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

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

BOULDER

Conferencia; International Wader Study Group Annual Conference; 2006

Wader Study Group

From February to April, San Antonio Oeste is an important stopover site for Red Knots. During the day, birds feed on ?restinga?, a broad intertidal rocky flat, ingesting small mussels Brachidontes rodriguezi visually taken from patches. However, during March?April 2004 they were observed unusually in an extensive intertidal sandflat feeding by probing into the substrate. Benthos sampling plus nine samples of 100?150 droppings and 34 pellets collected in March and April 2004 showed that: Restinga: mussel size distribution, density of total and selected sizes, benthos biomass of selected sizes (19.4 to 78.8 g Ash Free Dry Mass total/m2) and biomass equivalent per dropping (0.063 to 0.065 g AFDMtotal/drop) were in the same range than a similar study done in 1992. Sandflat: the main prey was the clam Darina solenoides with a size distribution ranging from 9?35 mm shell length. Despite the low clam biomass of selected sizes (0.886 g AFDMtotal/m2), biomass equivalent per dropping (0.068 to 0.091 g) and per pellet (0.088 g) were higher than those found at the restinga. Mussels eaten (4?20 mm) and positively selected sizes (6?13 mm) were smaller than sizes of clams eaten (16?35 mm) and positively selected (16?24 mm) sizes. Clams were a better quality food than mussels, with thinner shells relative to biomass (AFDM/Ash clam= 2 AFDM/Ash mussel p < 0.001). In 2004, breeding plumage scores ? as an index of Red Knot condition ? were significantly higher than in 2001? 2002 and 2003 seasons indicates that they fed on sandflat because of an unusual food supply of clams and not because of low availability of food in the restinga.