Intertidal community ecology is an extremely well developed field. However, sea ducks are frequently neglected in intertidal community studies, because they are viewed as transient or not sufficiently numerous to have an effect. This may be a mistake, because recent work with other aquatic birds indicates that they can be very important in structuring communities. We examined the importance of Common Eiders (Somateria mollissima) as predators in an intertidal invertebrate community in Passamaquoddy Bay, New Brunswick, Canada. Eiders are present here year-round and feed heavily on blue mussels (Mytilus edulis), a dominant intertidal invertebrate. We erected predator exclusion cages in disturbed and undisturbed areas of the lower ntertidal zone. Disturbance involved manually removing 80% of all biomass present, and was intended to simulated a catastrophic abiotic event. We sampled invertebrates from cages and paired control areas from 1994-1996. Total biomass and species diversity were compared among treatments, disturbance levels, and times using split- plot ANOVA. Variation in abundance of common species was examined using a similar multivariate model. Eider predation significantly reduced invertebrate biomass within 4 months in undisturbed sites, and after 8 months Eiders had eaten 40-50% of biomass in both areas. Disturbance delayed effects of predation, but ultimately allowed them to persist longer in the system. Predation had little effect on total species diversity, but did generate indirect effects on, and interactions with, other species. Exclusion of Eiders led to an increase in dogwhelks (Nucella lapillus) (a gastropod predator of blue mussels) within one year at undisturbed sites. This in turn obscured the impact of ducks on mussels in these areas. Eiders are also size-selective predators, preferring mussels of 15-30 mm in length. This affected size-frequency distributions of the prey population, and has implications for future Eider-mussel interactions, as well as indirect effects on the invertebrate community. Clearly, Common Eiders play an important role in this community, and our results show that sea ducks should not be neglected in community studies.