Birds in southern Öresund in relation to the wind farm at ... - Vattenfall

Common Eider Somateria mollissima
The aerial surveys and the boat counts show marked variations in the number of Eiders in the
area (Fig. 20 & 21). Maximum counts were between 7000 and 8000 both for the parts
covered by the boat counts and the southern area covered by aerial surveys. Eiders are
regularly wintering in southern Öresund with the largest concentrations in the waters south of
Falsterbo, where estimated numbers regularly are between 10 000 and 15 000 individuals
(Nilsson unpubl.). Wintering flocks are also found north of Falsterbo, but there the numbers
present are usually lower.
In spring, during the migration period, the distribution of Eiders is markedly different from
the winter distribution with large flocks both in the waters north of Falsterbo and in the
reference area south of Falsterbo (Fig. 22). Large flocks of migrating Eiders are regularly
found resting both in the Öresund proper and in the waters south of Falsterbo during this time
of the year. Moreover, the breeding colony at Saltholm with between 4000 and 5000 pairs in
2000 (Desholm et al. 2002) is situated a short distance to the north of Lillgrund and probably
these birds use the shallow areas also on the Swedish side of Öresund for foraging. Regular
movements of pairs and small groups of Eider between Saltholm and the waters around
Bredgrund and Lillgrund were observed during studies of in the area around the bridge
(Nilsson et al. 2009, 2010).
In May, three flights in 2006, 2008 and 2009 showed that the majority of the Eiders were at
this time in the northern area. In May most of the migration has passed and the majority of the
Eiders counted were probably local birds, mainly from the Saltholm colony. The majority of
those late Eiders were males, as the most of the females were incubating at this time.
There were marked differences in the local distribution of the Eiders in the Lillgrund area
when comparing the pre- and post-construction years (Figs 22, 23, 24). The density of Eiders
was markedly higher in the wind farm area before the construction of the wind farm than
anywhere else in the northern study area, both in spring and in winter (Fig. 25). Postconstruction
the densities in the wind farm area were much lower, while densities in the 2 km
buffer zone instead were higher than during the pre-construction period. The maps (Fig. 21,
22, 23) and the densities (Fig. 24) hence indicate a shift in the local distribution from the wind
farm area to adjacent parts of Lillgrund and its surroundings. Number of Eiders counted in the
aerial surveys showed much variation between years in all zones (Table A10, appendix). In
winter mean numbers varied between 1 and 22 Eiders within the windfarm area. In spring
high counts were obtained in 2006 (before) and 2011 (last year of study) with 296 and 280
Eiders counted, respectively. In the buffer zone mean numbers counted in the survey lines
varied between 26 and 327 in winter and between 391 and 1118 in spring.
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