The Birds Are Coming! The Birds Are Coming!
By Teri Shore
Published: November, 2001
I spotted the white-crowned sparrows in my Sonoma backyard in early October.
I was waiting for them. The little brown birds always arrive about this time to feed in the trees around my house during the winter. The black-and-white head stripes distinguish them from other sparrows. I don’t know where they come from, but their arrival always marks the golden transition from autumn to winter.
The sparrows are among the thousands, even millions, of birds that travel the Pacific Flyway every year. So it seemed a good time to investigate the flow of feathered traffic into the Bay Area’s common backyard, the San Francisco Bay.
The San Francisco Bay Estuary is the most important wintering estuary for aquatic birds on North America’s Pacific Coast. Nearly half of the waterfowl and shorebirds migrating along the Pacific Flyway pass through the Bay and Delta. Waterfowl, such as ducks, geese and swans, and shorebirds, including plovers, stilts, avocets and sandpipers, are now arriving in large flocks. The South Bay is one of the most important winter feeding grounds in North America.
As soon as I began my research, it became obvious that I could not share the wonder and expanse of the winter migration in one column. Instead, I’ll provide an overview and highlight some species and the threats they are facing.
I also suggest that ferry riders do a little research on their own by just looking out onto the Bay during these coming months and look for birds that you don’t normally see. Carry a pair of binoculars. Pick out a bird and look it up in a field guide. You might be amazed.
The Bay’s World of Birds
Over one million shorebirds inhabit bay marshes annually. Ducks, geese, swans and other waterfowl seek out shallow, sheltered open water such as sloughs and bays to nest and rest along shores and feed on plants, seeds, snails, clams and insects. Some, like the canvasback, seek out open water to rest.
Waterfowl The Bay Estuary is the winter home for 50 percent of the diving ducks in the Pacific Flyway, with one of the largest wintering populations of canvasbacks in North America. More than 30 species of waterfowl are found in the San Francisco Bay ecosystem. Almost the entire California population of migrating northern shovelers winters in the bay, as do two-thirds of the greater and lesser scaups.
Nearly 90 percent of the canvasbacks from Alaska winter in San Francisco Bay. Canvasbacks are about the size of the common Mallard. The male canvasback is tri-colored with a distinctive red head with blood-red eyes, mostly white body and black breast and tail. The females are less showy with a brown head and eyes and dusky gray body. They tend to congregate in the mouths or rivers or channels, large wetlands and brackish marshes. The protection of this species was one of the main reasons for establishing the San Pablo Bay National Wildlife Refuge in the North Bay.
Sadly, canvasbacks are decreasing. In the 1960s, 60,000 birds were counted. By 1990 the count declined to 24,000. Scientists believe that canvasbacks suffer from toxic contamination in the food chain, as they feed on clams that contain high levels of selenium. Boats, aircraft, people and pets also disturb canvasbacks.
Total waterfowl numbers in the San Francisco Bay Estuary has dropped from a record high of 1.3 million in 1977 to a low of 109,000 in 1982 (with a mean annual population over the same period of 390,532). Populations of dabbling ducks and geese are at an all time low due to drought, predation and habitat loss.
Shorebirds The SF Bay Estuary is an important wintering and migratory stopover site for shorebirds such as plovers, stilts avocets and sandpipers. It attracts more migrating shorebirds than any other wetland area in California. At least 34 species live, nest and forage for food along the Estuary’s shore, many equipped with long bills and legs useful for foraging in the mudflats. Some of these birds are thriving, and others struggling. The long-billed curlew is declining due to habitat loss and drought. The black-necked stilt, American avocet and Wilson’s phalarope appear to have been increasing in response to tidal marsh conversion into salt ponds suitable for nesting and raising young. Many shorebird populations, including the endangered clapper rail and black rail, are suffering from red fox (a non-native species) predation.
When it comes to ferry and vessel traffic, some researchers say that wintering ducks and other waterfowl seem fairly tolerant of boat disturbance. But the effect on these waterfowl has to be assessed and minimized, or avoided as the ferry system expands. The potential impacts on shorebirds are also an important consideration that must be studied before new ferry service begins.
For more information consult:
The San Francisco Bay Estuary Institute website www.sfei.org/
California Department of Fish & Game website, www.dfg.ca.gov
Baylands Ecosystem: Species and Community Profiles published by the San Francisco Bay Area Wetlands Ecosystem Goals Project (January 2001)
