?Restinga or sandflat?? Prey selection of Red Knots (Calidris canutus rufa ) in 2004, an unusual northern migration season at San Antonio Oeste, Rio Negro, Argentina?

ECHAVE, MARIA EUGENIA; BARZOLA, LUCIANA; ZAIDMAN, PAULA; GONZALEZ, PATRICIA

KNOCKADOON HEAD

Conferencia; . IWSG`s Annual Conference; 2005

Wader Study Group

From February to April, San Antonio Oeste is an importantstopover site for Red Knots after leaving their winteringgrounds in Tierra del Fuego. Usually, flocks of thousands ofknots feed by day on a restinga, a broad intertidal rocky flat,where they ingest the small mussel Brachidontes rodriguezi.These occur in patches attached to the substrate and arelocated visually. During March?April 2004, however, unusually knots were observed probe-feeding on an extensiveintertidal sandflat.Benthic sampling in March/April 2004 plus nine collections of 100?150 droppings and 34 pellets showed:a. On the restinga, mussel size distribution, total musseldensity, the density of sizes selected by knots, the biomassof selected sizes (19.4?78.8 g AFDM total biomass/m2)and biomass equivalent per dropping (0.063?0.065 gAFDM total) were in the same range as and not significantly different from those found in a similar study carried out in1992.b. In the sandflat, the main prey was the clam Darina solenoides with a length range of 9?35 mm. Despite the low clam biomass of selected sizes (0.886 g AFDMtotal/m2), biomass equivalent per dropping (0.068 to 0.091g) and per pellet(0.088 g) were higher than those found on the restinga.Mussels eaten (4?20 mm) and positively selected sizes(6?13 mm) were smaller than clams eaten (16?35 mm)and positively selected (16?24 mm).Clams were a better quality food than the mussels becausethey have thinner shells relative to biomass (AFDM/Ash ofclams = 2*(AFDM/Ash of mussels), p < 0.001). The knotsfed on mussels on the restinga only by day and it wasassumed that the reason why they did not feed there at nightwas because of an increased risk of predation due to the proximity of dunes and cliffs. In 2004, the knots fed on thesandflat both day and night and their breeding plumage scores(an index of body condition) were significantly higher thanin 2001?2003. It is concluded that they were reaping the benefit of an unusually good food supply of clams on the sandflatand were feeding there for that reason rather than because ofpoor availability of mussels on the restinga.