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DescriptionSan Francisco Bay is recognized as a Western Hemisphere Shorebird Reserve Network (WHSRN) Site of Hemispheric Importance for shorebirds – the highest possible ranking. San Francisco Bay holds higher proportions of the total wintering and migrating shorebirds on the U.S. Pacific coast than any other wetland. For eleven species, the Bay holds over half of the individual shorebirds detected, during at least one season of the year (see Table). San Francisco Bay is also the northernmost regular breeding area of the American Avocet and Black-necked Stilt on the Pacific coast of North America. About 10% of the U.S. Pacific coast population of the Western Snowy Plover breeds in the salt ponds of the South Bay. San Francisco Bay Overview Nearly half of California’s fresh water runoff finds its way to the San Francisco Bay Estuary. Sierra snowmelt and foothills Range rainfall are captured by the Sacramento and San Joaquin rivers which meet with ocean tides entering through the Golden Gate. Once, the estuary sprawled over more than half-a-million acres of mudflats and salt marsh: the largest contiguous tidal marsh system on the Pacific Coast. San Francisco Bay wetlands have a long history of human alteration, including development of adjacent uplands and seasonal wetlands, dredging of tidal mudflats, and changes in salinity and tidal regime. Today more than 90% of the original wetlands have been lost to urban development; converted to agricultural fields or salt ponds; or degraded by pollution, exotic species introductions, and habitat destruction. Despite the huge loss of natural habitat, the estuary’s remaining wetlands provide habitat for hundreds of thousands of shorebirds, waterfowl, and other water birds throughout the year. Habitats in San Francisco Bay Tidal flat is the primary foraging habitat of many of the region’s most abundant shorebirds, including the Black-bellied Plover, Semipalmated Plover, Willet, Long-billed Curlew, Marbled Godwit, Western Sandpiper, Least Sandpiper, Dunlin, and Short-billed Dowitcher. The main shorebird prey in the tidal flats are invertebrates, but many of these are introduced species that arrived through the release of ship ballast and other human actions. Salt Pannes and Salt Ponds Historically, about 645 ha of natural salt pannes, large open areas within the salt marsh vegetation, served as supra-tidal foraging and roosting sites for many shorebird species, and as nesting areas for plovers, stilts, and avocets. As the demand for salt rose in the mid-1800s, artificial salt ponds replaced the pannes.  Currently there are 13,943 ha of salt ponds in the estuary. The ponds vary in size, depth, salinity, and most importantly, invertebrate characteristics. Thus each type of pond varies in the vertebrate populations that are supported by the particular invertebrate assemblage found in that pond, resulting in the highest diversity of shorebird species of any other habitat in the Bay. Though the habitat value of the once extensive vegetated marsh was lost when the ponds were formed, the ponds and levees within the salt complex became significant roosting and nesting sites for a wide variety of non marsh-dependent species, and the ponds themselves became important foraging areas for millions of shorebirds and other species of waterfowl, sea birds and other waterbirds. Salt Marsh Shorebirds use salt marsh to a lesser degree than tidal flats, but under some tidal conditions, roosting birds do use this habitat. The larger non-vegetated channels in salt marsh are used as foraging habitat by the same species that feed on tidal flats. Some species, such as the Willet, Whimbrel, Long-billed Curlew, and Least Sandpiper, also forage on marsh plains with sparse or low vegetation. Species such as the Willet, Least Sandpiper, Dunlin, and Long-billed Dowitcher use salt marsh as diurnal and nocturnal roost sites, possibly to provide some protection from predators such as owls. There currently are about 16,265 ha of tidal marsh in the San Francisco Bay, a 79% decline from historic levels. Tidal marsh has been lost primarily to the development of salt ponds, agriculture land, and urban areas. |
Site Facts
Country, State, Province/Region: USA, California Relative Location: Estuarine wetlands in the San Francisco and San Pablo Bays Latitude/Longitude: 37º 28’ N, 122º 02’ W Category: Hemispheric Site Basis for Designation: Usage by more than 900,000 shorebirds annually Size: 22,489 ha. (55,571 acres) Joined: 1989 Site Owner/Steward: Multiple landowners: U.S. Fish & Wildlife Service, California Department of Parks and Recreation, California Department of Fish and Game Lands, East Bay Regional District, Midpeninsula Regional Open Space District, City of Mountain View, National Audubon Society Site Partners: Point Reyes Bird Observatory, San Francisco Bay Joint Venture Human Population within 100 km About 8 million Contact: Catherine Hickey, PRBO |
