Piersma, T.*; van Gils, J.; Dekinga, A.; Spaans, B.; Dietz, M.W.; Visser, G.H.: Physiology-based trade-offs in diet and habitat selection of a small shorebird

Red knots (Calidris canutus, Scolopacidae) are small, 120 g, shorebirds that breed on High Arctic tundra and spend the nonbreeding season in coastal intertidal areas, reaching much of the world’s coastlines in the course of their migrations. Red knots are sensory equipped to efficiently detect hard-shelled prey such as bivalves buried in soft sediments. They are also specialized in processing these prey that consist largely of salt water and shell. We have studied their diet and habitat requirement in the field and in the laboratory using a large array of techniques, often focussing on what we consider key physiological processes. Using energy budgets as the basis, we aim to contrast the great range of energy expenditure levels encountered by individual knots in the course of their annual cycle and by knots of different populations spending the nonwintering season in very different climates. Shifting focus to the components of energy budgets, we examine the different cost factors and the extent to which birds try to minimize these costs, and the gain functions of different kinds of prey and whether the birds try to maximize these gains. Underlying these considerations is the realization that the phenotypic flexibility of red knots and other shorebirds demands that external (environmental) and internal (physiological) factors always have to be studied in concert.