We examined shorebird use of mudflats, marsh islands, and nearby agricultural fields near Humboldt Bay in northwestern California between September 1988 and April 1989. Most species used fields for both foraging and roosting, including some species usually considered to be mudflat specialists. After seasonal rains began in late fall, Dunlins (Calidris alpina), Least Sandpipers (Calidris minutilla), Long-billed Curlews (Numenius americanus), and Marbled Godwits (Limosa fedoa) became opportunists and used fields at intermediate and high tides when mudflats were inundated. Black-bellied Plovers (Pluvialis squatarola) and Greater Yellowlegs (Tringa melanoleuca) were seasonal generalists during the two wettest seasons, using fields at all tides and mudflats at low and intermediate tides. Western Sandpipers (Calidris mauri) were mudflat specialists, and Willets (Catoptrophorus semipalmatus) were salt marsh opportunists that mainly used mudflats, but shifted to salt marsh at high tide. Killdeer (Charadrius vociferus) and Common Snipe (Gallinago gallinago) were field specialists and did not use the other two habitats in significant densities during any season. The presence of short vegetation and the presence or absence of standing water were the two most important characteristics influencing increased use of fields by all species.