Declines amongst breeding Eider Somateria mollissima numbers in the Baltic/Wadden Sea flyway

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Declines amongst breeding Eider Somateria mollissima numbers in the Baltic/Wadden Sea flyway
  Declines amongst breeding Eider  Somateria mollissima numbers in the Baltic/Wadden Sea flyway Johan Ekroos, Anthony D. Fox, Thomas K. Christensen, Ib K. Petersen,Mikael Kilpi, Jón E. Jónsson, Martin Green, Karsten Laursen, Anja Cervencl,Peter de Boer, Leif Nilsson, W  odzimierz Meissner, Stefan Garthe& Markus Öst  J. Ekroos & M. Öst, ARONIA Coastal Zone Research Team, Åbo Akademi University & Novia University of Applied Sciences, FI-10600, Ekenäs, Finland. E-mail:, A.D.Fox,T.K.Christensen,I.K.Petersen&K.Laursen,DepartmentofBioscience,Aar-hus University, Grenåvej 14, DK-8410 Rønde, Denmark. Email:,,  M. Kilpi, ARONIA Centre for Environmental Research, Åbo Akademi University and  Novia University of Applied Sciences, FI-10600 Ekenäs, Finland. A. Cervencl, IMARES Wageningen UR, Department of Ecology, Landsdiep 4, NL-1797 SZ ‘t Horntje, the Netherlands. Email:  P. de Boer, Dutch Centre for Field Ornithology, SOVON, NL-6573DG Beek Ubbergen,the Netherlands. Email:  J.E. Jónsson, The University of Iceland’s Research Centre at Snæfellsnes, Hafnargata 3,340 Stykkishólmur, Iceland. Email: M. Green & L. Nilsson, Department of Biology, Lund University, SE-22362 Lund, Swe-den. Email:,,AvianEcophysiologyUnit,DepartmentofVertebrateEcologyandZoology,University of Gdansk, ul. Wita Stwosza 59, 80-308 Gdansk, Poland. Email: S. Garthe, Research and Technology Centre (FTZ), University of Kiel, Hafentoern 1,25761 Buesum, Germany. Email: Received 29 April 2011, accepted 5 October 2011 WereportonthestatusoftheBaltic/WaddenSeaflywayEiderpopulationbasedontrendsin breeding and wintering numbers throughout the region, supplemented by changes inthe sex ratio and proportion of young Eiders as monitored in the Danish hunting bag. Atthe flyway scale, total numbers of breeding pairs decreased by 48% during 2000–2009,after relatively stable breeding numbers in 1991–2000. The majority of the populationnestinFinlandandSweden,wherethenumberofbreedingpairshashalvedoverthesame period. After initial declines in winter numbers between 1991 and 2000, during 2000– 2009, national wintering numbers increased in the Baltic Sea, but decreased in theWadden Sea. The annual proportion of adult females in the Danish hunting bag data de- Ornis Fennica 89:81–90. 2012  creasedfromca.45%(1982)toca.25%(2009)andsimultaneouslytheproportionoffirst-winter birds fell from ca. 70% to ca. 30%, indicating dramatic structural changes in theDanishwinteringnumbers.Theseresultssuggestthatthetotalflywaypopulationwillex- periencefurtherdeclines,unlessproductivityincreasesandthefactorsresponsibleforde-creasingadultfemalesurvivalareidentifiedandameliorated.Wediscusspotentialpopu-lationdriversandpresentsomerecommendationsforimprovedflyway-levelmonitoringand management of Eiders. 1. Introduction The Baltic/Wadden Sea Eider   Somateria mollis- sima  population winters mainlyin Denmark, Ger-manyandTheNetherlands,wherewinteringnum- bers declined by c. 36% between 1991 and 2000(Desholm  et al  . 2002). To identify the drivers of this decline, Desholm  et al  . (2002) reviewed the breedingstatusandnumbersofwinteringEidersinFinland, Estonia, Sweden, Denmark, southern Norway, Germany, Poland and the Netherlands,and compared changes in the numbers of both breeding and wintering Eiders throughout the en-tire Baltic/Wadden Sea flyway. Surprisingly, de-spite the dramatic decrease in wintering numbers,changesinbreedingabundanceshowednoconsis-tent trend (Desholm  et al  . 2002, BirdLife Interna-tional 2004).The discrepancy between declining winteringnumbers and stable breeding numbers at that timecould simply have resulted from count error or  bias.Alternatively,adeclineintheoverallpopula-tion size in this long-lived species may have beenmasked if there was a surplus of non-breedersavailable to recruit into the breeding population.Indeed, even under normal circumstances, the to-tal potential breeding pool of Eiders may include20–30% of individuals which only initiate bree-ding under suitable conditions and hence buffer the breeding numbers against overall populationdeclines (Alerstam  et al  . 1974, Almkvist  et al  .1974,Coulson1984).Suchfloaterswillnotbede-tected in bird censuses on thenesting grounds andhence a decrease in the wintering population maynotinitiallybeevidentamongstbreedingnumbers(Svensson  et al  . 1986).Desholm etal  .(2002)predicted thatdecreaseswouldeventuallybeevidentalsoinEiderbreedingnumbersattheflywayscale,giventhatthehypoth-esized buffering effect of non-breeders was real.Suchadecreasewouldsignalthatthebufferingco-hort of non-breeding Eiders had been depleted intheregion.Recently,nationaldeclinesinbreedingnumbers of Eiders have been reported from Fin-land (Hario & Rintala 2008), Estonia (Elts  et al  .2009) and Sweden (M. Green, unpubl. data), sug-gesting that such changes may indeed have oc-curredattheflywayscale.However,thebufferingeffect hypothesis was criticized by Hario andRintala (2009), who showed that the age of first breeding increased rather than decreased during adeclineinbreedingnumbersintheeasternGulfof Finland. Given current declines in breeding num- bers,anewassessmentofthepopulationdriversishighly timely.We here present more recent information onthe status of the Baltic/Wadden Sea Eider popula-tion, supplemented with data from 1991–2000whichwereunavailableduringtheearly2000sandhence omitted from Desholm  et al  . (2002). We present up-to-date estimates of breeding and win-teringEidersintheregionandassessthehypothet-icalpopulationdriverspresentedbyDesholm etal  .(2002) for the current population trajectory of thespecies.We also update changes in annual sex- andage-ratiosamongstDanishsamplesofbirdsshotinwinter. We discuss plausible mechanisms behindrecentchangesintheBaltic/WaddenSeaEiderpo- pulation by contrasting national numbers of bothwintering and breeding Eiders before and after 2000, and by comparing changes in sex ratios andoffspring production with changes in summer andwinter numbers. 2. Material and methods We compiled available information to estimatenumbersofbreedingpairsandwinteringEidersinthe different countries from published and other sources (Table 1).82 ORNIS FENNICAVol. 89, 2012  2.1. Assessing changes in breeding abundance Survey methods for monitoring breeding sitesranged from counts of nests and/or breeding fe-males in sample areas (e.g., in Finland) to surveyscovering 60–70% of the total breeding nationalnumbers (Estonia, see Kuresoo 1998). The vastmajority of breeding Eiders in the Baltic/WaddenSea occur in Finland and Sweden (Desholm  et al  .2002).Recentestimatesonannualchangeinabun-dance were provided by M. Green (unpublisheddata) and Hario & Rintala (2008) for Sweden andFinland, respectively. Annual Finnish populationestimates were based on nest count data(Koskimies & Väisänen 1988) from 36 sites dis-tributed evenly along the entire coastline (11 sitesfrom the Gulf of Finland, 10 sites from the south-westernarchipelago,8sitesfromthesouthernBayof Bothnia and 6 sites from the northern Bay of Bothnia; Hario & Rintala 2008).The selected sites were the most intensivelymonitored throughout the monitoring period, al-thoughannualcensusdatawerenotavailablefromallyearsfromeachsite.AnnualSwedishestimateswerebasedondatagatheredat41sitesmoreorlessannuallysurveyed,supplementedbymulti-annualcensuses from larger regions. In Sweden, nestcounts were undertaken in Blekinge, pairs presentat breeding sites monitored in Västra Götaland,and male counts undertaken in the Stockholm re-gion.Thesecountdatawerecombinedfollowingastandardized procedure (M. Green, unpubl. data)to yield a total estimate of breeding pairs. Thechanges in annual estimates presented here werederived using the software TRIM (Trends and In-dices for Monitoring data) (Pannekoek & vanStrien2003),whichfitslog-linearPoissonmodelsallowingforover-dispersionandserialcorrelationin the data, generating annual estimates with 95%confidence intervals. Based on these confidencelimits, it is possible to detect statistically differentannual estimates, despite large inter-annual varia-tion. The software also estimates an annual index between observationsbased on imputation of datamissing from sites in specific years.Annual estimates of national breeding num- bers were available only from Finland and Swe-den. Elsewhere, breeding numbers have been as-  Ekroos et al.: Declines in breeding Eiders in the Baltic/Wadden Sea  83 Table 1. The number of breeding and wintering Eiders in the Baltic/Wadden Sea flyway in 1991, 2000 andthe most recent available estimates (2009 unless otherwise stated).Country Breeding numbers (pairs) Wintering numbers (individuals)1991 2000 2009 1990 2000 2009The Netherlands 7,621 a 9,000 b,1 4,650 c,2 103,299 a 101,052 d 58,831 d Germany 971 a 1,166 a 1,200 e,3 236,451 a 248,663 a 297,000 f, 4 Denmark 25,000 a 24,000 a 25,000 g,5 797,000 a 370,000 a 500,000 g,6 Sweden 270,000 b,h 315,000 b,h 161,000 h 20,000 a 20,000 a, * 68,900 i,7 Norway 30,000 a 15,000  j,8 10,000 a 46,500 g,6 Finland 165,000 a,9 170,000 a 80,000 k,10 30 a 200 a 200**Poland 0 m 0 b 0 m 24,000 a 10,000 a 4,900 m,11 Estonia 12,000 a 12,000 a 5,000 n,12 100 a 100 a 60 n,13 Total flyway 480,592 561,166 291,850 1,180,880 760,015 976,391 a Desholm  et al  . (2002),  b BirdLife International (2004),  c Boele  et al  . (2011),  d  Arts (2010); the numbers for 2000 re-evaluated, e Südbeck  et al.  (2007),  f  S. Garthe & J. Wahl (unpubl.),  g Christensen & Bregnballe (2011),  h M. Green (unpubl. data),  i Nilsson(2009),  j Barrett  et al  . (2006),  k Hario & Rintala (2008),  m W. Meissner, pers. comm., n Elts  et al  . (2009). 1 Estimated number 8,000–10,000,  2 estimated number 4,300–5,000,  3 1,100–1,300 pairs in 2005,  4 99,000 in the North Sea(2008 for coastal data and 2006–2010 for offshore data) and 198,000 in the Baltic Sea (2008 for coastal data and 2006–2010 for offshore data),  5 estimate for 2010,  6 estimate for 2008,  7 combined estimates for 2009 (west coast), 52,000 individuals, and 2007(Baltic Sea), 16,900 individuals,  8 estimated breeding number in southern Norway in 2005,  9 estimated number 150,000–180,000, 10 estimate for 2007,  11 estimated number 4,800–5,000 in 2011,  12 estimated number 3,000–7,000 in 2003–2008,  13 estimatednumber 20–100 in 2003–2008, * the estimate for 2000 was probably too low; a survey conducted in 2004 resulted in 50,000 win-tering eiders and it is likely that this was the case also in 2000 (M. Green, unpubl. data), ** no data available, but no recentchanges according to an expert opinion (Aleksi Lehikoinen, pers. comm.).  sessed intermittently, typically once or twice per decade. In Denmark, breeding abundance was es-timated based on counts in 2010 from 191 sites,combined with data from 37 supplementary sitessampledin2007–2009(butnotin2010)toprodu-ceatotalnationalestimateofbreedingEidernum- bers in 2010. Nest counts were carried out at mostsites, with male counts (including those close to breedingislands)atsomesites,correctedtorepre-sent nest numbers based on the established rela-tionshipbetweenthenumbersofpairedmalesandnests from 20 sites undertaken in 2010 (Christen-sen & Bregnballe 2011).In Estonia, c. 70% of all breeding eiders have beencountedannuallyusingdirectnestcountsandthe current estimate of breeding numbers is con-sideredreliable(Elts etal  .2009).BreedingcountsinGermanyhavebeenconductedwithvariousme-thods (Desholm  et al  . 2002). Breeding counts in Norway were carried out by both pre- and post- breeding aerial surveys of males (Desholm  et al  .2002).IntheNetherlands,theWaddenSeaconsti-tutes a special monitoring area in which breedingseabirds are monitored annually. The programmestrives to achieve full coverage of the region’s Ei-der colonies (Boele  et al  . 2011). 2.2. Assessing overall population size ThemajorityofEidersintheregionwinterinDen-mark,GermanyandtheNetherlands.Annualmid-winter censuses have been carried out in thesecountries based on aerial surveys. In the WaddenSea, Eiders have been subject to coordinated mid-winter aerial survey annually under the TrilateralMonitoring and Assessment Programme (TMAP)of the Netherlands, Germany and Denmark since1993(Laursen etal  .2011)aswellasnationalcen-sus programmes in each country (e.g., Arts 2010,de Jong  et al  . 2010). Throughout Denmark, prior to2004,Eiderswerecountedinallareasusingthe“totalcountmethod” using aerialsurvey(Laursen etal  .1997),butsince2004,numbershavebeenas-sessed by a combination of this method in certainareas, complemented by transect counts subjectedtodistancesamplingandspatialmodellingtogen-erate density surfaces (and hence total estimateswith confidence intervals; Petersen  et al  . 2006).This change in survey method may mean we canonly detect relative, rather than absolute, changesinwinterabundanceofEidersinDanishwatersbe-tween 2000 and 2009 (Petersen  et al  . 2006). InSweden, land-based counts have been carried outduring the whole study period, as well as aerialsurveys covering all the important marine areas inthe country, comprising the southern parts of boththe west and east coasts (Nilsson 2009). In Ger-many,datawerecollectedincoastalareasbyaerialsurveys and by land-based counts. Offshore areaswere covered by both aerial and ship-based sur-veys. Combined, these surveys cover the vast ma- jority of Eiders wintering in the Baltic/WaddenSea. For a detailed description of census methodsused in the different countries, see Desholm  et al  .(2002). 2.3. Changes in productivity and sex ratio Danish hunters voluntarily contributed wingsfromshotbirdstolongestablishedsurveys(1982– 2009). These have provided an average of 1,838Eider wings per year (SD = 788.9, n = 51,468),from which the annual proportions of adult fe-males (of all adults) and juveniles (of the totalsample) were calculated and used as proxies for changes in these ratios in the Baltic/Wadden Sea population as a whole. The wing survey data areadjusted for a shortening of the hunting season of female Eiders since 2004, and have been shownnotto bebiased in terms of spatialor temporaloc-currence (Noer   et al.  1995, Christensen 2005; seealsoLehikoinen etal. 2008).LikewiseEiderhunt-ers in Denmark did not prefer males over females(Noer   et al.  1995, Christensen 2005). 3. Results Annual estimates of breeding numbers from Fin-land and Sweden (Fig. 1a–b) showed remarkablysimilar patterns throughout the study period, withan initial increase of ca. 40% from 1986 to theearly 1990s, and a subsequent decrease beginninginthelate1990s.Takentogether,astatisticallysig-nificant overall decline in numbers between thefirstand lastyear was apparentin both time-series(Fig.1a–b).InFinland,therateofdeclinehasbeenfairly constant from 1998 to 2007. Swedish num-84 ORNIS FENNICAVol. 89, 2012   Ekroos et al.: Declines in breeding Eiders in the Baltic/Wadden Sea  85 Fig. 1. Population developmentof the Eider in (a) Finland dur-ing 1986–2007 and (b) Swedenduring 1985–2010. Populationdevelopment is expressed asannual rates of change. Year 1986 (Finland) and 1985 (Swe-den) is set as the baseline withthe index value 1. Dashed linesrepresent the 95% confidenceintervals of annual change indi-ces.Fig. 2. The percentage of (a)first-year Eiders (of all individu-als) and (b) adult females (of alladults) shot in Denmark duringthe years 1980–2009 accordingto the Danish wing survey data.
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