2.1 Study area
The San Francisco Bay (SF Bay) is the largest estuary on the west coast of North America. It is designated as a Western Hemisphere Shorebird Reserve Network site of hemispheric importance because it is visited by over half a million wintering and migratory shorebirds each year (Morrison, 1981; Myers et al., 1987; Page, Stenzel, & Kjelmyr, 1999). Western sandpipers are one of the most abundant shorebird species in SF Bay, foraging on tidal mudflats throughout the non-breeding season (Page et al., 1999). Western sandpipers exhibit strong foraging site fidelity and have similar home range sizes in winter and spring in this region (Warnock & Takekawa, 1996). In addition to serving as an important wintering area for western sandpipers, SF Bay is a heavily utilized migratory stopover for sandpipers wintering at more southern latitudes (Bishop, Warnock, & Takekawa, 2006; Butler, Delgado, De La Cueva, Pulido, & Sandercock, 1996; Iverson, Warnock, Butler, Bishop, & Warnock, 1996). The influx of migratory western sandpipers in April leads to increased population densities and rapid depletion of invertebrate prey that could increase competition (Rowan, 2012). Our study was conducted on the Dumbarton shoal, an intertidal mudflat on the southwestern side of SF Bay. This mudflat supports a high biomass of benthic invertebrates and biofilm consumed by western sandpipers, but it’s invertebrate carrying capacity is exceeded in April (Rowan, 2012).