Status of the Danish breeding population of Eiders Somateria mollissima 2000-2002 Peter Lyngs (Med et dansk resumé: Status over den danske ynglebestand af Ederfugl Somateria mollissima 2000– 2002) Abstract A survey of the Danish population of Eider Somateria mollissima in 2000-2002 showed that numbers remained virtually unchanged compared with a similar survey during 1988-1993, at about 25 000 nesting females. Introduction Danish Eider population probably reflects a decline The Danish breeding population of Eider Soma- in the entire Baltic/Wadden Sea population since teria mollissima increased considerably during the mid 1990s (see Desholm et al. (2002), Kats the 20th century. Nationwide surveys suggest that (2007), and references therein). the population numbered some 1500 nesting fe- males around 1935 (Spärck 1936), 3500 around Material and methods 1960 (Paludan 1962), 7500 around 1970 (Joensen The Danish National Environmental Research 1973), 19000–20 000 around 1980 (Franzmann Institute organised a nationwide survey of nest- 1989) and 25 000 around 1990 (Lyngs 2000). ing Eiders during 2000–2002. The survey in- This paper presents a status of the breeding volved numerous local ornithologists and several population around 2000 and concludes that the govermental bodies (see Acknowledgements). population then numbered at least 25 000 nest- Most breeding sites were visited once during ing females, i.e. virtually the same number as in early May, where the number of nests/nesting 1990. Major differences between regions were females were counted. In Vadehavet (the Wad- evident in the population trends between 1990 and den Sea), however, adult males near the breeding 2000. In addition the general development of the sites were counted in late April – early May, and Dansk Orn. Foren. Tidsskr. 102 (2008): 289-297 290 Population status of Eiders 100 10 2 Fig. 1. Distribution of breeding Eiders in Denmark, 2000-2002. Ederfuglens yngleudbredelse i Danmark 2000-2002. the number of breeding females guesstimated by included hatched and deserted nests. If counts multiplying male numbers by 0.667 (assuming a from several years were available from a site, the male/female ratio of 60/40; Noer et al. 1995). On most detailed count close to 2000 was chosen. Ertholmene (Bornholm) and in parts of Sydfyn- In the following, 1990 refers to the survey car- ske Øhav, two counts of nests were undertaken, ried out during 1988–1993, while 2000 refers to one in late April and one in late May. On Saltholm the 2000–2002 survey. the number of nesting females was calculated from counts of nests done during early May 2000 Results in seven transects covering 5.4% of the colony The sites that had breeding Eiders in 1990 and/ (Noer & Christensen 1993, Christensen & Noer or 2000 are listed in Appendix 1, while the geo- 2001); the counts were timed to take place when graphical distribution of breeding sites in 2000 the numbers of incubating females were at their is shown in Fig. 1. According to Appendix 1, the seasonal maximum. At all sites, counts of nests 2000 survey produced a sum of 22 087–23113 Population status of Eiders 291 Table 1. Number of breeding Eiders counted in different regions of Denmark, 1988-1993 and 2000-2002, and the corresponding annual growth rate (r). Only the highest figures for both periods are presented. Antal optalte ynglende Ederfugle i forskellige områder af Danmark 1988-1993 og 2000-2002 sammen med den tilsvarende årlige tilvækst (r). Kun de højeste tællinger per periode er vist. Area 1988-1993 2000-2002 Years r (%) Område Antal år Vadehavet ≥457 644 10 3.5 Limfjorden 2 25 11 25.8 Kattegat 1274 1098 12 -1.2 N Sjælland 925 1615 9 6.4 Østjylland 4412 2220 11 -6.1 N Fyn 1588 3266 11 6.8 Lillebælt 244 466 9 7.5 Sydfynske Øhav 113 1690 11 27.9 Storebælt 2321 1936 10 -1.8 Smålandshavet 1092 931 11 -1.4 Nakskov Fjord 118 600 12 14.5 S Lolland 487 774 9 5.3 Øresund 7160 4770 10 -3.9 Bornholm 3000 2503 8 -2.2 Total 23193 22538 10 -0,3 used nests, suggesting an annual growth rate of Saltholm in Øresund (4300 females), Ertholmene -0.3% between 1990 and 2000 (Table 1). In 2000, off Bornholm (2400 females), Mejlø-Enø at Hinds- breeding Eiders were recorded at 42 sites where holm, Nordfyn (1000 females), and Rønø in Ise- none had been recorded during the 1990 survey. fjorden, Nordsjælland (1000 females). Breeding numbers at these new sites were 1351– 1357, corresponding to 6% of all nests counted in Discussion 2000. Breeding Eiders were not recorded at nine The 2000-survey covered almost all breeding sites where a total of 142 nesting females were sites for Eider in Denmark. Based on information counted in 1990. from local observers, the number of breeding Ei- The highest average annual growth rates be- ders on sites not covered during the survey can at tween 1990 and 2000 were recorded in Sydfynske most have been a few hundred. Øhav and in Nakskov Fjord (Table 2), while the Due to the difficulties of censusing breeding Ei- largest declines occurred in Øresund (Saltholm) ders (see Lyngs 2000), the methods used at most and in Østjylland (Stavns Fjord and Hov Røn). sites in the countrywide Danish surveys since The biggest colonies in Denmark in 2000 were 1935 have underestimated the true size of the Table 2. Population size (breeding females) and annual growth rates of the Danish breeding population of Eiders, 1935-2000. Bestandsstørrelse (ynglende hunner) og den årlige procentuelle vækst i den danske bestand af ynglende Ederfugle, 1935-2000. Year Population size Annual growth rate (%) References År Bestandsstørrelse Årlig tilvækst (%) 1935 1200-1500 Spärck 1936, Joensen 1973 1960 3000-3500 3.5-3.7 Paludan 1962, Joensen 1973 1970 7500 7.9-9.6 Joensen 1973 1980 19000-20000 9.7-10.3 Franzmann 1989 1990 23000-25000 2.3-2.8 Lyngs 2000 2000 23000-25000 -0.3 This study 292 Population status of Eiders breeding population. However, the surveys reflect outbreak of avian cholera in Isefjord (Nordsjæl- the general trends in the development of the Eider land) in 2001 did not affect the numbers recorded population (see discussion in Lyngs l.c.). Incidents during the 2000-survey, as the counts of breeding of extensive non-breeding among Eiders in the birds from this area derive from the year 2000. Netherlands during periods of low food availabil- Red foxes were recorded at 11 sites during the ity have recently been described (Kats 2007; see 2000 survey (Appendix 1) and had a strong influ- also Coulson 1984). If widespread non-breeding ence on the local Eider population. On Rågø in occurred in the Danish population during the sur- Smålandshavet almost 1000 Eider females bred in vey years, the nest counts will further have un- 1990, but only about 200 in 2000; at least some derestimated the population size. However, except of the birds appear to have moved to Vensholm for Saltholm (Noer & Christensen 1994) no data (Nakskov Fjord; 16 km WSW) where the popula- exist to elucidate the proportion of non-breeders tion increased from about 60 females in 1988 to in the Danish Eider population. almost 400 in 2000. Other Rågø birds may have Correcting the counts from 1980, 1990 and moved to Suderø (Smålandshavet; 25 km ESE), 2000 by factors of 1.15–1.30, as suggested by where the population increased from 35 females in Lyngs (2000), gives likely ranges of the Danish 1988 to about 340 in 2000. Similar displacements Eider population of 21000–23300 nests in 1980, related to the presence of foxes were observed at 25800–28 800 in 1990, and 25300–28 400 in Æbelø & Holmene (Nordfyn), where 850 females 2000. In conclusion, the Danish Eider population bred in 1990 but only 40 in 2000. Many of these remained practically stable between 1990 and birds apparently moved to Dræet, where no birds 2000, for the first time after 65 years of continued bred in 1990 but 550 in 2000. growth (Table 2). Eiders have been considered as the epitome of In many ways, the population trends observed "site fidelity" (e.g. Baillie & Milne 1989, Swennen in the 1980s continued during the 1990s: high 1990). The present data, however, suggest that a growth rates were found at breeding sites in fiords high degree of site fidelity is maintained only as and sheltered, shallow waters, whereas declines conditions permit. were the rule at sites in the open-water areas Apart from the rather obvious influences of where, during the 1960s and 1970s, a majority disease and foxes, little is known about the fac- of the Danish Eiders bred. Annual growth rates tors underlying the observed population changes exceeding 5% were noted at Sydfynske Øhav, in Denmark. It is, however, clear that the Dan- Nordfyn, Nakskov Fjord, Roskilde Fjord, Isefjord ish breeding population of Eiders is in a state of and Lillebælt, while the Eider population in Øre- change, and more knowledge is needed to evalu- sund, Østjylland and Bornholm declined (Table ate the significance and the reasons behind these 1, Appendix 1). The increasing importance of the changes. sheltered areas and fiords for the Danish Eiders are nicely illustrated by the growth around Fyn Acknowledgements (mainly Det Sydfynske Øhav, Odense Fjord and For their kind collaboration and collecting of data Hindsholm) – approx. 2% of the breeding popu- thanks are due to the National Forest and Nature lation were found here in the 1970s, 8% in the Agency, the National Environmental Research Insti- tute (University of Aarhus), the Danish Ornithological 1980s, and 24% in the 1990s. The proportion of Society, Christiansø Biological Fieldstation, Fyns Amt, the population breeding at the old, big colonies of Storstrøms Amt; and to Annelise Pedersen, Bent Stau- Ertholmene, Saltholm, Stavns Fjord and Mandø gaard Nielsen, Christian Thomsen, Finn Jensen, Frank decreased from 70% in 1980 to 35% in 2000.