The diets of sympatric wintering populations of Common Eider Somateria mollissima and King Eider S. spectabilis in Northern Norway

Abstract

Atotal of 2534 food items of 42 species (exclusive eggs of fish) were identified in the diet of Common Eiders Somateria mollissima (1650 items of 34 species) and King Eiders S. spectabilis (884 items of 35 species) from Troms in northern Norway in April/May. Both species had been feeding on much the same prey species but in quite different proportions . The diet of Common Eiders was dominated by Mytilus edulis (46.3% wet wt .), eggs of Cyclopterus lumpus (25.9%) and other molluscs, Modiolus modiolus (7 .3%), Modiolaria discors (2 .1%) and Arctica islandica (2 .8%) . King Eiders feed mainly on echinoderms, Ophiopholis aculeata (27.1%), Strongylocentrotus droebachiensis (22.7%) and Asterias rubens (15.1 %). Otherimportant King Eiderfood items were molluscs, Modiolaria discors (4 .8%), Modiolus modiolus (3 .6%) and Trophonopsis clathratus (4 .3%). King Eiders had a broader diet than Common Eiders (average number of food species per individual 6.9 vs . 3.6), and thecalculated diet overlap was 0.15. The differences in the diet observed is most likely a result of King Eiders feeding at greater depths than Common Eiders .