Predation on meiofaunal and macrofaunal invertebrates by western sandpipers (Calidris mauri): evidence for dual foraging modes

Western sandpiper (Calidris mauri) predation was examined by concurrent exp eriments and direct observations of foraging behaviour on high intertidal m udflats of the Fraser River estuary, British Columbia. Western sandpipers f oraged by either "pecking" on the surface (64% of observational time) or pr obing into sediment (29%:). The first experiment (probe-mark method) consis ted of collecting small-volume cores (21.3 cm(3)) of probed (experimental) and non-probed (control) sediment on the tidal flat, following a 22.5-min f eeding period. The second experiment (exclosure method) involved deploying exclosures immediately prior to the feeding period and subsequent collectio n of cores from inside (control) and outside (experimental) the exclosures. Sediment cores were analysed for both macrofaunal and meiofaunal size frac tions. Comparisons between macro- and meiofaunal invertebrate densities in experimental and control sediments revealed significant differences, attrib uted to shorebird predation? for both experiments. The probe-mark experimen t detected the removal of large infaunal polychaetes (similar to 20 mm), wh ile the exclosure experiment showed depletion of epifaunal harpacticoid cop epods (0.063-0.5 mm). Predation on macrofaunal cumaceans was detected in bo th experiments. Invertebrates selected by western sandpipers neither fell w ithin traditional infaunal size classifications (macro- vs. meiofauna; 500 mum delineation) nor corresponded to the highest densities of taxa. Rather, inference from experimental results and observations is that western sandp ipers forage in two modes, by: (1) surface gleaning of epibenthic copepods and cumaceans in the macro- and meiofaunal size ranges and (2) selective pr obing for larger infauna, such as polychaetes. These findings were facilita ted by the combination of methodologies employed.