Status of the Danish Breeding Population of Eiders Somateria Mollissima 2010
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Status of the Danish breeding population of Eiders Somateria mollissima 2010 Thomas Kjær ChrisTensen & Thomas BregnBalle (Med et dansk resumé: Status over den danske ynglebestand af Ederfugl Somateria mollissima 2010) Introduction found in open-water areas, where the majority of The Danish breeding population of Eiders Somate- the Danish breeding population was found during ria mollissima has increased considerably during the the previous decades (see Lyngs 2000, 2008). 20th century. Around 1935 there was an estimated During the 1990s the total Baltic/Wadden Sea 1500 nesting females (Spärck 1936), whereas na- flyway population of Eiders has shown a marked tionwide surveys around 1990 and 2000 revealed decline of approx. 35% when assessed from surveys totals of c. 25000 breeding pairs (Lyngs 2000, 2008). of the wintering population (Desholm et al. 2002). As also evident in the Baltic (cf. Desholm et al. However, apparently the total breeding numbers 2002), the increase in the Danish Eider population throughout this flyway remained unchanged dur- was most marked during the 1970s, with national ing the same period, which led Desholm et al. (op. estimates of 7500 around 1970 (Joensen 1973) and cit.) to suggest, as one among other hypotheses, 19000–20000 around 1980 (Franzmann 1989). Du- that the breeding population upheld its numbers ring the 1990s, the total number of breeding Eiders owing to supplementary recruitment from a hith- in Denmark remained stable, whereas changes in erto non-breeding population segment, which as colony distribution and dynamics occurred, with a special case could also include earlier first-time relatively high growth rates observed in fjords and breeding. However, in a continuously declining sheltered, shallow water areas, while declines were population, depletion of a non-breeding segment Dansk Orn. Foren. Tidsskr. 105 (2011): 195-205 196 The Danish population of Eiders would eventually lead to declines in breeding num- Male counts were applied at several sites with bers. Such declines in breeding numbers seem to the aim of estimating breeding numbers from the have occurred recently in both Sweden (M. Gren number of (paired) adult males observed attending unplub. data) and Finland (Ekroos et al. in press). As breeding colonies during the prebreeding or early a segment of the Baltic/Wadden Sea flyway popula- incubation period. Male counts allowed collection tion, the Danish Eider population would potentially of data from sites sensitive to disturbance from be subject to a similar developmental pattern fol- nest counting activity, and from sites where access lowing the stable situation. permits could not be obtained, e.g., some privately The present paper presents a status of the Dan- owned islands. Male counts were made from small ish breeding population in 2010 and discusses the boats or from adjacent mainland coasts and includ- most recent changes in distribution and breeding ed all adult males present within a 200-300 m zone numbers at selected sites. It also addresses the na- around known or potential breeding sites (islands, tional status in relation to the recent changes in the islets, peninsulas). Immature males and groups of flyway population in general. males recorded at larger distances from the breed- ing sites were counted separately, and were not in- cluded in the estimates of breeding numbers. Male Material and methods counts were generally performed in April or early A nationwide survey covering (almost all) known in May; in parts of Sydfynske Øhav, Kattegat and and potential breeding sites of Eiders was organised Smålandsfarvandet they were conducted as aerial by the Danish National Environmental Research In- surveys, performed from a twin-engine aircraft fly- stitute, Aarhus University (NERI) in 2010. For practi- ing at an altitude of 250 feet. cal reasons, some sites were surveyed before 2010. A total of 223 sites (covering 229 single localities) The two important breeding sites Stavns Fjord at were surveyed for breeding Eiders (2 in 2007, 6 in Samsø, and Saltholm, were covered in 2007 and 2008, 26 in 2009, 188 in 2010, and a single site – Nor- 2008, respectively. For the majority of sites in Syd- dre Rønner (Kattegat) – in 2011). A total of 49 sites fynske Øhav we use data collected during a survey were covered only by male counts, 154 sites were in 2009 (Bisschop-Larsen 2009). The vast majority of covered only by nest counts, while 20 sites were other localities were covered in 2010. The surveys covered both by nest and male counts. Male counts in 2009-2010 involved counts carried out by volun- were performed between 13 April and 16 May, with tary ornithologists, the Danish Hunters Association the majority (n = 47) carried out between 13 April (Christensen 2010) and personnel from the Nation- and 3 May (Fig. 2). Nest counts covered a longer time al Forest and Nature Agency (see Acknowledge- period than male counts, ranging from 24 April to 22 ments), who all reported their results back to NERI. June. Some sites do not have a count date, as they For a few additional sites, supplementary data from were covered several times during April and May, 2009-2010 were extracted from the observation while a few counts were reported without dating. database, DOFbasen, hosted by BirdLife Denmark. The present survey included 64 localities that had In the following, 2010 refers to the present survey not been covered by the surveys in 1990 and 2000, covering the period 2007-2010, while 1990 refers to whereas 13 previously surveyed sites were not cov- the survey carried out during 1988-1993, and 2000 ered. refers to the 2000-2002 survey. In calculating the number of breeding Eiders at The number of breeding Eider females was es- a given site, we used the most recent and robust timated for each site based on either nest counts counts, with nest counts taking precedence over or male counts. Nest counts were performed by male counts. At sites covered by nest counts more visiting breeding sites during the breeding season, than once in the same year, we used data collected and included recordings of the number of incubat- in May, preferentially in early May. ing females and nests that were either hatched or To calculate nest numbers from male counts, we deserted. Field personnel were asked to describe used a correction factor to adjust for the expected site coverage and to make on-site assessments of presence of non-paired adult males around the the minimum and maximum number of breeding breeding colonies. This factor was obtained from Eiders in the total area. Notes should likewise be analysing the relationship between male counts made on the presence/absence of predators, e.g., and nest counts carried out separately at 20 sites in fox, martens and mink, which could influence the 2010. These data showed a highly significant rela- number of breeding birds. tionship between numbers of adult males and nests The Danish population of Eiders 197 Fig. 2. The temporal distribution of male and nest counts. The male counts include the counts at 20 sites also cov- Fig. 1. Relationship between nest numbers and numbers ered by nest counts. of adult males recorded at separate counts at 20 breeding Den tidsmæssige fordeling af han-tællinger og rede-tællin- sites in 2010. ger. Han-tællingerne omfatter 20 lokaliteter, hvor der også Forholdet mellem antallet af adulte ederfuglehanner og blev optalt reder. antallet af reder registreret ved separate tællinger på 20 ynglelokaliteter i 2010. 2 (r = 0.931, F18,1 = 242, P < 0.0001), revealing an aver- survey – with the inclusion of male counts – could age of 0.68 nest per recorded male (95% confidence lead to an overestimation of the actual population. interval 0.59-0.77; Fig. 1). Consequently, the correction was applied solely to At a few sites the number of breeding Eiders was the numbers obtained from nest counts, using the assessed from observations of females accompany- conservative factor of 1.15. ing young. In each case, it was assessed whether the group could come from other colonies in the region, and hence not represent local breeding birds. For Results some observations reported to DOFbasen it could Calculated from the maximum breeding numbers not be excluded that the birds originated from assessed by observers, the total number of breeding other known breeding sites in the region, and such Eider pairs recorded during the 2010 survey was observations were not included in the population 22185 (18589 nest recordings and 3596 [95% c.i. estimate. 3110-4082] nests estimated from male counts). The In general nest counts were performed as total corresponding total based on the minimum num- counts, but for a few sites the total number of nests ber of directly observed nests was 21561. Nests re- was obtained by correcting counted numbers ac- cordings constituted c. 83% of the total, while the cording to the observer’s notes on site coverage. remaining 17% were based on male counts. More than half (52%) of the nest counts were per- The result of the 2010 survey is similar to the esti- formed during the optimal time period between 1 mates from 1990 and 2000 (Table 1), indicating that and 15 May, and 78% were counted at some time the Eider population has been stable during the during this month (Fig. 2). On the 13.5 km2 large past two decades. In accordance with this stability, island of Saltholm (Øresund), nest counts were per- the calculated growth rate of the population is very formed as special transect counts: a total of seven small (-0.3% for 1990-2000, -0.2% for 2000-2010; see evenly spaced east-west oriented transects (45 m Table 1). During the 2010 survey, breeding Eiders wide) was fully surveyed, covering 5.4% of the to- were recorded at 188 sites, of which 45 had no previ- tal area, and total nest numbers were obtained by ous record of the species.