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Despite habitat loss, the bay is a site of global importance to birds

At Work on San Francisco Bay

Nils Warnock, PhD

Caspian Terns roost on South San Francisco Bay's developed shoreline. ©San Francisco Bay National Wildlife Refuge

San Francisco Bay is a paradox. It is an estuary that has lost over 80% of its native tidal marshes and over 25% of its tidal mudflats. If you stand in just about any tidal or seasonal marsh or on any tidal flat in the bay, you cannot fail to see urban development--industry, housing tracts, highways, marinas, and more. But within the same view you are likely to see a swarm of 10,000 Western Sandpipers feeding on tidal flats in the shadow of San Francisco International Airport or an immense aggregation of scoters and scaup roosting and diving in waters below the bay's bridges. Tidal marshes, some nestled right in housing tracts, host significant breeding populations of Song Sparrows, Common Yellowthroats, various rails, and other bird species.

This abundance of bird life in San Francisco Bay has long attracted the attention of PRBO biologists. Beginning in the 1970s, we surveyed for breeding Snowy Plovers and found that the bay supported some 20% of California's coastal Snowy Plovers (down to about 5% now). In the 1980s, we conducted extensive rail surveys and the first comprehensive shorebird survey of the entire bay (see page 3). We study breeding marine birds on Alcatraz Island, endangered California Least Terns at Alameda, and cormorants on bay bridges. Our demographic studies of tidal marsh species, begun in the mid-1990s, continue today, as do bird surveys in over 100 tidal marshes throughout the bay. To help guide tidal marsh restoration and evaluate its success, we are now studying how bird populations interact with their environment at recently restored and unrestored sites in both the North Bay and South Bay. By studying the millions of birds that use the salt ponds, tidal marshes, tidal flats, and surrounding waters, we are developing ways to predict how habitat change in the bay will affect resident and migratory bird populations (see page 2).

"PRBO brings leadership, vision, and strong science to each of the many conservation projects and programs it's involved with in the San Francisco Bay area. PRBO has demonstrated the rare quality of being able to passionately advocate for conservation and at the same time work with innumerable partners to address multiple and sometimes competing objectives."--Nadine Hitchcock, San Francisco Bay Area Conservancy Program, California Coastal Conservancy

We are doing this work not only because of our avid interest in bird ecology and the San Francisco Bay ecosystem, but because the bay faces many changes and threats, as does the whole Pacific Flyway. Shorebird populations are declining throughout North America, and duck populations like San Francisco Bay's Lesser and Greater Scaup are also in decline. Clapper Rails, Black Rails, and Snowy Plovers face enormous pressure from introduced predators and habitat loss.

PRBO's ongoing studies of San Francisco Bay bird populations, with partners like the U.S. Geological Survey, U.S. Fish and Wildlife Service, and San Francisco Bay Bird Observatory, are vital to ensuring that the San Francisco Bay estuary--although a site of intense urban encroachment with accompanying habitat loss, change, and degradation--supports the greatest abundance and diversity of birds and other wildlife for generations to come.