Baltic/Wadden Sea Common Eider 167

Status of the Baltic/Wadden Sea population of the Common Eider Somateria m. mollissima

M. Desholm1, T.K. Christensen1, G. Scheiffarth2, M. Hario3, Å. Andersson4, B. Ens5, C.J. Camphuysen6, L. Nilsson7, C.M. Waltho8, S-H. Lorentsen9, A. Kuresoo10, R.K.H. Kats5,11, D.M. Fleet12 & A.D. Fox1

'Department of Coastal Zone Ecology, National Environmental Research Institute, Grenåvej 12, 8410 Rønde, Denmark. Email: mde0dmu.dk/tk0dmu.dk/tfo0dmu.dk 2Institut für Vogelforschung, 'Vogelwarte Helgoland', An der Vogelwarte 21, D - 26386 Wilhelmshaven, Germany. Email: g.scheiffarthiat-online.de ‘Finnish Game and Fisheries Research Institute, Söderskär Game Research Station. P.O.Box 6, FIN-00721 Helsinki. Finland. Email: martti.hario0rktl.fi ‘Ringgatan 39 C, S-752 17 Uppsala, Sweden. Email: ake_aSswipnet.se 5Alterra, P.O. Box 167, 1790 AD Den Burg, Texel, The Netherlands. Email: b.j.ensØaIterra.wag-ur.nl/r.k.h.katsØalterra.wag-ur.nl ‘Royal Netherlands Institute for Sea Research (Royal NIOZl, P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands. Email: kees.camphuysenldwxs.nl 'Department of Animal Ecology, University of Lund, Ecology Building, S-223 62 Lund, Sweden. Email: leif.nilssoniazooekol.lu.se “73 Stewart Street, Carluke, Lanarkshire, Scotland, UK, ML8 5BY. Email: clydeeider0aol.com 'Norwegian Institute for Nature Research. Tungasletta 2, N-7485 Trondheim, Norway Email: Shl0ninatrd.ninaniku.no '“Institute of Zoology and Botany, Riia St. 181, 51014, Tartu, Estonia. Email: akuresoo0zbi.ee "Department of Animal Ecology, University of Groningen, Kerklaan 30, 9751 NH, Groningen, The Netherlands. Email: r.k.h.kats0alterra.wag-ur.nl ,2Landesamt für den Nationalpark Schleswig-Holsteinisches Wattenmeer, Schlossgarten 1, D - 257832 Tönning, Germany. Email: fleet0nationalparkamt.de

©Wildfowl & Wetlands Trust W ild fo w l (2002) 53: 167-203 168 Baltic/Wadden Sea Common Eider

A dramatic decline in the number of wintering Common Eiders from c. 800,000 to c. 370,000 occurred in Danish waters between 1990 and 2000. Denmark represents the second most important wintering area for Eiders from the Baltic/Wadden Sea flyway, and m id-winter counts sug­ gest the total population could have fallen from c.1.2 million individuals in 1991 to c. 760,000 in 2000, implying major (c.36%) overall declines. However, although declines of similar magnitude have been detected in breeding numbers at some sites (eg Saltholm in Denmark), such a dramatic reduction is not generally evident amongst breeding numbers monitored throughout the flyway. Five hypotheses are offered to explain this dis­ crepancy, two of which are considered likely to contribute to the differences. These relate to shortcomings in our ability to monitor ade­ quately breeding and wintering numbers in both time and space, as well as to an unknown buffering effect of non-breeders (which are counted on the winter quarters, but which do not appear amongst assessments of breeding abundance). Parameters known to contribute to declines in population size include low duckling survival caused by viral infections, mass adult mortality due to Avian Cholera, and reduced adult annual survival rates due to mass mortality events on the wintering grounds. The population continues to be a quarry species in Denmark, Sweden, Norway and Finland. It is strongly recommended that the national mon­ itoring schemes should be standardised and synchronised, in order to establish future population abundance and change. It is further recom­ mended that population modelling is undertaken to understand the relationships between the numbers of Common Eiders in the Baltic/Wadden Sea flyway population and the different factors affecting their abundance.

Key Words: Conservation, hunting, Avian Cholera, duckling viral infection, mass-starva- tion, monitoring

The Baltic/Wadden Sea flyway popu­ and The Netherlands are resident or lation of Common Eider Somateria m. partly migratory whereas those in m ollissim a comprises breeding popula­ southern Norway, eastern Sweden, tions from Finland, Estonia, Sweden, Finland and the Baltic countries are Denmark, southern Norway, Germany completely migratory [Cramp & and The Netherlands. Birds nesting in Simmons 1977). Eiders of eastern Denmark, Germany, western Sweden provenance mix on the wintering areas Baltic/Wadden Sea Common Eider 169 in the western Baltic Sea, Kattegat, ing and wintering numbers in the range inner Danish waters, and in the states, and (c) briefly summarises the Wadden Sea from Denmark to The causes of mortality reported in recent Netherlands (Swennen 1990; Noer years. 1991 ; Fransson & Petterson 2001). The Common Eider has been widespread Methods and common throughout its range in the Baltic and Wadden Sea countries Different methods have been used for most of the 20th Century (Cramp & to count Eiders at different stages in Simmons 1977). Numbers have fluctu­ the annual cycle, and the sources of ated throughout the 20th Century, but data considered in the paper are pre­ increased continuously between the sented and assessed below. late 1940s and the 1990s (Almkvist ef at. 1974; Hario & Selin 1988; Denmark Camphuysen 1996). The Baltic/Wadden Sea flyway popu­ In Denmark, breeding pairs have lation was estimated to comprise been surveyed at least once every 1.35-1.70 million birds in 1991 (based decade since the 1960s (Paludan 1962; on extrapolation from midwinter Joensen 1973; Franzmann 1989; Lyngs counts), and was considered to be sta­ 2000; Lyngs in prep.). Generally, most ble at that time [Rose & Scott 1997). breeding sites were visited once during However, since the late 1980s, reports early May, and the numbers of have documented marked declines in nests/nesting females were counted. In specific breeding populations within some areas, colony size was estimated the Baltic/Wadden Sea flyway (Hario & from the numbers of males present Selin 2002; Hollmén 2002; Christensen around breeding islands during the & Noer 2001 ). To obtain better informa­ pre-breeding period. No long-term sys­ tion about the Baltic/Wadden Sea Eider tematic counts of migrating birds at population, the Seaduck Specialist specific Eider locations have taken Group of Wetlands International place in Denmark, but aerial surveys of arranged a workshop in Estonia during moulting Eiders in Danish waters took 17-21 April 2002 to compile information place in August 1987, 1988 and 1989. on the status and distribution of Eiders These covered coastal waters and off­ from all range states along the flyway. shore shoals/reefs to 10m depth This paper (a) summarises the out­ (Laursen et at. 1997). Since the 1970s, come of the workshop, (b) reviews the seven extensive aerial surveys have present status and most recent been conducted to estimate the total changes in the Baltic/Wadden Sea mid-winter population (Joensen 1974; Common Eider flyway population based Laursen ef at. 1997; Pihl ef al. 1992, Pihl on data on breeding, migrating, moult­ et at. 2001 ). Each survey was conducted 170 Baltic/Wadden Sea Common Eider

between January 5 and March 11, cov­ other Danish colonies, a demographic ering all Danish coastal waters and model using Leslie matrices was devel­ Kattegat with water shallower than 10 oped to compare the observed changes m (Joensen 1974; Laursen ef al. 1997; in population size on Saltholm with that Pihl ef af. 1992; Pihl ef al. 2001). In the predicted based upon selected breed­ latest national survey (winter ing parameters (Christensen & Noer

1999/2000) the offshore part of 2001). Kattegat was covered by transect sur­ veys. Only birds observed on transect The Netherlands lines were included from these flights, since the methods for density estima­ Common Eiders first nested in The tion are still in development. See Netherlands in 1906. Early breeding Laursen et al. (1997) and Pihl ef al. population censuses were estimates (2001) for more detailed descriptions based on incidental sightings of breed­ and comparisons of the aerial survey ing females. Until the early 1940s, methods. In 1996 and 2001, outbreaks Common Eiders only nested on of Avian Cholera epidemics caused Vlieland (53°15'N , 04°55'E), after initial mass mortality amongst breeding breeding attempts on Terschelling in females at several breeding colonies 1906. With most breeding Common (Christensen et al. 1997, NERI unpub­ Eiders in nature reserves, estimates of lished data). Eiders are hunted in breeding numbers in the 1940s, 1950s Danish waters from 1 October to 28 and 1960s were usually from reserve February. The number of Eiders shot by wardens, often without indications of census methods. In the late 1950s, hunters has been monitored by the Danish Bag Record Scheme since 1958 when the breeding population had (Strandgaard & Asferg 1980; Asferg increased to several thousands of breeding pairs, the 'differentiated 2001 ), while sex/age composition of the bag has been recorded through the count' became established, notably on Danish wing survey since 1982 Vlieland (Hoogerheide 1950; Swennen (Clausager 2002 and references here­ 1976). The differentiated count involved in). Based on these data, specific a census of Common Eiders roosting analyses of factors affecting the Danish near the colony, after adult females Eider bag have been undertaken (Noer had gone ashore to incubate eggs. et al. 1995, Christensen et al. in prep.). Differentiating between adult males A detailed population study of breeding (Am), immature males (Im) and birds in Eiders on the island , of Saltholm female plumage (F; adults and imma­ (55°39'N, 12°46'E) was carried out dur­ tures combined), and assuming a 1:1 ing 1993-2000 (Noer et al. 1993; sex ratio, the breeding stock (B in pairs) Christensen & Noer 2001). Based on was estimated as: parameters collected at Saltholm and B = Am - (F - Im ) Baltic/Wadden Sea Common Eider 171

This method was developed to avoid and co-workers between 1966 and 1991 disturbance in colonies, but it was concentrated on traditional Common deemed inadequate for large colonies Eider wintering grounds in the Wadden (Hoogerheide & Hooogeheide 1958). Sea (Swennen 1976, 1991). Since 1993, The differentiated method was a systematic aerial survey of Eiders in employed on Vlieland between 1962 Dutch coastal waters has been under­ and 1988 and occasionally on taken, covering the Wadden Sea and Schiermonnikoog (53°30'N, 06Ί0Έ). the North Sea coastal zone during mid­ Reserve wardens monitored all other winter (Koffijberg et al. 2001). In 2000, colonies, who produced annual esti­ 2001 and 2002 three aerial surveys per mates, usually based on restricted nest winter were conducted, attempting searches and extrapolations. In the late complete coverage by flying along tran­ 1980s and early 1990s, the monitoring sects at high tide. programme on Vlieland was discontin­ Beached bird surveys have been ued. On some small, easily covered carried out along the entire Dutch islands, complete nest counts were coast, in 19 sub-areas, on a systematic conducted (Oosterhuis & van Dijk basis since 1977 (Camphuysen et aí. 2002). Since 1991, breeding Eidernum- 2002), with a small number of less sys­ bers have been monitored annually by tematic surveys before this. Census counting nesting females in selected areas are searched monthly for all dead sampling areas in the Wadden Sea birds. Based on numbers found per one area, covering c. 20% of the Dutch kilometre coastline searched, the total breeding population (Dijksen & numbers of beached birds in The Klemann 1992). However, the methods Netherlands have been estimated employed probably preclude reliable (Camphuysen et a/. 2002). A number of population trend analyses. On the detailed studies of Eider population Dutch North Sea coast, observers size and structure, dispersion, winter­ make systematic observations of ing numbers, foraging, reproductive migrating seabirds. The numbers success, adult and juvenile mortality, counted along the North Sea coast diseases, and physiology have been reflect wintering numbers in the North undertaken (Everaarts et al. 1983; Sea, but cannot be used to estimate the Hoogerheide 1950; Hoogerheide & total number wintering in The Hoogerheide 1958; Swennen 1976, Netherlands. The first Wadden Sea 1983, 1990; Swennen & Smit 1991). aerial survey took place during severe winters in 1956 and 1963 and covered Germany ice-free areas in the western Wadden Sea (Over & Mörzer Bruijns 1956; The numbers of breeding Common Verwey 1956; Zweeres 1963). Eider throughout Germany were esti­ Subsequent surveys made by Swennen mated for the period 1982-1999 by various methods, depending on size 172 Baltic/Wadden Sea Common Eider

and accessibility of the areas involved 1985; Svensson et at. 1999). Aerial post­ (Hälterlein et a/. 1995; Wilkens 1999). breeding censuses were carried out The breeding data allows discrimina­ when males aggregate at traditional tion between the northern and western areas, prior to southward moult migra­ parts of the German Wadden Sea. For tion. The sex ratio was estimated several smaller areas longer time during spring migration and on the series are available, but they do not breeding grounds, and was used to enable the compilation of a national transform these male counts into estimate or trend (Behm-Berkelmann breeding numbers. No national esti­ & Heckenroth 1991 ; Berndt et at. 1993], mates of Eider breeding numbers exist Eider migration has not been studied for the last 18 years. Regionally, trends systematically in Germany, but the in breeding numbers have been moni­ moulting concentrations during the tored at several sites along Swedish post-breeding period (July/August) in coasts. Boat based surveys of pre­ the German part of the Wadden Sea breeding males have been carried out have been monitored by aerial survey at in the Stockholm archipelago (59°30'N, low tide (when Eiders concentrate in 19°00'E) in late April since 1985, when large flocks along tidal gullies) since pairs are close to their breeding island. 1987. Surveys of wintering Eiders have Each year, several groups of islands in also been conducted annually since the northern and the southern parts of 1987 in the German Wadden Sea. In the the outer Stockholm archipelago have Baltic, some aerial counts, combined been surveyed (Skargårdsstiftelsen with ground based counts, have taken 2002). In the Bullero archipelago place in most years (Nehls & Struwe- (59°10'N, 18“52'E), systematic nest Juhl 1998). Estimates of the numbers counts have been made six times since of dead Eiders along a selection of 1971. In the Lygne archipelago German North Sea beaches can be (59°32'N, 19°28'E], Eider nests have made for the period 1992/93 to 2001/02 been counted regularly by a private (Fleet 2001; Fleet & Reineking 2000, landowner and his heirs continuously 2001). Since 1992/93, beaches have since 1910. In the Baltic Småland arch­ been surveyed between October 1 and ipelago (57°45’N, 16°40'E) the breeding March 31, with the same search effort numbers of Common Eiders were sur­ and using the same methods, and can veyed between 1990 and 2000 be used for the calculation of the num­ [Johansson & Larsson 2001). On Lilia ber of dead birds found per kilometre. Karlsö (57°19’N, 18°04'E) and Stora Karlsö [57°17’N, 17°58’E), Gotland, Sweden numbers of breeding pairs have been counted since 1990. In southern The total Swedish breeding popula­ Sweden, land based counts of Eiders tion was estimated in 1973 (Almkvist et have been carried out annually in mid- al. 1974), and 1983-1984 [Andersson Baltic/Wadden Sea Common Eider 173

September since 1976. At Kåseberga the southernmost tip of the mainland (55°25'N, 14°05’E) in southern Sweden, has generated long-term count data of spring migration counts of Eiders have migrating Eiders (from 1979 onwards), been made annually between 1992 and which are currently the subject of 2000. Assuming the percentage of the analysis. During 1968-92, systematic Baltic-breeding Eiders which pass winter counts have been conducted on Kåseberga in spring each year is con­ four open-sea routes in the Aland Sea stant, these counts provide a (Flario et al. 1993). Detailed studies of representative measure of the annual breeding Eiders have been carried out total numbers. Volunteers in Sweden in the Söderskär bird sanctuary have conducted countrywide land- (60°07'N, 25°25'E) in the Gulf of Finland based winter surveys since 1991 since the 1950s. Data on breeding covering the southern part of both the numbers, clutch size, timing of breed­ east coast and west coast. In the ing, hatching success, female body Stockholm archipelago an experimen­ weight and adult and duckling survival tal study of the effect of the non-native have been collected (Flario & Selin American Mink Mustela vison on the 1988). Similar data have been collected Eider population has been carried out at four other Eider nesting areas on the between 1995-2001 at Bullerö- southern coast. Studies to develop Långviksskar. Counts of males, nests methods for the evaluation of effects of and habitat preferences on Mink- diseases, food limitation, and contami­ inhabited and on Mink-free control nants have been conducted at all these islands have been carried out annually. locations since the mid-1990s (Hollmén 2002, Franson et al. 2000). Finland Other countries The size and extent of the Finnish archipelago makes complete breeding The Norwegian Skagerrak winter censuses impossible. Eider popula­ population was estimated using land- tions are monitored in sample areas based surveys in the late 1980s (Nygård within the framework of the national et al. 1988) and in 2002. The breeding Archipelago Birds Censuses. These numbers for this region have been esti­ comprise six core areas where regular mated on the basis of aerial surveys of counts have been conducted for more pre-/post-breeding males. Breeding than 50 years. Since 1984-, an addition­ and wintering numbers are monitored al 27 areas (comprising 1550 islands) annually (Lorentsen 2001; Lorentsen & have also been counted. From these Nygård 2001). Estonian breeding sur­ counts, indices are produced to veys have been conducted annually describe the population trends using springtime nest searches within (Koskimies & Väisänen 1991). The nature reserves supporting 60-70% of Flanko bird station (59°50’N, 23°00'E) at the national total (Onno 1970; Renno 174 Baltic/Wadden Sea Common Eider

1993; Kuresoo et al. 1998). The total maximum of c.25,000 pairs in 1990 national breeding numbers are based (Lyngs 2000). In 2000, the population upon aerial and boat surveys of males comprised c.24,000 breeding pairs in spring performed at a few colonies (Figure 1, Lyngs in prep.). The similari­ outside of the nature reserves. In 1990- ty in numbers of breeding Eiders in 2002, annual winter ground counts of 1990 and 2000 does not reflect a stable Estonian Eiders have been made, with situation between or within colonies. an aerial winter survey covering the During this period, a few old, large whole coast in 1993. in Latvia sporadic colonies have experienced marked observations in possible breeding areas decreases, while increases have have been made. The Latvian coast is occurred in a number of small and counted every winter, and observations newly established colonies (Lyngs in of migrating sea birds are made annu­ prep.). Danish breeding Eiders are res­ ally during spring and autumn. For ident or partly migratory (Lyngs & Poland, wintering numbers used in this Christensen, in prep). Females are paper are those submitted to the highly philopatric as documented by Seaduck Specialist Group Database. ringing data (Lyngs & Christensen, in prep.) and a DNA-study (Tiedemann & Statistical analysis Noer 1998), but males show high natal dispersal. Marked declines in numbers Statistical analyses were carried out of breeding Eiders have been recorded using SAS for Windows. Spearman at several Danish colonies during the Rank Correlation Coefficients were cal­ 1990s. The most obvious declines were culated. Trends in the Finnish and related to outbreaks of Avian Cholera in Norwegian breeding populations and 1996 and most recently in 2001. Avian Norwegian wintering populations are Cholera, caused by the bacteria analysed using TRIM software (Trends Pasteurella multocida, was recorded in and Indices for Monitoring data), with Eiders from several colonies in south­ stepwise models being used to identify west Kattegat (Stavns Fjord (55°54'N, significant changes in index trends 10°39'E), Hov Røn (55°54'N, 10°17'E), (Pannekoek & van Strien 2001; Flario & Svanegrund (55°50’N, 10°21 Έ ) , Alrø Rintala 2002; Lorentsen 2001; Polder (55°52'N, 10°07'E), Mågeøerne Lorentsen & Nygård 2001). (55°35'N, 10°07’E), Søby Rev (55°53’N, 10Ί4 Έ )) in both 1996 and 2001, but was Results also recorded in one colony in eastern Denmark in 2001. The number of Denmark Eiders affected by the Avian Cholera The Danish breeding population of epizootics in 1996 and 2001, was esti­ Common Eiders increased from c. mated to total 3,500-4,000 females and 1,200 pairs in 1935 (Spärck 1936) to a C.400 males (Christensen et al. 1997, Baltic/Wadden Sea Common Eider 175

30000 • The Netherlands 4 Germany ■ Denmark □ Estonia

-B-

····· ···

t t —T- r~T t t t T T τ O ) CM in CO r^- co g> r^- cp s co O) O) O) O) O) o> §

Figure 1 National annual total numbers of breeding Eider females in The Netherlands, Germany, Denmark and Estonia for 1960-2001 (see Methods section for description of data collection techniques and constraints on interpretation). Each year is identified by the January of each count, hence “1986” refers to the winter 1985/1986 etc.

NERI unpublished data). At individual only a reduction in adult survival could colonies, the proportion of females result in a match between the observed dying ranged from 30% and 90%. On a and expected population decline, cor­ national scale, the epizootics were esti­ responding to a change in mean annual mated to have reduced the total survival rate from 0.87 to 0.81 breeding population in 1996 and 2001 (Christensen & Noer 2001). by c.8-10%. In eastern Denmark, the Counts of moulting Eiders in Danish Saltholm colony has decreased by 38% waters during August in 1987-89 found during 1993-2000 (Figure 2, total numbers between 70,000 and Christensen & Noer 2001) without 135,000 individuals. The major moult­ being affected by Avian Cholera. For ing grounds were located in Kattegat, this colony, modelling showed that the Belts and in the Wadden Sea, and observed changes in breeding parame­ showed a similar distribution in all ters during the study period could only years (Laursen et al. 1997). Eiders win­ account for a 3% decline in breeding tering in Danish waters comprise birds female numbers. Given this result, the from Sweden, Finland, other Baltic population model was used to assess countries and local Danish breeders the effect of potential changes in first (Noer 1991). Based on aerial mid winter time breeding distribution (age at first surveys, the number of Eiders winter­ breeding), and changes in adult sur­ ing in Danish waters was estimated in vival. Multiple model runs showed that 1970/71 at 300,000-500,000 (Joensen 176 Baltic/Wadden Sea Common Eider

Figure 2. Annual numbers of female Eiders breeding at the Saltholm colony, in Øresund, southeast Denmark, during 1993-2000. Estimates are based on extrapolation of sampled nest-search transects carried out using identical methods each year.

1974), increasing to c.800,000 in the 2000-2002 found high infestation rates late 1980s and the early 1990s (Laursen (80-95%), with a mean of c. 170 para­ et at 1997). In 2000, the winter popula­ sites per bird (25 parasites per bird in tion was estimated to comprise adult males). Preliminary analyses c.370,000 individuals (Figure 3, Table 1, showed no indication that body condi­ Pihl ef at. 2001 ), suggesting a dramatic tion was affected by parasite infection decline in mid-winter numbers during in either adults or juveniles (NERI the 1990s. Comparison of regional unpubl. data). numbers shows that declines in win­ Denmark has a traditional sea duck tering numbers have occurred most hunt during autumn and winter. Eider markedly in Kattegat (91%) and Great numbers bagged by hunters increased Belt (71 %), while five other regions con­ during 1958-1970 from c.100,000 to tributed a combined decrease of 30% c.140,000 per season. During the 1970s (see Pihl et at. 2001). The overall and 1980s, the size of the bag was rel­ decrease of c.50% between 1990 and atively stable, but it has declined during 2000 corresponds to an annual the 1990s to the present level of decrease of c.5% over the last ten c.80,000 individuals (Figure 4, Noer et years. al. 1995; Asferg 2001). Recent analyses Studies of the occurrence of intesti­ from the period 1983-1999 showed that nal acanthocephalan parasites in the annual Eider bag is mainly affected Eiders shot by Danish hunters during by the number of hunters, since varia- Baltic/Wadden Sea Common Eider 177

Table 1. S ummary table showing available national estimates of winter (individuals] and summer (pairs) numbers of Eiders in the Baltic/Wadden Sea flyway population for 1991 and 2000. *= extrapolated figure since no firm data exists for the given time period.

Winter Summer 1991 __2000__ 1991 _ 2000 The Netherlands 103,299 97,657 7,621 ? Germany 236,451 248,663 971 1,166 Denmark 797,000 370,000 25,000 24,000 Sweden 20,000 20,000 ?? Norway ? 10,000 ? 30,000 Finland 30 200 150-180,000 170,000 Poland 24,000 10,000 ? ? Estonia 100 100 12,000* 12,000*

Total flyway 1,180,850 756,620 215,592 237,166

tion in this parameter explains 78% of declined due to pesticide discharges in the variation in bag size. Reproductive the river Rhine (Swennen 1972; success of Danish breeding Eiders (the Koeman et al. 1969). The population ratio of juvenile birds per adult female subsequently recovered during the bagged in October), and the number of 1970s and 1980s (Figure 1). In the late days with hunting (affected by weather 1990s and early 2000s, local population conditions), were found to explain an declines have been documented while additional 8.1% and 5.0% of the varia­ the breeding population as a whole tion, respectively (Christensen et al. in numbered probably between 5,000 and prep). Thus, a decline in the number of 10,000 breeding pairs. The breeding hunters reporting shot Eiders from population is sedentary staying gener­ c.14,000 in the early 1980s to c.7,000 in ally within the Wadden Sea (ef. 1999 seems to be the main reason for Swennen 1990), so its size cannot be the decline in Eiders shot in Denmark. determined using migration observa­ tions. Females are highly philopatric, The Netherlands but males show high natal dispersal. The majority of Eiders wintering in The The number of breeding Eiders in Netherlands originate from Baltic The Netherlands increased markedly breeding areas, which mix with local from C.10 pairs in 1930 to c.6,000 in the breeders (Swennen 1991). The total 1960s. In the mid-1960s the population number of wintering Eiders has 178 Baltic/Wadden Sea Common Eider

900000 ^Denm ark „ 800000 A ■ ■ A Germany ■o 700000 LU #The Netherlands j? 600000 ■ £ 500000 c

5 400000 ■ S 300000 ■Ω A A * A A A I 200000 A A A A A z 100000 * . * . · . · O 1986 1988 1990 1992 1994 1996 1998 2000 2002

Figure 3. Annual mid-winter counts of Eiders in The Netherlands, Germany and Denmark from 1987- 2002. Note that 'Germany' includes only data from the former West Germany, thus excluding Mecklenburg-Vorpommern. Each year is identified by the January of each count, hence "1987" refers to the w inter 1986/1987 etc.

2ÜUÜUU 180000 160000 140000 120000 100000 80000 60000 40000 20000 0 o CNI CD CO Ο CM CD CO 0 0 o o OO CO 0 0 CD CD CD CD CD CD CD CJ> CD CT> (T> CD CD CD CD T— T “ χ— χ— Τ­ T— T— χ— T— T—

Figure 4. Estimated size of the annual Danish Eider bag during 1980-1999. Each year is identified by the January of each count, hence "1986" refers to the winter 1985/1986 etc. Baltic/Wadden Sea Common Eider 179 increased markedly from c. 10,000 dur­ apparently avoided the massive die-offs ing the 1950s to a peak of c.170,000 in in Dutch waters. Examination of dead the early 1970s. Since then, numbers birds collected during the winter have varied between c .1 00,000 and 1999/2000 and 2001/2002 showed that 170,000. Despite large year to year vari­ all were emaciated and carried large ation, counts suggest a decline during numbers of intestinal acanthocephalan the late 1990s (Figure 3, Table 1). parasites (Kuiken 2001; Borgstede During the 1970s and 1980s, wintering 2001 ; Smaal et al. 2001, van den Berk et Eiders were mainly confined to the al. 2001). Flowever, as heavy parasite Wadden Sea with few recorded in North infestation has also been found in Sea coastal waters. In the early 1990s, 'healthy' Eiders shot by hunters in the distribution of Eiders changed Denmark, it seems that a general markedly, with a higher proportion of shortage of food (specifically bivalves) birds recorded along the North Sea in the Wadden Sea is the most obvious coast, especially during the winters explanation for the observed mass 1992-93 and in 2000-2002. mortality. Documented over-exploita­ Normally c.3000 beached dead tion of the shellfish stock (blue mussel Eiders are found each winter in and cockles) in the Dutch Wadden Sea beached bird surveys, but extremely occurred in the early 1990s. At this time high numbers occurred during the Eiders began to occur in large numbers three most recent winters, 1999/2000, on Spisula banks in the North Sea, indi­ 2000/2001 and 2001/2002, with a peak cating a general food shortage in the of c.22,000 during 1999/2000 Wadden Sea (Camphuysen et al. 2002). (Camphuysen 2001; Camphuysen et al. In the North Sea, however, Spisula is 2002, Figure 5). Compared to the long­ being increasingly exploited by humans term average, relatively large numbers (Sm aal et al. 2001; Piersm a & of beached Eiders were also recorded Camphuysen 2001), resulting in years during the winters 1990/91 and 1991/92 when Eiders have difficulties in obtain­ (Figure 5). Given the areas in which ing adequate alternative food dead Eiders were found and recoveries resources. In the winter of 1999/2000 of ringed birds (Camphuysen unpub­ very few large Spisula occurred along lished), it seems that a large proportion the entire Dutch North Sea coast. of the birds that died belonged to the Dutch breeding population. Recoveries Germany of dead birds ringed in Denmark and Finland showed that these were mainly The German Wadden Sea breeding juveniles and immatures (Lyngs in population has been stable at 1000- prep.). Thus, despite the fact that 1300 pairs since 1 982 (Figure 1), migrants out-number local birds dur­ although there have been shifts from ing winter, the migratory adult Eiders northern to western areas in recent years (Hälterlein et al. 2000). At most, 180 Baltic/Wadden Sea Common Eider

6,00

CM CD CO o CM CD o o O CM OO o o c o 0 0 CDCD 5 CD CD o O r^ CD T“ CO m CD T“ CO LO CD τ - h- r^ o o 0 0 0 0 CO 0 0 CD CDCDCDCD Ο c d CD CDCD CD CD CD CDCDCDCD CD o T“ r— ■*“ ■τ- T ” v ~ ■*— T— T— CN Winter

Figure 5. Total number of dead Eiders found in winter per km beach searched during the beached bird surveys in The Netherlands, 1978-2002 and in Germany, 1993-2002.

19 breeding pairs have been recorded aerial winter counts along the coast of in the German part of the Baltic Sea, so Mecklenburg-Vorpommern (54-°12’ N, the contribution from these birds is 12°05’E] started in the early 1990s (Nehls negligible in a flyway perspective. The 1994; Nehls & Stru w e-Ju hl 1998), but moult concentrations in the German have not been conducted on a yearly Wadden Sea have shown a significant basis. Wintering numbers were con­ decrease from 258,000 in 1989 to stant around 70,000 Common Eiders 160,000 in 2001 (rs=-0.891, P=0.001, during 1997/1998-2000/2001. However, n=10). Excluding w inter 2001/2002, the no counts are available for the winter German winter population has been 2001/ 2002. stable over the last 15 years, showing The numbers of dead Eiders regis­ no long-term trends in any region tered along German North Sea beaches (Figure 3, Table 1, Brager & Nehls in winter increased tenfold from an 1987; Brager et al. 1995; Nehls 1989; average of 433 birds in the period Scheiffarth et at. 2001). In w inter 1991/92 to 1994/95 to 4265 birds in 2001/2002, Eider numbers were the 2001/02. The increase began in lowest seen since the start of regular 1997/98, two years after the extremely aerial counts throughout all former cold winter 1995/96, when 783 were West German coastal waters in 1987 found. Numbers in 2000/01 were lower (more than 100,000 less than the for­ than in both the previous and the fol­ mer minimum numbers). Systematic lowing winter, due to prevailing Baltic/Wadden Sea Common Eider 181 southeasterly winds that winter, which Islands in the Stockholm archipelago prevented birds being beached in the showed an increase from 1948 to 1979. German Bight. Because awareness of During the last ten years (1992-2001) the recent dramatic increase in Eider the breeding numbers at Lygne have mortality could inflate the detection decreased by c. 25% from 1,320 to 980 rate of this species, the numbers of pairs (Figure 7). There was no signifi­ dead Eiders registered in winter on only cant trend in the Eider breeding a selection of German North Sea numbers on the Småland archipelago beaches have been compared in this between 1990-2000, although there analysis. These are sites using the were negative trends amongst 10 out of same methods and effort since 39 other breeding avian species 1992/93. Numbers reported from these (Johansson & Larsson 2001). No signif­ sites increased from 1.4 birds/km in icant trends in breeding numbers were 1992/93 to 4.6 birds/km in 2001/02 found from the two islands near (Figure 5). The increase was greater in Gotland (P>0.05). From the Swedish Niedersachsen (0.9 birds/km 1992/93; west coast, September counts of Eiders 7.6 birds/km 2001/02) than in showed no significant trends between S ch lesw ig-H olstein (1.7 birds/km 1992-2001 (rs=-0.62. P=0.05, n=10). 1992/93; 3.2 birds/km 2002/01). Between 1992 and 2000, there was no overall significant trend in the time Sweden series of migration data from Kåseberga (Figure 8; rs=0.067, P=0.86, The total Swedish breeding popula­ n=9), although reported numbers have tion was estimated at 170,000 pairs in been lower in more recent years. Since 1973 and 270,000 pairs in 1983-1984, an 1991, the peak in winter numbers along annual increase of c.6%. The Swedish the southern coasts was c.16,000 in population is mainly migratory, wintering 2000 (Figure 9, Table 1), with no signif­ in Danish waters and the Wadden Sea icant trend over the whole period (Fransson & Petterson 2001). In 1973, (Figure 9). the Stockholm archipelago was esti­ In most areas of the Stockholm mated to support c. 50% of the Swedish archipelago where the Mink have been breeding population. Breeding num­ present in high densities for 10-30 bers in the northern Stockholm years, there has been a substantial archipelago have increased (rs=0.78, decrease in numbers of breeding Eider P=0.0002, n=17), but decreased in the ducks. The choice of nesting habitats south between 1985 and 2001 (rs=-0.53, has also changed dramatically since P=0.03, n=17) (Figure 6). Nest counts at the arrival of the Mink. Eiders have Bullero showed an increase from 1971 abandoned bushy and wooded islands, to 1989, followed by a decrease in the and moved to gull colonies or joined years from 1989 to the most recent solitarily nesting gulls on small open count in 2001 (Figure 7). The Lygne 182 Baltic/Wadden Sea Common Eider

10000 9000 8000 ® 7000 E 6000

h0 . 5000 E 4000 1 3000 z 2000 1000 0

CO 00 oo oo oo CD CD CD CD CD CD CD CD CD C D O O CD CD CD CD CD CD CD CD CD CD CD CD CD CD CD O O t - CM CM

Figure 6. Trends in numbers of breeding Eiders in the northern and southern parts of the Stockholm archipelago, Sweden, as estimated from ship-based male surveys undertaken in late April. Five island areas were not surveyed in some single years and where this occurred, the average of the year before and immediately after were substituted.

1600 ¡2 1400 '5 “ ■ 1200 σι A A ▲ ▲ i 1000 ▲ SIέ 800 A Lygne o 600 ■ Bullero k . oOCN-M-tDCOOCSI-'J-tOCOOCM-riOCOO O)(j)(j)0)0)(j)0)0)(j)(j)cnœo)0)cj)0

Figure 7. Eider nest counts at Bullero and Lygne in the archipelago of Stockholm, Sweden, 1970-2001.

isLets. In areas more recently colonised Finland by Mink, Eider populations have shown In 2001, the total Finnish breeding stable or even increasing trends, whilst population was estimated to be areas without Mink show consistent 150,000-180,000 pairs. Approxim ately increases in numbers of breeding 150,000 breed in the southwest archi­ Eiders. The Swedish annual hunting pelago, while c.10,000-20,000 breed in bag amounts to 3-5,000 Eiders. Baltic/Wadden Sea Common Eider 183

the Gulf of Finland, and less than es occurred in the late 1980s and 10,000 breed in the Bay of Bothnia. The 1990s, and the population is estimated population is entirely migratory. The to be declining at a rate of 8-16% per index of breeding Eiders in Finland year (Figure 10). Decreases in numbers increased from the 1970s to the late of nests were first reported from the 1990s. The increase was most rapid Gulf of Finland in the late 1980s, during the 1970s and 1980s, averaging whereas marked declines in nest abun­ 7% to 10% per year. No further increas­ dance in the southwest archipelago

Figure 8. Number of spring migrating Eiders counted at Kåseberga on the southern coast of Sweden, 1992-2000.

18000

<£ 16000 0) I 14000 c 12000 Π) ■£ 10000

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 Figure 9. The number of wintering Eiders along the west cost and southern part of the east coast of Sweden, 1991-2000. 184· Baltic/Wadden Sea Common Eider

2 η

1

1 1988 1992 1996 2000

Figure 10. Indices of Eider population trends in Finland according to nest-count data, 1986-2001. Vertical lines show upper 95% confidence limits of the indices (from Hario & Rintala 2002).

only started during the mid 1990s both adult female Eiders and ducklings (Flario & Rintala 2002). Regular winter in Finland, but the role of parasites as a counts from the Aland Islands have mortality factor seems to be associated shown that only 20-200 Eiders remain with other predisposing factors, such in ice-free areas, depending on the as impaired feeding ability or immuno­ severity of the winter (Table 1). suppressive viruses (Hario et al. 1995, Studies at the Söderskär bird sanc­ Hollmén et at. 1999, 2000). A decrease tuary have documented poor fledging in clutch size during the period of pop­ success during the period of population ulation decline cannot explain the decrease, although adult mortality has population trajectory (Hario & Selin, in remained stable (Flario & Selin, in press), nor is it associated with the press). Low duckling production has effects of gull predation on ducklings been related to viral infections in 1996 (Hario & Selin 1991). Predation by feral and 1999, causing mass mortality with­ Mink has had only limited effect on the in the first weeks after hatch (Hollmén density of incubating females 2002). Intestinal infection with acantho- (Niemimaa & Pokki 1990; Nordström et cephalan parasites has been found in at. 2002). However, the recovery of the Baltic/Wadden Sea Common Eider 185

White-tailed Eagle HaÍiaeetus aÍbicilía 2001 (J.A. Rundtorn pers. comm.). The population may be an important factor Estonian breeding population (Kilpi & Öst 2002). increased from c.3,000 pairs in 1965 to Common Eiders are legal quarry in c .15,000 in 1995 (Leibak et al. 1994), Finland in both spring and autumn. and subsequently decreased to c. Spring hunting is strictly licensed and 12,000 pairs in 2001 (Eve Magi and Arvo is only permitted on males. Flunting Kullapere pers. comm.). Between 1990 practices differ between mainland and 2002, ground counts have shown Finland and the Aland Islands, due to that Estonia supported fewer than 100 differing legislation. On mainland wintering individuals annually (Table 1); Finland, c. 7,000 males were formerly winter aerial survey of the entire coast bagged in spring, but the current annu­ in early 1993 found only 75 Eiders. al quota has been reduced to 2,000 Common Eiders have never been males. Another 10-12,000 Eiders are recorded breeding in Lithuania, due to taken in autumn, of which c. 25% is lack of suitable nesting habitat. This male. On the Aland Islands, spring species does not regularly winter there, hunts take 7,500-9,000 males, but although a few have occasionally there is no open season for females occurred in recent years. The species and yearlings. The total Eider bag is a little more frequent during spring am ounts to 25,000-27,000 birds of and autumn migration, but numbers which c. 18,000 (70%) are males. are low, with no regular staging sites Contrary to the situation in Denmark, (R. Zydelis, in litt.). Common Eiders the Finnish bag size shows a strong have never been observed breeding in correlation with the size of the breeding Latvia (Viksne 1983). The maximum population (Flario & Selin 1987). count of wintering Eiders was 14 in 2001, with autumn and spring maxima Other countries of 5 and 50 respectively counted along the Latvian coast (A. Stipniece pers, The Norwegian Skagerrak popula­ comm.). According to the Polish rarities tion of breeding Eiders is estimated at committee, Eiders have bred only very 20-30,000 pairs based on male counts. sporadically in Poland. The Polish coast The number of breeding pairs has offers little wintering habitat for this shown an overall significant increase species, which usually numbers fewer since 1988 (Lorentsen 2001), although than 1,000 (W. Meissner pers. comm.). breeding numbers since the mid 1990s have stabilised. The wintering numbers Assessing the overall size of the have been stable at c.10,000 Eiders Baltic/Wadden Sea Flyway Population (Table 1), with minor increases in some sub-areas. In Norway, a mean of 9,108 For 1991 and 2000, reliable breeding individuals were bagged annually in the estimates exist for Germany, Denmark, Skagerrak region between 1998 and Estonia and Finland. For the numerous 186 Baltic/Wadden Sea Common Eider

Swedish population, there are no national breeding population estimates nationwide surveys from these two since the mid-1980s, a country known time periods and a reliable breeding to host approximately one-third of the estimate for The Netherlands is avail­ Baltic breeding population (Almkvist et able only for 1991 (Table 1). Summing al. 1974), and trends in breeding num­ the breeding figures excluding Sweden, bers at a few local areas may not be Norway and Poland, results in representative of those in the total pop­ c.215,000 pairs for both periods. ulation as a whole. Given that better Comprehensive winter surveys have count coverage is achieved in winter, been conducted in Denmark, The we consider that the c.36% decline in Netherlands, Germany and Norway. In total numbers offers the best assess­ 1991 the total number of wintering ment of population change available Eiders amounted to c.1,200,000 birds and therefore gives some cause for and during the following 10-year period concern, in trying to answer the ques­ a decrease to c.750,000 birds has taken tion of why there is an apparent place (Table 1). This decline is appar­ mismatch between the reductions in ently due entirely to changes in the size wintering numbers and apparent sta­ of the Danish wintering population. bility in local breeding numbers, it is suggested that the discrepancy could Discussion be explained by one or more of the fol­ lowing five (not necessarily competing) Status of population and evaluation of hypotheses: methods Hypothesis A: declines in breeding Although the Baltic/Wadden Sea numbers at nesting sites are buffered by Common Eider population increased the availability of substantial numbers of steadily in numbers from the 1970s young non-breeding birds. onwards, this review suggests that the These normally non-breeding situation has begun to change during Eiders are able to recruit into the the last ten years. The total flyway esti­ breeding population during periods of mate of the wintering population has reduced competition for pre-nesting decreased by c.36% between 1991 and feeding habitat or breeding habitat (eg 2000 (Table 1), although most national as a result of increased adult female breeding trends show no equivalent mortality). This would sustain (at least marked decrease since the mid-1990s. in the short term), stable local breeding Unfortunately, the lack of national numbers (eg by a simple decrease in breeding estimates, precludes an over­ age of first breeding) despite an overall all assessment of changes in the total decrease in total population number. flyway breeding population between Alerstam & Högstedt (1982) were the 1991 and 2000. There are no Swedish first to propose that the relative extent Baltic/Wadden Sea Common Eider 187 of breeding habitat to those habitats breeders. Common Eiders are capital used to survive between breeding sea­ breeders (Meijer & Drent 1999) and sons, shaped the migratory and females store large energy reserves (up reproductive strategies of different bird to 20% of body mass) for egg formation, species. They characterised the egg laying, and for fasting during incu­ Common Eider as a species that bation. Prior to breeding, females need exploits a highly limited breeding habi­ to feed intensively in order to reach the tat, but a surplus of abundant and minimal body mass for a successful widespread non-breeding survival breeding attempt. If this weight is not habitat (a classic 'S-species' of attained, females may decide not to Alerstam & Högstedt 1982). Strong breed, or to breed and risk abandon­ competition for breeding habitats ment of the clutch. Local food supply results in deferred breeding and high and the level of competition may there­ annual survival. This 'buffer' element fore affect the pre-nesting increase in of the population can constitute at least body mass. The observed increase of 20-30% of the total population first-breeding females in the year after (Almkvist et al. 1974; Alerstam et aí. the die-off of adult females could also 1974; Coulson 1984). The buffering indicate less competition and more effect was evident after the Danish out­ available food for inexperienced and breaks of Avian Cholera, which killed less competitive first-year breeders. 90% of all breeding females in two colonies, in the years following the out­ Hypothesis B: declines in flyway winter­ breaks, breeding numbers fell by 75% ing numbers are caused by a significant and 65% at Stavns Fjord and Rønø, (but currently undetectedÌ decline in the respectively (NERI unpublished data), Swedish breeding population. ie less than would have been expected Since no national breeding esti­ if there was no pool of non-breeders mates exist from Sweden for the last awaiting the opportunity to attempt to ten years, this population could have nest. These non-breeders contribute in decreased significantly without the number to the winter counts, but are trend being detected. Three out of four not detected during normal breeding studies of local Eider breeding ecology surveys. In this way, winter surveys in the Stockholm archipelago have would detect a general flyway popula­ shown negative population trajectories, tion decline before this was apparent whereas a fourth from the northern from counts of breeding birds. In addi­ part of the archipelago showed stable tion to competition for suitable nesting numbers. The results from these local sites, competition for food prior to breeding studies, and the fact that the breeding may have an even greater archipelago of Stockholm was formerly impact upon the success of a breeding known to host c.50% of the Swedish attempt by first-year or experienced breeding population of 270,000 Eider 188 Baltic/Wadden Sea Common Eider

pairs, indicate that Hypothesis B could No major changes in survey meth­ contribute to the observed differences ods or extent have occurred or been in trends between breeding and winter­ reported for the breeding counts during ing numbers. the period, and hence, we consider that all figures from the ten year period are Hypothesis C: recent winter surveys in reasonably comparable. Hypothesis D Denmark have not counted ail Eiders, is therefore considered unlikely to con­ either because of difference in coverage tribute to the explanation for the or problems with the counting proce­ observed patterns. dures. Hypothesis E: changes in winter distrib­ The geographical coverage of mid­ ution of Eiders have involved expansion winter Eider counts has not been into previously non-surveyed areas. changed between the surveys. The switch from systematic total coverage Since most wintering areas are sur­ to a transect survey in the Kattegat off­ veyed regularly and because Eiders shore area in 1999/2000 cannot explain winter relatively close to coastlines eas­ the magnitude of the decline in Danish ily covered from either land or aircraft, wintering numbers between 1991/1992 any significant changes in winter distri­ and 1999/2000. Additionally, all other bution should have been detected. Food areas in Danish waters outside supply and the energetic constraints of Kattegat showed significant declines. feeding in deeper water restrict the In general, the aerial survey method opportunities for large numbers of used for the Danish winter counts is Eiders to winter offshore or elsewhere in considered to be reasonably robust, as the range. Although large numbers for­ suggested by the low variability merly wintering in the Wadden Sea have between the three survey totals from moved out into the North Sea in recent 1989, 1991 and 1992, when total Danish years, the probability of this occurring Eider numbers ranged between undetected elsewhere in the winter 779,000 (in 1989) and 797,000 (1991, range remains small. Hypothesis E is Figure 3). It is therefore considered that therefore unlikely to explain the factors relating to survey technique are observed patterns. However, the relative highly unlikely to have contributed sig­ low number of wintering Common nificantly to the dramatic decline in Eiders in the German Wadden Sea in Danish wintering numbers of Common winter 2001/2002 coincided with a lack Eiders that was found in 1999/2000. of midwinter survey in the Mecklenburg- Vorpommern region. This involved Hypothesis D: previous breeding counts c.100,000 Eiders which underlines the have missed a large proportion of the need for a greater degree of co-ordina­ breeding pairs detected in more recent tion of winter counts throughout the years. flyway. Baltic/Wadden Sea Common Eider 189

It seems probable that Hypotheses an increase during the 20th century A and B are the most likely explana­ until the early or mid 1990s, followed by tions for the differences in trends either stability, or in most cases, between the wintering and breeding declines, either in breeding or winter­ figures. Whatever the true explanation, ing numbers (Figures 1, 6, 7, 8, 10). In it is clear that it is possible to count Finland, the annual breeding index was birds effectively on the wintering not significantly different between the grounds, and that this is likely to be the two breeding seasons of 1991 and 2000, best point in the annual cycle at which but in the years between, numbers to measure and detect major changes increased up to 1997, after which a in overall flyway population number. steep decline ensued to 2001. In winter Birds on the wintering grounds are 1999/2000 and 2001/2002, relatively aggregated, so if the problem s of large numbers of dead Eiders from the adopting common count techniques local breeding population have been and co-ordinating international census recorded in the beached bird surveys in effort can be overcome, this represents The Netherlands (Camphuysen pers, the most important monitoring goal for comm.). the future. If Hypothesis A is correct, it In conclusion, it is considered that might be expected that, once the pool the c. 36% decline in winter numbers of non-breeders is exhausted, declines between 1991 and 2000 reflects a sig­ w ill also be detected on the breeding nificant population decrease grounds. At that stage, the throughout the flyway, not distinguish­ Baltic/Wadden Sea Eider population able at present in the trends of national will be even more sensitive to changes breeding numbers due to the buffering in annual adult survival, disease epi­ effects of the non-breeding element of demics and winter and pre-breeding the population. food shortage. For this reason, it is rec­ ommended that there is continued Factors affecting population size vigilance on the breeding areas, and that common standards for conducting Besides the dramatic decline in the breeding surveys are devised and Danish wintering population from adopted. 1991-2000, other parts of the wintering Even though no significant decrease range of the Baltic/Wadden Sea popu­ in national total breeding numbers has lation Eiders have also seen substantial been recorded between 1991 and 2000, declines. For example, declines were several indications strongly suggest observed during 2000-2002 at two large declines in local sub-populations. German wintering areas: the Wadden Amongst most countries in the Sea and in the Baltic off Schleswig- Baltic/Wadden Sea flyway, a common Holstein. In the Dutch Wadden Sea, a trend in population trajectory is seen: long-term decline in wintering num­ bers has occurred. Many local and 190 Baltic/Wadden Sea Common Eider small-scaLe population studies along Mink invasion (Nordstrom et at. 2002). the flyway suggest that the population Nevertheless, the fact that American dynamics of Eiders have been adverse­ Mink have spread to most parts of the ly affected by a variety of human Eider breeding range suggests that it is induced and natural factors. Modelling a potential threat to the population and has shown that small changes in adult may have contributed to the present survival have far greater impacts on decline. Hence, Mink predation repre­ annual changes in the num bers of sents an important subject for study in Common Eiders (Christensen & Noer coming years. Other ground predators, 2001) compared to far greater changes such as rats Rattus spp. and Red Foxes in annual reproductive success Vulpes vulpes, also affect Eider nesting (Swennen 1983). Nevertheless, a pro­ success. In spring 2002, considerable longed decrease in recruitment rate egg-predation by rats was observed at due to either persistent low fecundity or the breeding colony on Vlieland in The low chick survival will eventually result Netherlands (R.K.H.Kats unpublished in lower numbers of immature non­ data). The presence of Foxes at breed­ breeders, reductions in recruitment ing areas on the mainland is also and will also finally reduce population thought to contribute to declines in size (Hario & Selin, in press). nesting density there. The observed In the breeding season, both adult shift from nesting in closed habitats to survival and recruitment can be more open areas and to islands with adversely affected by different factors, the presence of gulls may improve eventually leading to a decline in the adult survivaL, but may increase the population. Intensive predation studies predation of eggs and ducklings and as on the American Mink in Sweden have a consequence reduce reproductive shown significant reductions in local output. densities of breeding Eiders. To what Diseases can lower both adult sur­ extent Mink presents a general prob­ vival and recruitment during the lem to sustaining the current size of the breeding season, as reported for Avian Baltic/Wadden Sea Eider population Cholera in Denmark (Christensen et al. and whether the Eiders are able to 1997), for intestinal occlusion in males adapt to this invasion, remains an open in Finland (Grenquist et al. 1972), and question. In Finland, long-term the outbreaks of duckling virus in removal of Mink from two archipelago Finland (Hollmén 2002). Since little is areas (125 km2 and 72 km2, respective­ known about diseases and their effects ly), did not alleviate declines in on the Eider population dynamics, we breeding Eider populations, whereas it need to monitor disease in Eiders as a greatly enhanced re-colonisation by part of an international flyway-monitor- smaller waterfowl species that had ing programme. As a first step, a study been extinct from the areas since the of the causes of death and an assess- Baltic/Wadden Sea Common Eider 191

ment of how many colonies are infect­ bag size will not change even though ed, when and where shoutd be made an the flock sizes are changing, since a immediate priority. Studies found high maximum of one to two Eiders are nor­ levels of acanthocephalan parasite mally bagged independent of flock size. infestation in healthy wintering Danish The fact that the kill used to be far birds, with no relationship between greater in Denmark at a time when the parasite numbers and individual body population still increased dramatically, condition, suggesting that high parasite suggests that the Danish hunting activ­ loads were not responsible for mass ities were sustainable, at least at that mortality events in the Wadden Sea. time. Indeed, there was no relationship Amongst potential contaminants, lead between the Danish bag size and the exposure needs to be monitored over size of the Söderskär Eider population the entire range. Among nesting during the years of the steep population females in the Gulf of Finland, highest increase (1973-84-). Thus, there is no exposure rates were documented in indication that the Danish hunting areas of population declines. Severe or pressure responded to the Finnish pop­ sub-clinical lead poisoning was diag­ ulation growth (despite responding nosed in 23% of females that died of significantly to annual changes in emaciation after incubation (Franson et recruitment rate at Söderskär, Hario & at. 2000, 2002, Hollmén 2002). Selin 1987; Noer et at. 1995). The Baltic/Wadden Sea population Other human activities could poten­ of Common Eider remains legal quarry tially increase the mortality of in Denmark, Norway, Sweden and full-grown Eiders unintentionally; eg Finland, amounting to an annual total by-catch in gill nets and collisions with flyway kill of c.1 15,000 individuals. high-speed vessels (which are travel­ Whether or not this level of hunting ling increasingly fast in inshore waters) pressure is sustainable, is at present and with offshore structures such as unknown. Analysis of the Danish bag bridges and wind turbines. These statistics indicates that the bag size is potential sources of mortality have more related to the number of active increased in European offshore areas hunters than to the size of the Eider in recent years, but nothing is known population (Christensen et ai. in prep.). about the magnitude of such mortality This apparent lack of relationship and its individual and cumulative between bag size and population size effects on migratory bird populations can be explained by the hunting such as Eiders. method predominantly used in Danish The Common Eider in the Wadden waters, where hunters approaching a Sea is at its southern limit of winter flock of Eiders in motor boats are bag­ distribution where it is almost wholly ging one or two individuals from each dependent upon shellfish stocks for its flock when they flush, in this way, the food supply. The staple food items are 192 Baltic/Wadden Sea Common Eider

the Blue Mussel MytiLus edulis and the weight at the start of incubation, clutch Cockle Cerastoderma edule, as w ell as size and egg size did not change during Spisula in some areas (Leopold et al. the extended period of population 2001). However, food quality is crucial increase followed by recent decrease for these birds, in particular the rela­ [based on data from individual females tion between she LI mass and flesh breeding both in 1982-85 and 1986- mass (Nehls 1995, 2001). Flesh mass is 1991, Hario & Selin in press). This is highly dependent upon winter tempera­ likely to result from Eiders being capi­ tures and shows large fluctuations tal breeders (Meijer & Drent 1999), between years (Beukema et al. 1993; determinant egg-layers (Swennen et al. Zwarts 1991; Zwarts & Wanink 1993). 1993), and that egg size varies with age Therefore sufficient stocks of alterna­ and clutch size (Swennen & van der tive prey are important for long-term Meer 1992). This may suggest that the survival, as has been shown for the blue mussel stocks exploited in winter Oystercatcher Haematopus ostralegus, in Denmark have not changed in quali­ [Zwarts et al. 1996). Since only a small ty or size, as Finnish Eiders rely heavily fraction of the total shellfish stock is on stored (subcutaneous) fat transport­ available for exploitation by Eiders ed from Denmark to the northern (Nehls 1995), they are sensitive to large breeding sites in spring. By contrast, scale destruction of mussel and cockle food intake on the breeding grounds, beds as has occurred in several parts during the relatively unpredictable peri­ of the Wadden Sea in recent years od between the ice break-up and (Piersma & Koolhaas 1997; Piersma & laying, is mainly invested in female Cam phuysen 2001). In com bination self-maintenance (Hario & Ost 2002). with a series of mild winters with low Further evidence for the importance of food quality and poor shellfish recruit­ the Danish mussel stock to northern ment in the subsequent summer, this Baltic Eiders comes from the fact that human impact on food stocks can have the strong salinity-induced west-east substantial adverse effects on the Eider decline in mussel abundance in the population. In addition to direct adult Gulf of Finland (Ost & Kilpi 1997; mortality caused by lack of food, stud­ Westerbom et al. 2002) is not reflected ies have shown that the reproductive in parallel trends in Eider breeding output may also be adversely affected parameters along the Gulf. Since by reductions in food resources (e.g. Eiders from the central Gulf of Finland through delayed nesting, increased (with poor mussel stocks) fare equally desertion rate and reduced hatchabili­ as well as those in the western Gulf ty, Oosterhuis & van Dijk 2002). (with ample mussel stocks), during the Among the strictly migratory early breeding season, this suggests Finnish Eider population, food-related that both these Eider 'sub-populations' parameters such as female body exploit the same winter food resources Baltic/Wadden Sea Common Eider 193

(ie those in Denmark) which are ade­ better co-ordinated. We would particu­ quate at the present time. On the other larly urge, for example, those hand, Finnish breeding numbers have countries, which survey the winter pop­ declined, suggesting that fewer ulation every second or third year, to females are able to feed and store suf­ synchronise their counting years and ficient nutrients in Denmark to fly to increase the num ber of years in which Finland and breed. If this is the case, the whole Baltic/Wadden Sea Eider where are the females that failed to population is surveyed simultaneously. acquire sufficient stored nutrients to A monitoring programme of winter breed? Perhaps this suggests that the numbers on the flyway-scale is of the food supply in Denmark has declined in highest priority, if the further future guality or extent, and fewer females decline is to be adequately tracked and return to the colonies to breed quantified. The possible alternative (although those that do so, especially options for future monitoring pro­ the experienced older birds, continue to grammes to assess changes in return in good condition). abundance and distribution could Despite the fact that several studies include migration counts, breeding have shown that different local factors surveys or a combination of these two are having adverse effects on the approaches. In general, surveys of sea breeding population, their relative ducks such as the Baltic/Wadden Sea importance is unknown. Forthis reason Common Eider population are difficult it is important that future research pro­ to achieve comprehensively, due to the posals include a modelling approach, wide and often scattered distribution which will enable a better understand­ throughout the annual cycle. During ing of the relative importance of, and winter, Eiders aggregate in coastal and potential sensitivity to, the different fac­ near offshore waters, making assess­ tors affecting the population size and ment of numbers easier, if not hence enable a prioritisation of future particularly easy logistically. Flowever, conservation actions. in sum m er they nest on thousands of islands scattered over vast areas, mak­ Recommendations for monitoring ing complete surveys impossible. Designing census protocols that opti­ During the compilation of this mise coverage in both time and space review it became clear that national to minimise the extent of unsurveyed and local monitoring programmes are habitat and avoid double counting due performed in many areas along the fly­ to poor coverage and/or redistributing way. The usefulness of the compiled individuals between counts will be diffi­ data would, however, be greatly cult, but not impossible. enhanced if the methods used by differ­ Monitoring the overall population ent schemes were harmonised and trajectory through any form of direct 19Λ Baltic/Wadden Sea Common Eider breeding counts is unlikely to be feasi­ available for the study period of this ble, for the reasons discussed above. If review (per L. Nilsson). the breeding areas are to be the subject The Baltic/Wadden Sea Eider popu­ of a future monitoring scheme, the lation is a classic meta-population combination of post breeding aerial system with widely differing migratory surveys of male numbers and sex ratio and life-history strategies depending estimates, as performed in the Baltic in on breeding provenance. It is therefore the 1970s, seems to be the best poten­ unlikely, that the population processes tial option. Searching for nests in a of sedentary Dutch-breeding birds systematic or sampled fashion is both reflect those of the long-distance time consuming, disruptive and poten­ migratory Finnish birds. Local studies tially suffers from many pitfalls, of the breeding biology of the Eider although a properly designed stratified throughout the flyway cannot con­ sampling programm e could be used to tribute directly to the monitoring of generate an index of relative breeding overall changes in population size. abundance and hence trends overtime. Flowever, they are vital to understand Another possibility, rather than and interpret the reasons for the attempting to count birds extensively on observed changes in numbers and dis­ the breeding grounds, would be to tribution. Local studies invariably focus count attention intensively in include some assessment of local and areas where a representative number relative abundance that can represent pass through on m igration. If counts of important time series for tracking local migrating Eiders could be used to mon­ change in relation to flyway population itor the size of the Baltic breeding changes. Although the wing ratios in stock, spring represents the best peri­ the hunting bag from Denmark offer an od because the spatial and temporal index of annual reproductive success, distribution is relatively limited. Such a there is no regular measure of breed­ migration monitoring approach is only ing output in the population, so even feasible for the truly migratory popula­ simple measures of reproductive suc­ tions breeding in Finland and Sweden, cess would represent a major and fails to account for changes in the conservation contribution. Annual ring­ numbers of resident Eiders in The ing of even relatively sm all numbers of Netherlands or the partial migrants in females on nests, and their subsequent Denmark. The study by Alerstam et at. recapture histories can now be used to (1974) could form the methodological generate robust estimates of annual basis for developing such a migration adult female survival (and other para­ monitoring scheme. An historical per­ meters, such as site fidelity, annual spective is potentially available in the breeding probability). Systematic ring­ form of a radar dataset from the east ing of ducklings also enables an coast of Sweden which is known to be assessment of long-term changes in Baltic/Wadden Sea Common Eider 195

age of first breeding, a vital parameter and partly inadequate. Adequate moni­ in understanding the dynamics of the toring programmes are needed to track recruitment, discussed above. winter population trajectories in the However, in practice, this requires future in a way that can verify changes massive ringing programs due to the over time. If remedial actions are to be very high early mortality of ducklings; implemented for this species, aimed at and even when recruited, the first-time restoring the population to favourable breeders need to be located with confi­ conservation status, it is essential that dence - a demanding task in the field. the monitoring programmes can detect Measures of annual fluctuations in these changes. Establishment of such these local and regional breeding para­ monitoring programmes needs inter­ meters among the different national agreement, co-ordination and sub-populations are of enormous value adoption of common standards. The in interpreting population change. This available evidence suggests that a is especially true if the studies are number of factors are responsible for spread over larger areas and generate observed declines in the Baltic/Wadden longer time series, and if study m eth­ Sea Common Eider population, several ods are standardised between the of which have adversely affected num­ various different research groups. If bers on a local scale. It is known that conservationists are to understand disease has reduced adult female Baltic/Wadden Sea Eider population annual survival and duckling survival in dynamics, we need to know a great deal specific areas, but this may have gone more about the different sub-popula­ undetected elsewhere. Geographical tions, in order to identify critical shifts and mass-mortality events prob­ processes in the annual life cycle. ably caused by starvation have been Nevertheless, it is important to remain reported from parts of the wintering cautious and consider pattern and range, most notably in The Netherlands scale, since in the Swedish studies, two and the German Wadden Sea. Eiders adjacent study areas in the Stockholm are still hunted in annual numbers archipelago showed very different pop­ three times as high as the numbers of ulation trends. Care should therefore Eiders known dying in the mass mor­ be taken when extrapolating to a talities and it is still unknown whether national or international scale from the current level of hunting is sustain­ such local or regional studies. able. Other factors may also be involved which have not been fully considered Conclusions here (such as the general increase in shipping and shellfish exploitation To conclude, the monitoring mecha­ throughout the range]. The multi-facto­ nisms currently used to track changes rial explanation for the declines in abundance of the Baltic/Wadden Sea necessitates an appropriate under­ Common Eider population are uneven standing of the relative importance of 196 Baltic/Wadden Sea Common Eider these different factors if we are to References effectively prioritise and target resources to restore favourable conser­ Alerstam, T., Bauer, C. & Roos, G. 1974-. vation status of Common Eiders. Spring migration of Eiders Somateria mol­ Modelling of population processes is lissima in southern Scandinavia. Ibis 116: needed to fully understand the relation­ 194-210. ship between the Common Eider Alerstam, T. & Flögstedt, G. 1982. Bird population dynamics and the different migration and reproduction in relation to factors affecting its abundance and dis­ habitats for survival and breeding. Ornis tribution. Scandinavica 13: 25-37.

Almkvist, B., Andersson, A., Jogi, A., Acknowledgements Pirkola, M.K., Soikkeli, M. & Virtanen, J. 1974. The number of adult Eiders in the We are extremely grateful to all the Baltic Sea. Wildfowl 25: 89-94. people who have provided us with data Andersson, A. 1985. Östersjöns ejdrar räk- for the present review, without whom nade. Svensk Jakt 123: 500-503. nothing could ever have been written. Thanks to the following persons or Asferg, T. 2001. Vildtudbyttet i Danmark i institutions for providing the necessary jagtsæsonen 2000/2001 (The Danish Bag Record 2000/2001]. The Danish National data: B. Hjernquist, N. Kempf, P. Lyngs, Environmental Research Institute. Report M. Rinaldo, J.A. Rundtorn, A. Stipniece, no. 393, 36 pp. Gotland County Administration, National­ parkamt Schleswig-Holsteinisches Behm-Berkelmann, K. & Fleckenroth, FI. Wattenmeer, Nationalparkverwaltung 1991. Übersicht der Brutbestand- sentwicklung ausgewählter Vogelarten Niedersächsisches Wattenmeer, 1900-1990 an der niedersächsischen Staatliche Vogelschutzwarte Nieder­ Küste. Naturschutz Landschaftspfl. sachsen, Staatliche Vogelschutzwarte Niedersachs. 27: 1-97. Schleswig-Holstein and Stockholm County Administration. We also thank F. Berndt, R. K., Nehls, G. & Kirchhoff, K. 1993. Eiderente. In: Berndt, R. K. u. Bairlein, P. Potei, N. Kempf, S. Pihl and Busche, G. (eds.): Vogelwelt Schleswig- B. Hälterlein for their comments that Holsteins. Band 4: Entenvögel II. Karl improved the manuscript. Finally, we Wachholtz Verlag, Neumünster, pp. 53- would like to thank all the people 73. involved in the fruitful discussions at Beukema, J. J., Essink, K., Michaelis, Fl. & the Roosta Workshop in Estonia and for Zwarts, L. 1993. Year-to-year variability in all their good ideas, inspiration and the biomass of macrobenthic animals on support for this Common Eider status tidal flats of the Wadden Sea: how pre­ review paper. dictable is this food source for birds? Netherlands Journal of Sea Research 31: 319-330. Baltic/Wadden Sea Common Eider 197

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