PREY SELECTION WHILE DIVING BY COMMON EIDERS, SOMATERIA-MOLLISSIMA

Common eiders, Somateria mollissima, wintering in the Gulf of St Lawrence, Canada, dive for prey over a variety of substrate types. In one, the Agarum zone, eiders choose at different stages during a dive whether to reject ubiquitous urchins, Stronglylocentrotus droebachiensis, of low energetic value in favour of continued search for rare crabs, Hyas araneus, of higher energy content. A dynamic programming model is developed to predict dive duration as a function of prey choice. Observations on diving behaviour by eiders are compared with predictions of the model. The model predicts that dives terminated with the capture of a crab should be shorter in duration than those terminated with the capture of an urchin. Fruitless dives, which occur when urchins are rejected, should be the longest in duration. The fit between dive duration predicted by the model and that observed in the field is reasonable in shallow foraging zones. However, the model overestimates the amount of time spent underwater in deeper waters. It is shown that variability in crab total energy content, one factor that could potentially reduce the duration of deep dives, has no influence on dive duration. Deep dives may be shorter than predicted for a number of reasons. For instance, diving may be more energetically costly than currently estimated, or travel speed may increase with depth. More work is needed to evaluate the relevance of these possibilities in eiders. © 1992.