RESOURCES FOR LONG-DISTANCE MIGRATION - INTERTIDAL EXPLOITATION OF LITTORINA AND MYTILUS BY KNOTS CALIDRIS-CANUTUS IN ICELAND

We investigated the resource basis and energy budget of migrating knots Calidris canutus on spring stopover in NW Iceland. where large concentrations of birds deposit extensive fuel reserves for long distance flights to high arctic breeding grounds. Prey composition was determined on the basis of crushed shell and other fragments in faecal droppings. The daily foraging time, and dropping frequency, in two main intertidal habitats - seaweed beds on rocky shores and mudflats - was monitored in relation to the tidal cycle. Main food sources were 4-15 mm periwinkles Littorina spp. (providing 147 kJ of daily gross energy intake) taken from seaweed, and 8-23 mm common mussels Mytilus edulis (134 kJ), taken from both seaweed and mud. Additional contributions obtained froin mud include chironomid larvae (9 kJ), small gastropods (3 kJ) and probably also unidentified softbodied prey (about 73 kJ). Food resources at the stopover site are probably not limiting for the population of migrating knots. Estimated seasonal prey depletion rates were low, only 1-4% for Littorina, 0.5-1.7% for Mytilus and 11% for chironomid larvae. The knots' estimated daily energy intake was below the predicted limit for maximum metabolisable energy intake. Although this indicates that the knots' rate of fuel deposition probably was constrained by food availability and foraging time rather than the metabolic capacity, it could not be ruled out that the knots at least on some days reach their metabolic ceiling for energy accumulation. A combination of factors may explain why the study region offers highly favourable stopover conditions for transient knots, and why the surplus of resources are not exploited instead for breeding or winter survival by resident shorebirds: (a) macrotidal variation; (b) pronounced seasonality; (c) combined exploitation of different intertidal habitats (seaweed and mud) during different tidal phases; (d) exuberant food availability. The shore crab and limpet are absent from the study region, which may explain the unusually high Littorina densities as a result of significant release from predation and competition, respectively.