Size-Selective Predation of Blue Mussels (Mytilus edulis) by Common Eiders (Somateria mollissima) under Controlled Field Conditions

We studied size-selective predation by Common Eiders (Somateria mollissima) feeding on blue mussels (Mytilus edulis). Selection varied with location, season, and prey availability, and for the most part ducks preferred smaller mussels than those that would have been the most energetically profitable on a per prey basis. We tested predictions from two related hypotheses concerning optimal prey selection: (1) the shell-mass minimization hypothesis, which states that predators select food that minimizes shell ingestion, as opposed to maximizing energy intake per prey item; and (2) the risk-averse foraging hypothesis, which, assuming large prey are not preferred (because of high shell content, low energy value, or some other reason), states that predators feed on smaller prey when the risk of mistakenly taking large prey increases. We found support for both hypotheses, although the risk-averse foraging hypothesis should be tested further due to conflicting results and small sample sizes. During most of the year, Common Eiders selected relatively small mussels that minimized shell ingestion, even though larger available prey would have provided higher net energy gain per prey item. In winter, differences among length classes in shell ingestion became small, and birds switched to feeding on larger prey that provided more energy per unit work and probably were more profitable. During times when small and mid-sized prey were preferred, ducks foraging where large mussels were abundant usually selected smaller length classes of prey than did those feeding where large mussels were less common. Birds avoided the risk of inadvertently ingesting large prey by selecting smaller mussels. Our results provide insight into the mechanisms of prey selection by Common Eiders and may also help explain some of the discrepancies reported in previous studies of prey-size selection in this species.