Spring diet of common eiders (Somateria mollissima) in Breiðafjörður, West Iceland, indicates non-bivalve preferences

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Breiðafjorður is an important molting, breeding, and wintering area for about 25 % of the Icelandic common eider (Somateria mollissima) population. However, feeding habits of eiders in this area have not been investigated until now. Prey selection was analyzed from 192 stomach samples (esophagus and proventriculus) collected in spring 2007–2010. Thirty-five prey species were identified; the highest percentage occurrences were of gastropods (79 %), chitons (polyplacophorans) (58 %), crustaceans (43 %), bivalves (26 %), and echinoderms (8 %). The most common food species was the mottled red chiton Tonicella marmorea (58 %), followed by the common whelk Buccinum undatum (40 %), the spider crab Hyas arenarius (39 %), and the chink shell Lacuna vincta (35 %). The majority of the food items was of small size and consumed in high quantity. The chitons and mussels were of similar average sizes (11.7 and 13.4 mm, respectively), which might suggest that prey size could be as important as species in food selection. There were some inter-annual differences in dominant prey classes between years. For example T. marmorea was found in 60–70 % of birds in the years 2007 and 2009 but only in 30 % of the birds in the other years investigated. Diets of males and females were equally diverse and similar when all months and years were pooled. Prey selection was highly variable but most individuals focused on few or a single species in the hours prior to collection. Results indicate that the most common prey species for common eiders is a chiton and not blue mussels as reported elsewhere.