s Eider f o n populatio g breedin h Danis e th f o s Statu Somateria mollissima 1988-93

PETER LYNGS

(Med et dansk resumé: Status over den danske ynglebestand af Ederfugl Somateria mollissima 1988-93)

Introduction calculated from counts of nests made during early The Danish breeding population of Eiders Soma- May 1993 in seven transects covering 5.4% of the teria mollissima has been increasing since the start colony (Noer & Christensen 1993); the counts of the 20th century. Nationwide surveys suggest were timed so that the maximum number of nests that the population numbered 1500 nesting fe- in use were recorded. A detailed survey, including males around 1935 (Spärck 1936), 3500 around mapping of nests and trapping of practically all 1960 (Paludan 1962), 7500 around 1970 (Joensen nesting females, was carried out at - (Franz 0 198 d aroun 0 19000-2000 d an ) 1973 during April-June 1992 (Lyngs 1993). At all sites, mann 1989). counts of nests included empty (hatched or desert- This paper presents a status of the breeding ed) nests. If counts from several years were population around 1990 and concludes that the available from a site, the most detailed count population at this time numbered at least 25000 among those closest to 1990 was chosen. nesting females. Thus the growth continued during In the following, 1980 refers to the survey carri- the 1980s, although at a slower rate than in the ed out during 1980-83, while 1990 refers to the preceding two decades. . survey 3 1988-9

Material and methods Results A nationwide survey of nesting Eiders was carried A list of known breeding sites 1980 and 1990 are out during 1988-93. Data from wildlife and nature given in Appendix 1; see also Fig. 1 and Table 1- reserves were received from the National Forest 2. The few sites where no information was avail- and Nature Agency, the Wildlife Reserve Bureau able assumingly held only small numbers of breed- and the National Environmental Research Insti- ing Eiders. According to Appendix 1 the 1990 tute. Local ornithologists and the Danish Ornitho- survey produced a sum of 23 094-23 193 nesting logical Society supplied data for all other parts of females at 135-150 sites, implying an annual the country. growth rate of 2.3% between 1980 and 1990 (Table Generally, most breeding sites - including 1). A total of 57-61 new breeding sites holding Stavns Fjord (Samsø), but excluding the two 1553-1637 nesting females (7% of the total popu- largese t colonieswhil , , period Salthol e th g m (Øresunddurin d ) anestablishe d e Ertholwer ) - lation mene (Græsholm, Christiansø and Frederiksø, only 4 sites (holding 9 nesting females) were aban- ) - were visited once during early May doned. The highest growth rates occurred around and the number of nests/nesting females counted. Fyn, in northern Sjælland, in Fjord and in On the number of nesting females was southern (Table 1, Fig. 2). Dansk Orn. Foren. Tidsskr. 94 (2000): 12-18 3 1 Eiders of population Danish The

Fig. 1. Distribution of breeding Eiders in 1988- Fig. 2. Annual growth (%) 1980-90 of the breeding Eider . Denmark f o s region s variou n i n populatio . 93 yngleudbredelse i Danmark 1988-93. Procentvis årlig tilvækst i antallet af ynglende Ederfugle ynglende af antallet i tilvækst årlig Procentvis 1988-93. Danmark i yngleudbredelse Ederfuglens Danmark. af områder forskellige i

Discussio. O . N . n 1993) s (Lyng s area n bush-grow y densel Censusing Eiders is difficult. Investigations on Preuss (in litt.) found that as little as 50% of all Ertholmenn i t e havcoun e e showsingl a ng that durin du d e to a sprearecorde e b dy of ma 2 -s nest 2.5 months in laying dates a maximum of 75-80% May in the colony at Rågø in Smålandshavet. t coun e singl a t a d recorde e b d woul s nest e th f o The methods used at most sites during the s nest l al f o % 5 e som , Furthermore . May 1 d aroun Danish countrywide surveys obviously have n i y especiall , visit e singl y an t a d overlooke e b y ma produced underestimates, although more informa-

Table 1. Numbers of breeding Eiders counted in different 1980-83 and 1988-93 and the corre- sponding annual growth rate. Only the highest figures for both periods are presented. Antal optalte ynglende Ederfugle i forskellige områder af Danmark 1980-83 og 1988-93 sananen med den tilsvaren- de bestandsvækst. Kun de højeste tællinger per periode er vist.

Area 1980-83 1988-93 Years Annual growth (%) Område Antal år Årlig tilvækst (%)

Vadehavet 611 457 9 -3.2 Limfjorden 0 2 10 675 1274 9 7.3 N Sjælland 200 925 9 18.6 Østjylland 4597 4412 10 -0.4 N Fyn 266 1588 9 22.0 Lillebælt 34 244 10 21.8 Sydfynske Øhav 3 113 10 43.8 Storebælt 1724 2321 9 3.4 Smålandshavet 802 1092 9 3.5 d Fjor v Naksko 0 118 8 S Lolland 140 487 11 12.0 Øresund 6906 7160 13 0.3 Bornholm 2600 3000 12 1.2 Total 18558 23193 10 2.3 Eiders of population Danish The 4 1

Table 2. Population size (breeding females) and annual growth rates of the Danish breeding population of Eiders 1935- 90. 1935-90. Ederfugle ynglende af bestand danske den hos tilvækstrater og hunner) (ynglende Bestandsstørrelse

Yeae r siz n Populatio Annual growth rate (%) References Ar Bestandsstørrelse(%) tilvækst Årlig

1935 1200-1500 3 197 n Joense , 1936 k Spärc 1960 3000-3500 3.5 -3 3.7197 n Joense , 1962 n Paluda 1970 7500 6 9. - 9 7. Joensen 1973 1980 19000-20000 9.7 - 10.3 Franzmann 1989 1990 25000 2.3 - 2.8 This study

tion ism needefro e d beforestimat ew thlo ey accuracprobabl yd anan de reliabilitcrud a y yb od f cause - s term e quantitativ n i d assesse e b n ca s count e th , Rasmussen a Malth . (L ø Mand t a y colon n mai e th s method d an g timin , efficiency r observe n o . e.g pers. comm.); the population general- used to census each breeding site, and the rate of ly remained stable during the 1980s (Fleet et al. g relayin d an ) 1984 n Coulso e (se g non-breedin e th g durin h growt t ne e th f o d thir e On . 1994) e th d correcte ) (1989 n Franzman . predation r afte 1980s occurred in Storebælt, Smålandshavet, Øre- 1980e figurewer s s brate yh a factogrowt e r th 1.06-1.0 t bu , 8 to obtaiErtholmene nn ao totad an ld sun of 19000-2000n O . decade 0 g nesting precedin females e th n i n . Atha r correspondlowe s time - 0 4-5 ing adjustmenw ne y tman os fa th s e 199time 0e totathre lt producealmos , s a hand figur r eothe e th - Thor . females g nestin ) (24500-25000 0 2500 f o e th g durin s a d establishe e wer s site g breedin ough counts on Ertholmene in 1980 and 1990, 1970s; most new sites were in sheltered areas such f o r facto n correctio a t tha t sugges , however as Fjord, Sydfynske Øhav, o t g tryin n whe e appropriat e mor s i 0 1.15-1.3 e th f o d thir e on r fo t accoun h whic d Sjællan N d an g Makin . population g breedin e th f o e siz e th s asses o t d localise g bein r remainde e th , growth l tota such a correction (except for the already corrected Lillebælt, N Fyn (Æbelø/Mågeøerne), the area be- e rang e possibl a s yield ) Ertholmene m fro s figure S d an , Kattegat , Jylland E d an ø Sams n twee - 300 0 2 f o n populatio r Eide h Danis l tota e th r fo y virtuall d ha s region r latte e th f o t Mos . Lolland 23 300 nesting females in 1980 and 26100-29 200 stable growth rates throughout the 1970s and in 1990. 1980s. s Eider g breedin f o n populatio h Danis e Th y primar e th t tha d argue ) (1989 n Franzman t bu , 1990 d an 0 198 n betwee e increas o t d continue cause of the high growth rates observed in the g precedin e th n i n tha r lowe s wa e rat h growt e th n eutrophicatio d increase n a s wa s 1970 d an s 1960 0 1960-198 n i y annuall % (8-10 s year y twent g increasin n a o t g leadin , waters h Danis e th f o - develop e Th . 2) e Tabl ; 1980-1990 n i % 2-3 s versu production of phytoplankton and, in turn, of the s way r othe n i d differe o n populatio e th f o t men s duckling r thei d an s Eider r fo d foo l principa % 65 n tha e mor s 1970 e th n I . periods e th n betwee (mussels, small snails and crustaceans). This t larges e thre e th n i d occurre h growt l tota e th f o explanation fitted well with the observed popula- ) Ertholmene d an m Salthol , Fjord s (Stavn s colonie tion development in Denmark, and has also been with the highest growth rates outside these colo- generally accepted as the main reason behind the - Store n souther n i g occurrin ) annually % (>20 s nie widespread population increase of Eiders observed bælt and in Smålandsfarvandet (SW and S of Sjæl- elsewhere in since around 1950 - de r o ) annually % (<4 s rate h growt w Lo . land) (e.g. Hagemeijer & Blair 1997). Suggested contri- s region x si n i d note e wer s 1980 e th g durin s cline y man f o t establishmen e th e includ s factor g butin s rate h growt h hig d showe e fiv h whic f o ) 1 e (Tabl reserves on islands used by breeding Eiders and a s (Stavn ø Sams n o n populatio e Th . 1970s e th n i better overall protection during the breeding f o e presenc e th o t e du d halve t almos e wer ) Fjord season. e th n i e declin t apparen e Th vulpes. Vulpes s foxe Around 1990 it seems that some Danish regions Wadden Sea, however, is likely to be an artifact, (e.g. Ertholmene, Øresund and Storebælt) were 5 1 Eiders of population Danish The

g (usin s Eider r fo y capacit g carryin r thei g reachin g supplyin r fo s ornithologist l loca s numerou d an y Societ the term in a broad sense), implying that eutrophi- data and for their kind collaboration. I gratefully received cation no longer provided a surplus of food in these additional information and comments from Pelle Ander- h growt n populatio e th 0 198 d aroun o t p U . areas sen-Harild, Steen Flex, Jens Gregersen, Poul Henrik mostly occurred at sites in rather open waters Harritz, Kurt Due Johansen, Leif Bishop Larsen, Hans around old, well-established colonies. The fiords Lind, Lars Maltha Rasmussen, Bent Staugård Nielsen, f o h muc e wher , areas w shallo , sheltered r othe d an Morten Nielsen, Henning Noer, Mogens Ribo Petersen - Tho o t e du e ar s thank l Specia . Thelander l Michae d an d ha y mostl , 1980s e th g durin e plac k too h growt e th o wh n Christense d Overgår s Jen d an e Bregnball s ma few - or no breedinBregnbal s g EiderThoma . sdata untie th f lo recentlyh muc d , althougcollate y h originall r earlie d eutrophicate n bee e hav t mus s area e thes n a d improve y kindl r Kjæ s Thoma d an p Kamp j Ka , le - expla e possibl A . waters n ope e mor n i s area n tha earlier draft of the paper. d sheltere e th t tha s i n occupatio e lat r thei r fo n natio waters in Denmark are rather unstable environ- ments where, for example, periods of ice cover . spring n i s Eider r fo s supplie d foo d reduce n i t resul Annual growth rates exceeding 20-35% must in- Resumé volve immigration (cf. population parameters Status over den danske ynglebestand af Ederfugl presented by Franzmann (1980) and Coulson Somateria mollissima 1988-93 (1984))- . It thulandsdæk n se f appeara e s that immigratioresultatern t kor r n beskrive o l f Eiderartike e s Denn - increas y markedl g showin s water d sheltere e th o t kende optælling af ynglende Ederfugle foretaget i 1988- ing populatione dansk n de s g afteste 0 r 199 198 g o 0 0 did198 nom t occumelle t r untiti-åre I l. 1993 manye open ynglend wate0 r areas26100-2920 l werti 0 e reachin20300-2330 g a fr thei d r bestan . capacity g carryin . året m o % 2 g omkrin å p g stignin n e l ti e svarend , hunner Denne stigningstakt er væsentlig lavere end i de fore- % 8-10 d me g ste d bestan e dansk n de r hvo , år e tyv e gåend om året. I disse år var det især bestanden i de tre store Acknowledgments kolonier (Saltholm, Ertholmene og Stavns Fjord) samt i Thanks are due to the Wildlife Reserve Bureau, the Na- Storebælt og Smålandshavet, der steg. I 1980erne stag- tional Forest and Nature Agency, the National Environ- nerede eller faldt bestanden i disse områder, og to tredje- l Ornithologica h Danis e th , Institute h Researc l menta dele af den samlede vækst fandt sted andre steder. Antal- Eiders of population Danish The 6 1

let af nyetableredm. e ynglelokalitete(Somateria s Ederfuglen r va : r desude1980 . n trN.-E e , gange Franzmann - Nak , Fjord e Odens I . 1970erne i d en e 1980ern i e højer mollissima) ynglebiologi og populationsdynamik på e 1970ern i r de r hvo , Øhav e Sydfynsk t de g o d Fjor v sko Christiansø 1973-77. - Unpubl. ph.d. thesis, Universi- kun ynglede få Ederfugle, steg bestanden med over 20% ty of . - indvan n e r ske r de s hvi t mulig r e n ku t hvilke , året m o Franzmann, N.-E. 1989: Status of the Danish breeding population of the Eider Somateria mollissima 1980- dring fra andre steder. Høje vækstrater fandtes også i for 83, with notes on general population trends in northern e Tilsyneladend . Nordfyn å p g o d Fjor e Roskild l eksempe . 62-67 : 83 . Tidsskr . Foren . Orn k Dans - . Europe - vækstcen t være e havd e tidliger r de , områder e fler e nåed C EBC e Th : 1997 ) (eds r Blai . J . M & . M . J . E , Hagemeijer tre, deres mætningspunkt i 1980eme, mens der stadig var atlas of European breeding birds: their distribution and basis for en vækst i andr. e områderLondon , . DePoyser t. erD bemærkelses. A & . T - . - abundance værdigt, at lavvandede fjordsystemer som Joensen, A. H. 1973: Ederfuglen (Somateria mollissima) Fjord, Naksko- v Fjord, OdensVildtundersøgel e e FjorDansk - d . og det SydfynskDanmark i l e ynglefug m so Øhav først er blevet koloniseret i 1970erne og 1980erne. ser 20: 1-36. Årsage- n hertialkeun g l o e r- ikke klarlagtEderfugle : , me1992 nø øjensynliChristians g: foretræk1993 . P , - Lyngs e botanisk g o r ynglefugletællinge t sam r dersøgelse , vand e dyber d me r område i e yngl t a e Ederfuglen r ke registreringer på Græsholmen. - Skov- og Natursty- e ikk r de , fødetilgang l stabi e mer n e r ha e diss i ford e måsk relsen, Miljøministeriet. . isvintre f a g påvirkeli r e d gra j hø å s i - investiga e Base-lin : 1993 n Christense . K . T & . H r Noe . 1993 e May-Jun , Øresund n i s Eider g breedin f o s tion , Øresundskonsortiet o t d submitte t Repor I NER - September 1993. Reference- . s farvande e dansk e d i e Ederfuglen : 1962 . K , Paludan e th f o s dynamic n populatio e Th : 1984 . C . J , Coulson Danske Vildtundersøgelser 10:1-87. f o e evidenc d an mollissima Somateria k Duc r Eide Spärck, R. 1936: Om antallet af ynglende Ederfugle i : 126 s Ibi - . ducks t adul y b g non-breedin e extensiv Danmark til belysning af reservaternes betydning for 525-543. bestandens størrelse. - Dansk Orn. Foren. Tidsskr. 30: Fleet, D. M., J. Frikke, P. Sudbeck & R. L. Vogel 1994: a Se n Wadde - . 1991 a Se n Wadde e th n i s bird g Breedin 20-22. Ecosystem No. 1, Common Wadden Sea Secretariat.

Appendix 1 Breeding sites for Eiders in Denmark 1980-83 f o r numbe l tota e th d an 3 1988-9 d an ) 1989 n (Franzman . females g nestin Ynglesteder for Ederfugl i Danmark 1980-83 (Franzmann 1989) og 1988-93 med angivelse af det tota- le antal rugende hunner,

? No information Ingen information Probably not breeding Ynglede sandsynligvis ikke + Known to breed, but only few pairs Ynglede, men Limfjorden kun få par Nørskov Vig, Venø 1980 - 1990 2 Vadehavet

& m Ballu , Højer Kattegat 5 1 1 199 0 15-2 0 198 d Forlan e Rib 3 2 1 199 0 35-5 0 198 ø Røm 1983 87 1989 126 1981 2 1991 Deget 1981 + 1988 ? Mandø 1981-83 400-500 1991 300 Nordre Rønner 1980 220 1989 533 Fanø 1982 5-6 1991 2-3 Læsø 1980 200 1987-90 360 Keldsan5 d 15-2 8 198 198 0 3 50-6 25-3 0 0 198 1991 10 0t Anhol 1982 2 1991 15 1980 6-8 1990 15-30 Skallingen 1980 1 1991 Hesselø 1980 50-100 1990 200 Total 485-611 456-457 Total 613-675 1249-1274 7 1 Eiders of population Danish The

N Sjælland Lillebælt

Isefjord & Roskilde Fjord 1980 24 0 19811 8 Bågø 1980 2 1988 2 Øksneholm 1980-82 - 1991 15 Bastholm 1980 - 1991 59 Rønnen 1980-82 - 1991 2 Småholme 1980 2 1991 19 Marsvineholme 1980-82 - 1991 6 Årø 1980 5 1990 6 Rønø 1980-82 - 1991 850 Linderum 1980 1 1990 43 Illumø 1980 - 1990 3-4 & Langø 1982 150-200 1991 7 Horsehoved 1980 - 1 1990- 1 Eskilsholm 1980-82 - 1991 45 Total 34 242-244 Sejerøbugten Sejerø 1980 + 199v 0 Øha e + Sydfynsk Nekselø 1980 + 1990 + Mejlholm 1980 - 1989 1 Total 150-200 925 Græsholm v. Drejø 1980 - 1992 1 Hjelmshoved 1980 - 1992 2 Odden 1980 - 1989 1 E Jylland & Samsø Buddiken 1980 - 1989 2 1980 + 1988 1 Helgenæs 1980 - 3 1991- 0 Nyland 1980 - 1988 3 Tved& m Eghol e Stor 1980 - 1990 3 0 198 d Øhove t Dejre 15 1990 m ?Eghol e Lill 1980 - 1989 4 Begtrup Røn 1980 30 0 19910 0 Halmø 1980 - 1990 1 Stavns Fjord, Samsø 1980 3000 1990m 1575Græshol e Lill 1980 - 1988 15 Tunø 1980 ? 1991 2 Bredholm 1980 - 1991 46 Hov Røn 1981 828 1990 1800 Grensholm 1980 - 1991 16 Søby Rev 1981-83 350-375 1990 9 Bondeholm 1980 - 1989 1 Hjarnø 1981 23 1991 50 Vogterholm 1980 - 1989 2 3 1980-8 e Poldern & ø Alr 150-190 7 19954 1 StrynøKalv 1980 - 1991 2 Møllegrunden 1980 ? 1990 5-10 Storeholm 1980 3 1989 11 1981 11 1990 13 Monnet 1980 - 1990 0-1 Svanegrunden 1980-83 75-125 3 19930 0 Total 3 112-113 Total 4482-4597 4405-4412 Storebælt

Romsø 1980 - 1990 N Fyn Musholm&Nordholml980 12 1994 60 Sprogø 1980 1 1995 8 Mågeøerne 1980 69 1990 206 Lejodden 1980 - 1992 10 e Holmen & ø Æbel 1980 42-88 1990 850 Vresen 1980 300 1991 326 Tornen 1980 2 1989 15 Smørstakken 1980 - 1990 10 Mejlø/Enø 1980 63 1990 250 Egholm 1983 200 1990 500 Ægø 1981 5 1989 1 Kidholm 1980 - 1991 2 Bogø 1980 18 1990 100 Skælskør Yderfjord 1980 - 1990 6 Vejlø 1981 10 1989 5 Mindeshoved 1980 - 1990 8 VejløKalv 1981 5 1989 5 Agersø 1983 630 1990 620 Flintholm 1980 3 1989 7 ø Om 1980 50 1990 1-5 Roholm 1980 - 1989 4 Halten, Sylten, Vigelsø 1980 - 1991, Vejle 8 , 5 Stigsnæs Dørholm 1980 1-20 198 199 ø 1 Seved 29& t Drage 40 1990 26 Kyholm 1980 - 1989 11 SevedøFed 1983 25 1990 20 Mågeø & Sorteø 1980 - 1989 4 Næbbet, Stenfed Esbechholme 1980 - 1989 10 & Basnæs Nor 1983 360 1990 >525 Pludderholm 1980 - 1988 1 Glænø Vesterfed 1980 58 1990 20 Stenøerne 1980 2 1989 3 Sandholm 1983 35 1990 130 Trindelen 1980 3 -198 198 d 8 Østerfe 2ø Glæn 13 1990 45 Total 219-266 1588 Total 1724 >2321 18 The Danish population of Eiders

Smålandshavet Øresund

Knudshoved 1980 1988 2 Ægholm 1980 30 1989 120 Suderø 1980 ? 1988 35 1981 15-40 1990 8 Enø Overdrev 1980 1988 1 Tyreholm 1980 1992 49 Vigsø 1980 1990 66 5 4- 1980 5 1984- 9 Femø 1980 1988 + Ølsemagle Revle 1980 ? 1990 Avernakke Hage Aflandshage 1980 5 1990 &Fejø 1980 1 1988 10 Det Inddæmmede 1980 25 1990 1 Skalø 1980 ? 1988 3 Dragør Sydstrand 1980 1 1990 Rågø Kalv Saltholm 1981 6800 1993 6977 & Rågø Sand 0 70 1981 3 19997 0 Total 6880-6906 7159-7160 Vejrø 1980 51-100 1990 + Onsevig Sand 1980 1988 2 Total 752-802 1092 Bornholm

Ertholmene: Nakskov Fjord Græsholm 1982 600 0 19942 2 Christiansø Vensholm 1980 1988 58 & Frederiksø 1980 2000 1992 2580 Albuen 1980 0 1 1988 Coasts of Bornholm 1980 ? 1992 + Enehøje 1980 1988 5 Total 2600 3000 Vejlø 1980 1988 1 Rommerholm 1980 1988 9 Slotø 1980 1988 2 Munkeholm 1980 1988 8 Dueholm 1980 1990 21 Smedeholm 1980 1990 3 Kåreholm 1980 1988 1 Total 0 118

S Lolland

Hyllekrog 1980 40-100 0 19815 8 Drummeholm 1980 1991 1 Storeager 1980 18 1991105-130 Lilleager 1980 1991 57-60 Hylleholm 1980 5-10 1991 9-15 Tjørneholm 1980 ? 1991 56-70 Kalveholm 1980 1 1991 7-15 9 199 r Novembe 8 1 d Accepte Store Skåne 1980 1 1991 12 Lindholm 1980 1991 1-3 Rødsand 1980 10 1991 30 Peter Lyngs Kalvø 1980 1988 1 National Environmental Research Institute, Field Station Total 75-140 429-487 Christiansø, DK-3740 Svaneke, Denmark