Functional response curves of avian molluscivores: high intake rates maintained even at low prey density

Despite its low energy density, the blue mussel Mytilus edulis is a regular prey of
various seaduck species. As a result, seaducks must ingest large quantities of mussels to meet
their energy requirements. In this study, we modeled the functional response curve for a large
avian molluscivore, the common eider Somateria mollissima, by measuring intake rates of captive
individuals foraging in diving tanks under different mussel densities and at different attachment
strengths. We estimated the mean maximum intake rate to be 45 prey min−1 (with a mean bottom
time ± SD of 5.034 ± 3.793 s), which is relatively high compared to intake rates of other diving duck
species and prey types. However, we found no significant effects of density and attachment
strength on intake rates, indicating that eiders can maintain maximum intake rates even at low
mussel densities. These results could explain the depletion of mussel beds sometimes observed in
the wild, as well as the large negative impact that seaducks may have in aquaculture farms.
Seaduck · Blue mussel · Attachment strength · Prey depletion