Inusual selección del área de alimentación por el playero rojizo en San Antonio Oeste

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

Puerto Madryn

Jornada; VI Jornada de Ciencias del Mar; 2006

From Feb to Apr, San Antonio Oeste is an important stopover site for Red Knots after leaving their wintering grounds in Tierra del Fuego. In the 1990s, flocks of thousands of knots fed on a ?restinga?, a broad intertidal rocky flat, ingesting the small mussel Brachidontes rodriguezi taken from patches attached to the substrate. However, from 2004 Red Knots also fed on an extensive intertidal sandflat. Benthos sampling and dropping analysis from Feb to Apr 2006 showed: a. On the restinga, mussels eaten had bimodal distributions of shell length (5?7 and 10 mm) in seven samples, but a single mode (5?7 mm) in two samples taken on 22 April just before departure. These selections could not be explained by benthos distribution of mussel sizes. In the 1990s, the 10 mm size-class was positively selected by the knots that fed only on the restinga (González et al. 1996), while small sizes (< 0.016). Knots did not feed on the restinga early or late in the passage, and earlier short-staying knots never feed on the restinga. It is expected that just after arrival and before departure the birds have small gizzard mass because migration is less costly if the digestive apparatus is small. Therefore at such times the birds are not able to feed on hardshelled mussels so they avoid the restinga. The bimodal size-distribution of ingested mussels on the restinga probably reflects two different strategies: knots having a larger (time minimizing) gizzard mass that can fuel faster on lower quality food, and knots with a smaller (energy maximizing) gizzard mass that fuel faster on higher quality preys as polychaetes and small mussels. Small mussel sizes are better quality food because they have lower shell mass relative to biomass (0.29 AFDM/ash mass) compared with the 10 mm size (0.23 AFDM/ash mass), though the latter is more profitable in terms of total biomass.