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The article assesses the feeding modes of Calidris mauri or western sandpiper shorebird over the migratory cycle. The bird's demonstration of mudflat surficial biofilm feeding is described. The possible occurrence of a downward change in trophic position during the 2004 northward migration from the birds' overwintering areas to the Arctic breeding grounds is considered. Trophic positions of adult western sandpipers at their migratory stopover on Roberts Bank in British Columbia were analyzed to retrospectively determine their major diet source types. The advantages of biofilm over macrofaunal feeding are also discussed, including its greater availability and the digestibility of the carbohydrates needed by the birds for flight.