Diet composition can be influenced by age- and sex-related factors including an individual’s morphology, social status, and acquired skills; however, specialization may only be necessary when competition is intensified by high population densities or increased energetic demands. The western sandpiper is a small (22-35 grams) migratory shorebird that exhibits female-biased sexual size dimorphism with a 5 percent greater body size and a 15 percent longer bill in females compared to males. It is considered a generalist with a diverse diet that includes benthic invertebrates and biofilm – a thin layer of microphytobenthos, bacteria, and detritus encased in a polysaccharide-rich matrix of extracellular polymeric substances that forms on the surface of mudflats at low tide. In San Francisco Bay, CA, USA, western sandpipers are one of the most abundant shorebird species foraging on tidal mudflats throughout the non-breeding season. Stable carbon and nitrogen isotope data and were collected from western sandpipers and their potential prey on the Dumbarton shoal, an intertidal mudflat on the southwestern side of San Francisco Bay that supports a high biomass of benthic invertebrates and biofilm consumed by western sandpipers. Morphometric data were used to assign sexes and age classes to western sandpipers.