Dynamics of habitat use by shorebirds in estuarine and agricultural habitats in northwestern California

We examined shorebird use of mudflats, marsh islands, and nearby agricultur al fields near Humboldt Bay in northwestern California between September 19 88 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 Sandpi pers (Calidris minutilla), Long-billed Curlews (Numenius americanus), and M arbled Godwits (Limosa fedoa) became opportunists and used fields at interm ediate and high tides when mudflats were inundated. Black-bellied Plovers ( Pluvialis squatarola) and Greater Yellowlegs (Tringa melanoleuca) were seas onal generalists during the two wettest seasons, using fields at all tides and mudflats at low and intermediate tides. Western Sandpipers (Calidris ma uri) were mudflat specialists, and Willets (Catoptrophorus semipalmatus) we re salt marsh opportunists that mainly used mudflats, but shifted to salt m arsh at high tide. Killdeer (Charadrius vociferus) and Common Snipe (Gallin ago gallinago) were field specialists and did not use the other two habitat s in significant densities during any season. The presence of short vegetat ion and the presence or absence of standing water were the two most importa nt characteristics influencing increased use of fields by all species.