During its fall migration stopover on mudflats in the upper Bay of Fundy, Canada, the Semipalmated Sandpiper (Calidris pusilla) is thought to feed primarily on the amphipod Corophium volutator (mudshrimp). Semipalmated Sandpipers typically use a peck-probe foraging strategy and, until recently, there had been little evidence of variability or opportunism in their foraging habits during this stopover. From 2006 to 2008 we recorded data on the sandpipers' foraging behavior and food availability at three commonly used mudflats. Behavior and food availability varied considerably at one site in one year. In 2006 at Grande Anse, where mudshrimp densities were exceptionally low and ostracod densities very high, the peck-probe strategy was almost completely abandoned for “skimming,” a foraging behavior novel in this species. Because of similarities between skimming and grazing, used by Western Sandpipers (Calidris mauri) to consume biofilm, we hypothesized that birds switched to skimming to feed on biofilm. However, chlorophyll a concentration in the top 2–3 mm of sediment, an index of biofilm abundance, was not a good predictor of proportion of time spent skimming. Instead, skimming had a strong, positive relationship with ostracod density, suggesting that the sandpipers skimmed opportunistically to feed on ostracods rather than to target biofilm. Thus Semipalmated Sandpipers are capable of adapting to changes at traditional staging areas by using novel foraging mechanisms, apparently to forage opportunistically on alternative prey. If staging habitats continue to change, alternative foods and foraging modes may become increasingly important to the success of this species' migration.