Comparative Feeding Ecology of Steller's Eider and Long-Tailed Ducks in Winter

We examined the flexibility in feeding ecology of Steller's Eider (Polysticta stelleri) by comparing its diet to the taxonomically relatively close Long-tailed Duck (Clangula hyemalis) during two winter periods (early and late) when the two species occurred together. Steller's Eider generally fed on the same type of prey in early and late winter, with a dietary overlap of 43% between the two periods (57% among shelled prey). The mean number of prey species per bird was 7.8 in both early winter and late winter. In contrast, the Long-tailed Duck changed completely from feeding on benthic invertebrates in early winter to spawning Capelin (Mallotus villosus) in late winter. The dietary overlap between the two bird species was 38% in early winter. In early winter, the mean number of prey species was 5.5 in the Long-tailed Duck, and not significantly different from the Steller's Eider. Steller's Eider seemed to be restricted to small prey, both compared to availability of sizes and also to the prey size taken by the Long-tailed Duck. Thus the worldwide rare Steller's Eider seems to be a specialist with limited ability to exploit different food resources, while the numerous Long-tailed Duck is an opportunistic generalist. Low flexibility in feeding ecology indicates that winter habitats for Steller's Eider may be limited, and protection of these areas would probably benefit the species.