Variation in the diet of the Red Knot during northward migration at Playa Colombo, Peninsula Valdes, Argentina

The Red Knot (Calidris canutus rufa), uses Playa Colombo (42°38′ S, 64°13′ W) as a feeding and resting place during their northward migration. The diet of Red Knot was compared in that beach in 2007 and in 2014. The diet was reconstructed by analysis of feces (n = 38 in 2007 and n = 30 in 2014), collected after observing monospecific flocks feeding in a sector of the beach. In addition, we studied the trophic supply of the same beach sector from benthic invertebrate samples. The diversity of prey did not vary between years, but its densities. The clam Darina solenoides recorded low densities in 2007 (between 11 – 117 ind /m²) and high in 2014 (between 552 and 1285 ind /m²), the polychaete Travisia olens showed similar densities at two years (between 117-733 ind /m² in 2007 and between 43-637 ind /m² in 2014). The reconstruction of the diet indicated that D. solenoids was present in all feces during the two years, whereas T. olens was present in all feces in 2007, but only in 13.3% in 2014. On average, each feces had markedly fewer clams in 2007 (0.5±0.8 clams /feces) than in 2014 (8.23±4.76 clams /feces). The clams were always selected by the Red Knot, but depending on the availability of this prey, the amount ingested and the differential ingestion of alternative species varied such as T. olens, which is relevant in contexts of deficit of the main prey.