Assessing the quality of migratory shorebird stopover sites requires good measures of food availability. We developed simple methods to measure biofilm grazing by migrant western sandpipers (Calidris mauri), a species for which biofilm is an important dietary component. We used a field-portable chlorofluorometer to measure the density of chlorophyll-a(Chl-a) in surficial biofilms on Roberts Bank, a large intertidal mudflat in British Columbia, Canada, during northward migration.Chl-adensity begins at a low level during each diurnal emersion period, and increases steadily during emersion at 4.1 mg m−2h−1for a total of ∼24.6 mg m−2over a typical 6 h emersion period and ∼41 mg m−2over a 10 h emersion period. Western sandpipers grazed at 1.35–1.45 mg Chl-am−2min−1, thus biofilm production supports 17.6 min m−2of grazing time during a 6 h low tide period and 29.3 min m−2during a 10 h period. During peak northward migration, the average grazing intensity of western sandpipers over an intertidal emersion period was 3.3–6.4 min m−2, suggesting that biofilm accumulation was 2.7–8.8 fold greater than the amount consumed. We found Chl-adensity was highest (∼65 mg per m2) within 40mof the shoreline. Grazing intensity was lowest close to shore, where predation risk from falcon attacks is highest. Grazing intensity peaked at 240mand then declined, lowering Chl-adensity at greater distances to a uniform level of ∼54 mg m−2. These results indicate that interactions between biofilm production and sandpiper grazing underlie spatio-temporal patterns in biofilm abundance on Roberts Bank.