Common Eiders Somateria mollissima in Shetland are believed to be essentially resident within the archipelago, and may be closer to S. m. faeroensis than to S. m. mollissima, the subspecies found in mainland Scotland. Surveys during the late summer moulting period indicated the population in Shetland declined from an estimated 15,500 birds in 1977 (subsequently revised to 17,000) to c. 6,000 by 1997. Further surveys in August 2009 and August 2012 located 5,782 and 4,627 birds, respectively. The 2009 count suggested little change in numbers since the late 1990s, whilst the 20% difference between 2009 and 2012 is believed to represent a genuine decrease rather than any artefact of survey coverage or accuracy. The distribution of moulting Eiders changed fundamentally during the 2000s, from traditional sites on the exposed, outer coastline to the vicinity of shellfish and finfish aquaculture sites on the sheltered, inner coastline; by 2009-12 approximately two-thirds of the moulting population was associating with aquaculture sites. The reason for the recent decrease in the population is unknown, but possible contributory factors discussed include mortality from oil pollution, deterrence measures taken at aquaculture sites, and predation by marine mammals, especially by Killer Whales Orcinus orca.

The Common Eider Somateria mollissima is an abundant coastal-breeding seaduck with a holarctic distribution. In the Western Palearctic, its breeding range extends from Franz Josef Land and Spitzbergen south through Scandinavia and the Baltic to The Netherlands and Brittany, and through the northern parts of the British Isles northwest to the Faroes and Iceland. While some populations are migratory, those breeding in the British Isles, the Faroes and Iceland are essentially resident, making local, seasonal movements only (Cramp & Simmons 1977). There is considerable geographic variation in body size and plumage, and in the northeast Atlantic del Hoyo et al. (1992) recognised S. m. mollissima in the British Isles, mainland Europe and the Russian Arctic; S. m. faeroensis in the Faroes; and S. m. borealis in Iceland and northern Norway. Cramp & Simmons (1977), however, noted that those in Orkney and Shetland may be closer to faeroensis than mollissima. A recent comparison of biometrics, DNA and plumage features suggested that Shetland birds would be better classified as faeroensis (Furness et al. 2010). Although differences between faeroensis and mollissima may be clinal, and while faeroensis has yet to be accepted onto the British List, Common Eiders in Shetland are now treated separately from those elsewhere in the UK in some status reports (e.g. Holt et al. 2012).

Large numbers of Common Eiders (hereafter 'Eiders') had been killed by oil pollution incidents in the North Sea and the Baltic Sea in the late 1960s and early 1970s (Greenwood & Keddie 1968; Swennen & Spaans 1970; Joensen 1972; Joensen & Joensen 1977). With this in mind, and with a decision having been taken in 1973 to site a major oil-exporting terminal in the north mainland of Shetland, the Nature Conservancy Council began counts of Eiders in Shetland in the mid 1970s. The surveys focussed on late summer when flocks of moulting, flightless birds were known to gather in particular areas, typically at headlands or small islands remote from human disturbance, where birds could feed, roost ashore, and quickly find shelter from varying wind directions. Adult males began to gather at these locations in early June, were joined by females by August, and numbers declined through September, presumably as birds regained flight and dispersed (Jones & Kinnear 1979). It was noted that the sex ratio varied between locations, and that females and juveniles were usually grouped together rather than being scattered among males. A total of 13,800 birds was counted at 24 sites in August 1977, with an estimated total of 15,500 birds being present in Shetland (Jones & Kinnear 1979; but see below). Moult flocks were found in the same general areas each summer, but the number of Eiders using any particular area could vary considerably from year to year. Thus, when the Shetland Oil Terminal Environmental Advisory Group (SOTEAG) began a programme of seabird monitoring in 1978, periodic surveys of all known moulting locations in a single year were used to derive population estimates.

These surveys recorded a decline through the 1980s and 1990s, from a revised estimate of 17,000 birds in 1977 (based on numbers recorded regularly in the 1980s at sites not covered in 1977) to 6,000 by 1997 (Heubeck 1987, 1993; Dunnet & Heubeck 1995; Pennington et al. 2004). Oil pollution in the winter of 1978/79 and abnormal but unexplained mortality the following winter probably accounted for the loss of c. 5,000 birds, but the cause(s) of a further 40% decrease in the population between 1984 and 1991 was largely unknown. From surveys carried out in 2001-02 and 2005-06 it became evident that a proportion of the moulting population had moved from traditional locations on Shetland's exposed, outer coastline and was spending the late summer close to aquaculture sites in the sheltered voes (sea inlets) of the inner coast. Here, Eiders were feeding and roosting at Salmon Salmo salar farms with increasingly large and automated cage installations, or feeding at farms cultivating Blue Mussels Mytilus edulis on long-line tethered ropes. This paper describes the results of further surveys in August 2009 and 2012, and compares the numbers and distribution of Eiders with previous years.

For assisting with counts in 2009 and 2012, we are grateful to: Penny and Sheila Gear (Foula); Newton Harper and Helen Moncrieff (RSPB); Alistair Nayar; David Parnaby and Deryk Shaw (Fair Isle Bird Observatory); Rory Tallack and Howard Towll (Shetland Amenity Trust). We also thank boatmen Jim Dickson, Brian Edwardson, Victor Gray, the late Ronnie Johnson, Alan Longmuir, Edmund Nicolson, Jerry Ramsay, George Lamont Williamson, and Jonathan Wills. Harry Scott of Pica Design kindly produced the maps. Andy Foote (Uppsala University), Karen Hall (Scottish Natural Heritage, Lerwick), Paul Harvey (Shetland Biological Records Centre), and George Williamson provided helpful comments on a first draft, and Ib Krag Petersen and an anonymous referee improved the submitted manuscript. The SOTEAG seabird monitoring programme is funded by The Sullom Voe Association Ltd.

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