Food, feeding, and refuelling of Red Knots during northward migration at San Antonio Oeste, Rio Negro, Argentina

We studied the food and feeding ecology of Red Knots Calidris canutus rufa on an area of rocky flat, or restinga, near San Antonio Oeste in the northwest of Golfo San Matias, Provincia de Rio Negro, Argentina in March 1992. These Red Knots are on their way north, from ''wintering'' areas in Tierra del Fuego (Argentina and Chile) to the arctic breeding grounds in Canada. Three waves, each containing 5000-12,000 Knots, moved through the area. On the restinga they fed on banks of the small mussel Brachidontes, rodrigezi, virtually the only prey taken. Variably sized (1-28 mm) Brachidontes, of which those between 5 and 20 mm were eaten (8-12 mm long mussels being positively selected), occurred in high biomass densities (23-117 g ash-free dry mass [AFDM]/m(2)). Due to its abundance, visibility ease of detachment, slender shell form, and relatively weak shell, Brachidontes appear very suitable prey for Red Knots. Red Knots produced about 0.42 droppings/min. On the basis of a biomass equivalent of the droppings of approximately 62 mg AFDM, for conditions of balance (shell mass in=shell mass out) the intake rate was estimated at 0.433 mg AFDM/s. Making a few more assumptions, this translated into a net intake rate of 2.81 W, which would allow for a far deposition rate of less than a gram of fat per day. Using more optimistic estimates of the various parameters in the energy budget model, a fat deposition rate of 5 g could be achieved. This is the order of magnitude predicted for Red Knots staging at San Antonio Oeste for 6-8 d and refuelling for the 1700 km long flight to the next known major staging area in southern Brazil.