Digestive capacity and toxicity cause mixed diets in red knots that maximize energy intake rate

In energy-maximizing animals, preferences for different prey can be explained by ranking them by their energetic content. However, diet choice also depends on characteristics of the predator such as the need to ingest necessary nutrients, and constraints imposed by digestion and toxins in food. In combination, these factors can lead to mixed diets in which the energetically most profitable food is not eaten exclusively even when it is abundant. We studied diet choice in red knots (Calidris canutus canutus) feeding on mollusks at a West-African wintering site. At this site, the birds fed primarily on two species of bivalves, a thick-shelled one (Dosinia isocardia) that imposed a digestive constraint, and a thin-shelled one that imposed a toxin constraint (Loripes lucinalis). The latter species is toxic due to its symbiotic association with sulphide-oxidizing bacteria. We estimated experimentally the parameters of a linear programming model that includes both digestive and toxin constrain...