THE INFLUENCE OF WATER DEPTH AND SEDIMENT TYPE ON THE FORAGING BEHAVIOR OF WHIMBRELS

Prey-size selection by Whimbrels (Numenius phaeopus) feeding on an int ertidal polychaete was investigated to determine how different substra te conditions affect the foraging of Whimbrels feeding on an intertida l mudflat. Whimbrels preying almost exclusively on large polychaetes ( Perinereis gualpensis) in an estuarine mudflat in Chile (Queule River estuary 39-degrees 20'S, 73-degrees 13'W) were studied. Birds were hig hly selective: the average prey size found on droppings was 53.6 +/- 3 .7 SE (n = 914) mm total body length, and 95% of them were larger than 40 mm. Whimbrels obtained their prey almost exclusively by deep probi ng because large worms were found mainly at depths below 3 cm in the s ubstrate. Observations of feeding Whimbrels were made in five sediment conditions: (1) shallow water, sediment covered by a 2-10 cm water la yer; (2) water film, sediment covered by a water film <2 cm deep; (3) wet sand with no water layer; (4) dry sand; and (5) mud. Foraging was significantly more successful in sandy areas than in muddy areas. This result corresponded with an observed lower density of large polychaet es (>40 mm) in the mud. Within sandy areas Whimbrels had the highest f oraging success rate in wet sand. Wet sand was suspected to be optimal for visual detection of prey because, in water-covered sediments, wat er movement probably interfered with visual detection of prey. Whimbre ls did not feed in dry sand, which suggested that prey were not detect able or were unavailable to the birds due to burrowing depth and incre ase in sediment compaction.