Status of the Danish Breeding Population of Eiders Somateria
Total Page：16
File Type：pdf, Size：1020Kb
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 Ertholmene - (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- Bornholm) - 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 Nakskov Fjord and in On Saltholm the number of nesting females was southern Lolland (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 Denmark 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 regions of Denmark 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 Kattegat 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 Wadden Sea 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 Odense Fjord, Sydfynske Øhav, Nakskov Fjord 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 als 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 northern Europe since around 1950 - de r o ) annually % (<4 s rate h growt w Lo .