This dark plumaged duck breeds on fresh water (in the breeding season it can be found in small lochs in N and W Scotland) but spends most of its time during the rest of the year on the sea, often in large flocks. Wintering and moulting birds gather in large numbers in shore, sandy-bottomed bays feeding on small fish and invertebrates.

Significant numbers of scoter can be found in Carmarthen Bay and this area is of major importance for this species. Carmarthen Bay has been designated a ‘Special Protection Area’ for this species.

Numbers of common scoter counted on ground-based surveys peaked at 6189 in 2007/08. This was substantially fewer than in previous winters (numbers peaked at 24,460, 20,287 and 14,412 in 2004/05, 2005/06 and 2006/07, respectively). The British Trust for Ornithology, who undertook the 2007/08 survey, identified that, although the five-year mean (average) for the site remained above the 16,000 number which meant it is classified within the ‘favourable condition’ threshold, it would drop below this value within one or two winters if numbers remained at similar levels. Fortunately, the 08/09 ground counts, undertaken by the Wildfowl and Wetland Trust in February 2009, recorded a peak count of 22,930 birds, providing reassurance that peak numbers were comparable to those of a few winters previously, and that the bird population numbers remain in favourable condition. There are gaps in our knowledge, however, regarding what is causing fluctuations.