovementintheannualcentresofgravity and t + 1 and the mean January temperature at
of abundance in wintering distributions towards Schiphol Airport (52.3°N 4.76°E) in the Nether-
thenorthandeastduringthetimeseries,weesti- lands where most (40–45%) of the population
mated the annual and decadal centres of abun- overwinters (data from http://www.tutiempo.net/
dance as the weighted mean longitude and clima/Amsterdam_Airport_Schiphol/62400.htm).
weightedmeanlatitudeusingthecountsestimated Thismeteorologicalstation also lieswithin 150–
fromTRIMasweights. 200 km of the decadal centre of gravities of the
Finally,weplottednationalTRIMindicesfor winteringpopulation(seeFig.4).
mid-wintercountsfromcountriesalongtheentire
flywaytotestthepredictionundertheshortstop- 2.5.Predictingbetweenyearchanges
ping hypothesis that Wigeon wintering in the inabundancebasedonreproductive
northeast part of the winter distribution should successandwinterweather
showgreatestincreasesandincountriesprogres-
sively south and west along the flyway would Inthecourseofthisanalysis,weshowedastrong
showdeceasingtrendsasbirdsrelocatednearerto linear relationship between the proportional
thebreedingareasinmid-winter. changeinflywaypopulationabundanceinyeart
(P)comparedtoyeart–1(P )andtheratioof
t t–1
young to adults in the wing surveys (w ) pro-
t–1
2.2.Flywaytrends videdbyhuntersinDenmark(Fig.7).Totesthow
wellvariation in breeding success contributed to
changesinyear-to-yearabundance,wegenerated
ATRIMmodelwasalsofittedtotheentiredataset
anindependenttimeseriesofannualestimatesof
fromallcountriestogenerateaflywayindexofto-
theWigeonflywaypopulationsize(P)inyeartus-
tal estimated individuals for the period 1988 to
ing the regression modelled proportional change
2012 using the methods described and estimates
inpopulationsizepredictedbyreproductivesuc-
generatedbyNagyetal.(2014).
cessinthepreviousyearthus:
P =P (aw –b) (1)
2.3.Effectsofannualreproductivesuccess t t–1 t–1
onbetweenyearchangesinabundance
where P is the northwest European population
t
sizeintheJanuaryofyeart,w istheageratioin
t–1
Age ratios from the Danish hunter wing survey thehunterwingsurveysbasedonbirdshatchedin
(Christensen&Fox2014)wereusedasameasure summert–1andaandbareconstants(seetheac-
ofannualproductionofyounginthispopulation tualformulainFig.7).Thetimeseriespresentedin
forthesameperiod,sincethesearethoughttopro- Fig.8wasgeneratedstartingwiththeknownP
t–1
videareasonablemeasureofannualproductionof in 1988 (777,343 individuals from Nagy et al.
younginthenorthwestEuropeanpopulation(Fox 2014)togenerateP for1989andeachsubsequent
t
etal.2015,Foxetal.2016).Theratiooffirstwin- P estimatedinthesameway.
t
terbirdstoadultfemalesinthewingsampleswere We also took the same relationship from the
usedasameasureofrelativeannualbreedingout- Schiphol airport mean January model and incor-
puttoinvestigatethehypothesisthatthepopula- porated this into a multiple regression model to
tion in agiven winter washeavilyinfluenced by generateanothertimeseriescombiningthemod-
thereproductivesuccessintheprevioussummer. elledeffectsofbreedingsuccessandJanuarywin-
Foxetal.:ChangingwinteringWigeonabundanceinnorthwestEurope 17
Fig.2.MeanannualnumberofWigeoncounted Fig.3.Generalisedadditivemodel(GAM)ofKend-
from1990to2009aggregatedinto1°by1°grid all’sTauonlog-transformedWigeoncountsfrom
squaresbyaddingcountsfromallsitesfallingwith- theInternationalWaterbirdCensusfromacrossEu-
ineachgridcell.CountsoriginatefromtheInterna- ropebetween1990and2009againstagradient
tionalWaterbirdCensusfromallEuropeancoun- fromthesouthwesterncorner(0)tothenortheast-
triesplottedhere.Missingcountswereimputedus- ernmostcorner(2)oftheNorthwestEuropeanfly-
ingTRIMversion3.54(Pannekoek&vanStrien way(sensuAtkinson-Willes1976andseeDalby
2005)beforecalculatingmeangridcellvalues. 2013).SitebasedKendall’sTauvalueswereaggre-
NotedataarenotshownforIrelandorPortugalbe- gatedto1°by1°gridsquaresbytakingthemean
causefulldatafor1990–2009werenotavailable ofallsiteKandall’sTauvaluesbeforerunningthe
fromtheseschemessogriddedmeanswouldnot analysis.Estimatesareshowninwhitewith95%
becomparablewiththerestofEurope. confidenceintervals(shadedgreyarea).
terseverity.Wetestedgoodnessoffitofpredicted successofseveralbirdspecieshasbeenshownto
populationsizegeneratedfromthetwomodelsby be influenced by NAO (e.g. Forchhammer et al.
fitting general linear models to the annual pre- 1998,2002,Hüppop&Hüppop2003).Sinceposi-
dicted population size and those generated from tiveNAOindicescharacterisethemorenortherly
count data, with the expectation that the best trackingofthejet-streamandstormtracksinthe
model would have the slope closest to unity and North Atlantic, such conditions bring warmer,
we compared slopes using procedure of Zar drier, cloudless conditions to central Eurasia
(1999).Wealsotestedforsignsofdensitydepend- whereWigeonbreed(Follandetal.2009),likely
enceinthetimeseriesbyseekinganinverserela- enhancingbreedingsuccessinsuchyears;incon-
tionshipbetweenbreedingsuccessinyeartandln trastnegativeindicesreflectcolder,wettercondi-
transformedpopulationsizeinJanuaryoft–1. tionsonthebreedingareasandwouldthusbeex-
Finally, to testwhether breeding successwas pected to be associated with poor Wigeon bree-
linked with climatic conditions on the breeding dingsuccess.
grounds(asalsoconfirmedforWigeonintheIce-
land population, Gardarsson & Einarsson 1997)
3.Results
we used the combined May–July North Atlantic
Oscillation index (National Oceanic and Atmo- 3.1.Short-stopping
sphericAdministration2015)totestforacorrela-
tion between this summer NAO index and bree- Wigeonarehighlyconcentratedinmid-winterin
ding success as measured in the Danish hunter northwestEurope,with40–45%ofthewintering
wing surveys. The phenology and reproductive birdsinthisflywayoccurringintheNetherlands,
18 ORNISFENNICAVol.93,2016
centdeclines(Fig.5)anddeclinesatthewestern
endoftheflyway(significantdecreasesinIreland
andSpain).
3.2.Flywaytrend
Followingincreasesinabundanceduringthelate
1980sand1990s,Wigeonnumbershavestabilised
inthenorthwestEuropeflywayanddeclinedsince
themid-2000s(Fig.6a).Annualreproductivesuc-
cessasmeasuredbytheratioofjuvenilestoadult
femalessampledfromshotbirdsinDenmarkhave
fluctuated since 1988, but were generally below
averageduring8ofthe11yearsduring2002–2012
(Fig.6b).
3.3.Effectsofannualreproductivesuccess
Fig.4.CountweightedmeancentroidsofWigeon
onbetweenyearchangesinabundance
countsfromtheInternationalWaterbirdCensus
acrossEuropeforeachyearbetween1990and
Theproportionalchangeinsuccessivewinterpo-
2009.Blacktrianglesareshowingthedecadal
pulationestimateswaspositivelyandsignificantly
weightedmeancentroid.Missingcountswereim-
putedusingTRIMversion3.54(Pannekoek&van correlatedwiththeannualratioofyoungtoadults
Strien2005)beforecalculatingtheweightedcen- intheDanishhunterwingsurveyintheprevious
troids.Thesoliddiamondindicatesthelocationof yearduring1988–2012(r2=0.34,F =11.4,P=
themeteorologicalstationatSchipholAirportused 1,22
0.003,Fig.7).Theannualratioofyoungtoadult
toprovidemeanJanuarytemperaturesusedinthis
WigeonintheDanishhunterwingsurveyshowed
study.
a significant positive correlation with the May–
35%intheUnitedKingdomandjustunder10%in JulyNAO index for thepreceding summer, sup-
Germany,6%innorthFranceand4%inBelgium portingthehypothesisthatclimateaffectsthere-
(Fig.2).Thisdistributionhaschangedlittlesince productivesuccessofthispopulation(y=5.640+
Monval&Pirot(1989)andRidgill&Fox(1990), 0.510x,r2=0.27,F =9.97,P=0.01).Breeding
1,22
although the GAM analysis clearly showed that successinagivensummerwassignificantlynega-
thereweresegmentsoftheWigeonflywaywhere tively correlated with ln transformed population
trendsincreasedmoreonthenorthandeastofthe sizeinthepreviousJanuary(y=104.6–7.124x,r2
winter range than the south and west where de- =0.32,F =34.9,P=0.004).
1,22
clines were more evident (deviance explained =
42.8%,F =26.82,P<0.0001,Fig.3).The 3.4.Effectsofwinterweather
7213,308
weighted mean centroids varied between years onbetweenyearchangesinabundance
(Fig. 4), but there was no convincing temporal
trend through the years plotted here, the decadal Theproportionalchangeinwinterpopulationesti-
centreofgravityforthe2000sbeing43.9kmtothe matesalsoshowedamarginallysignificantposi-
NEofthatinthe1990s.Increasingnumberswin- tiverelationshipwiththemeanJanuarytempera-
teringinthenortheastofthewinteringrange(sig- tureatSchiphol(r2=0.17,F =4.4,P=0.05).
1,22
nificant increases from fitted TRIM models in
Sweden, Norway, Denmark, Germany and Swit- 3.5.Predictingbetweenyearchanges
zerland,seeFig.5)contrastedstabletrendsinthe inabundancebasedonreproductivesuccess
centre of the range (Belgium, Netherlands, the andwinterweather
UnitedKingdomandFrance,wherethevastma-
jorityoftheindividualsinthisflywaywinter,al- Modelling the year-on-year change in flyway
thoughthesecountriesallshowsomesignsofre- abundancebasedontheoutputfromthemodelin
Foxetal.:ChangingwinteringWigeonabundanceinnorthwestEurope 19
Fig.5.NationalTRIMindicesforWigeonbasedontheInternationalWaterbirdCensusdatafromselected
Europeancountriesfrom1990to2009,withindicessetto1in1990.Notedifferentscalesintheindices
axesbetweencountries.
Fig.7showedanextremelygoodfittothetimese- estimatedaslopeof1.013(generallinearmodelr2
ries although the combined modelled effects of =0.433,F =16.89,P=0.0005).Themodelin-
1,22
this and incorporation of the relationship with corporating both reproductive success and
Schiphol January temperatures worsened the fit Schiphol January temperature gave a significant
(Fig.8).Themodelusingreproductivesuccessto model with a slope of 0.909 (r2 = 0.434, F =
1,22
generatepopulationsizeshowedasignificantcor- 16.90,P=0.0005).Thetwoslopeestimatesdif-
relationwiththeobservedpopulationssizewithan feredsignificantly(t =5.6,P<0.001).
44
20 ORNISFENNICAVol.93,2016
Fig.6.(a)Combined
annualTRIMindices
withgeneratedstan-
darderrorestimates
(seevanRoomenetal.
2011formethods)for
Wigeonfortheyears
1988–2012basedon
theInternational
WaterbirdCensusdata
fromthenorthwestEu-
ropeflywaypopulation.
(b)Annualratioofju-
venileWigeontoadult
femalesinwingsamp-
lesvoluntarilysubmit-
tedbyhuntersinDen-
mark,1982–2014for
comparison.
4.Discussion ingnumbersinthesouthandwest(c.85,000)of
the range are relatively modest compared to the
During 1990–2009 there was good evidence for morethanonemillionwinteringindividualsinthe
Wigeonshowingpatternsofshortstoppingintheir centralpartoftherange,whererelativeabundance
winterdistributioninnorthwestEurope,withthe haschangedlittle,explainingthegenerallackof
establishment and expansion of a new wintering responseinthepositionoftheannualanddecadal
population in Sweden and increases in Norway, centresofgravityofthiswinteringpopulation.The
Denmark,GermanyandSwitzerland,stabletrends countdatashowthatthereisnowan established
inBelgium,Netherlands,theUnitedKingdomand wintering population in southwest Sweden that
France (where the vast majority winter) and de- wasnotpresentbeforethe1990s.IWCdataalso
clinesinIrelandandSpain.Interestingly,winter- show thatWigeon remain almostcompletelyab-
ingnumbershavetendedtoincreasealongtheBal- sent as wintering birds in the Baltic States (A.
ticcoastofGermany,incontrasttodeclinesinthe St(cid:1)pniece unpubl. data) and Finland (A. Lehi-
German Wadden Sea in very recent years (JW koinenunpubl.data).Giventheseverityofwinters
unpubl. data). However, overall, the increasing there,weconsiderthisisalsolikelytobethecase
numbersinthenortheast(c.200,000)anddecreas- throughoutEuropeanRussia.Forthisreason,we
Foxetal.:ChangingwinteringWigeonabundanceinnorthwestEurope 21
WigeonharvestedinDenmark(Fig.6bandChris-
tensen & Fox 2014) and the patterns of annual
variationintheproportionsofyoungbirdscorre-
latedpositivelywiththesummerNAO,sothere-
centdeclineinreproductivesuccessmaybepartly
explained by climate at large spatial scales. De-
clines in abundance of breeding Wigeon in Fin-
landhavebeenalsobeenrelatedtoeutrophication
ofbreedinglakewaters,changesinpredatorabun-
dance(Pöysäetal.2013)andtheeffectsoflocal
landuse,especiallyagriculture(Arzeletal.2015).
However, these effects are likely to be relatively
restricted geographically compared to the exten-
Fig.7.Graphoftheproportionalchangeinyearto sive breeding range in the Russia taiga where
yearwinterpopulationestimatesfortheyears changesinlandusearelikelytohavehadlessim-
1988–2012showninFig.6aplottedagainstthera- pact.Theresultsoftheanalysesherealsosuggest
tioofyoungtoadultsintheDanishhunterwingsur- thatsomeevidenceforweakdensitydependence
veysinthefirstofthepairedyears.Thefittedre-
inbreedingsuccess.Hence,therearelikelytobe
gressionmodelhastheformulay=0.0384x
–0.1754,r2=0.34,F =11.4,P=0.003. otherlocalorregionalfactorsaffectingannualre-
1,22 productivesuccessaswellasthedominantinflu-
enceofclimate.Thenegativerelationshipbetween
conclude that, although short-stopping is clearly wintertemperatureandannualchangesinflyway
occurringinthisspecies,ithasnotresultedinwin- abundancealsosuggestswinterseveritymaycon-
tering Wigeon occurring in Finland and Russia tribute very slightly to annual survival, although
wheretheywouldnotbecountedwithintheIWC thiscontributionwasmarginaloverthemajorcon-
network.Hence,wehaveconfidencethatthecur- tributionfromdifferencesinreproductivesuccess
rent count network is effective at sampling alone.Hence,onthebasisofparsimony,thefact
changes in local abundance throughout the Wi- thatthemodelbasedonreproductivesuccesspro-
geonwinterrangeandthatthecollatedtrendesti- ducedthebestpredictionsagainstobservedpopu-
mates are reflective of the overall abundance in lationsizeandthereducedstrengthofthemodel
thisflywaypopulation. incorporatingbothparameterstopredictobserved
Basedonthesamereasoning,wecanalsotake population size, we conclude that reproductive
someconfidenceinthefactthatafteranincreasein successinagivenyearmakesthegreatestcontri-
Wigeon abundance during the late 1980s and butiontoyearonyearchangesinWigeonflyway
throughthe1990s,thenumbershavestabilisedin abundance.
recentyearsandfallensincethemid-2000s(Fig. Thegreatweaknesswiththistypeofanalysis,
6a).Thecorrelationbetweenchangeinwinterpo- however, continues to be our ignorance of the
pulation estimates fromone year to the nextand inter-annualdifferencesinannualsurvivalofthe
the breeding output measured by proportions of species.Theabsenceofringingrecoveryorcap-
first winter birds harvested in Denmark enabled ture-mark-recapture as an independent means to
the retrospective reconstruction of the current estimateannualsurvivalremainsamajorimpedi-
trendinoverallpopulationsizebasedonobserved mentinourabilitytotrackandinterpretchangesin
abundance in 1988 and annual age ratios since. thedemographyofthecommonerdabblingduck
Thisconfirmedtheresultsofstudiesonthebree- species, such as Wigeon, thatare veryimportant
dinggroundsinIceland,namelythattheyear-on- quarryspeciesacrosstheEuropeancontinent.Our
yearchangesinthesizeoftheWigeonpopulation capacitytomodeltherelativeandactualeffectsof
are related to annual reproductive success (Gar- climate and environmental change also remains
darsson&Einarsson1994,1997).Thesummersof limitedaslongaswelackbagstatisticsandmore
2002–2010 (excluding 2004) wereyearsof rela- geographicallydispersedmethodsofgaugingan-
tively low reproductive success in amongst nualreproductive success (Elmberg etal. 2006).