A large molluscivore bird (Common Eider, Somateria mollissima)is able to discriminate quality of blue mussels (Mytilus edulis) based on size and provenance

Molluscivore birds that forage on abundant but low-quality food have to ingest large quantities of food to achieve energy balance. Such a strategy is often associated with important digestive constraints limiting predator’s ingestion. Thus, these predators may use prey selection to ingest better-quality individuals among a generally low-quality prey population. Using captive Common Eiders (Somateria mollissima (L., 1758)) diving in a constant environment, we were able to examine their preferences for blue mussels (Mytilus edulis L., 1758) of varying qualities (different sizes or provenances). In addition, we studied the consequences prey selection had on Eiders’ energy intake rates and ingestion of flesh and shell material. Eiders selected 10–20 mm mussels and were able to discriminate and to select cultivated mussels from intertidal mussels. Prey selection allowed, in certain conditions of prey-size abundance, higher flesh and energy intake rates without increasing the ingestion of shell material. This study confirmed the energetic advantage that Eiders have when foraging in aquaculture sites, which explain the large depredation of preferred mussel sizes. Key words: prey selection, intake rate, prey size, intertidal, aquaculture, Common Eider, Somateria mollissima.