For birders in the Bay Area, a trip to Suisun Marsh or Don Edwards National Wildlife Refuge during the mid-winter duck migration is more of a pilgrimage than an ordinary outing. San Francisco Bay is a key point on the Pacific Flyway, a major north-south migratory route for waterfowl and other birds that extends from Alaska to Southern California and farther south. Beyond its popularity with bird-lovers, the San Francisco duck population also provides a tool for gauging the health of the bay, as well as a glimpse of the Pacific Flyway's health. Overall, the mid-winter duck migration in and around San Francisco Bay is lower than it was in the early part of the 1990s; however, long-term trends are difficult to determine.

Findings
NRDC researchers relied on data gathered by the California Department of Fish and Game, which conducts yearly counts of the mid-winter duck population in and around San Francisco Bay. The counts include both types of ducks found here: diving ducks, including scaup, scoter, and canvasback, which feed on mollusks and other animals on the floor of the open bay; and dabbling ducks, such as mallards, which feed in shallow waters and wetlands on algae and other vegetation. The data reveal significant variations among duck species over the past 20 years. Total populations since 1980 ranged from a low of approximately 56,000 ducks in 1982 to a high of 455,000 in 1984. Between 1998 and 2000, total numbers ranged from 275,000 to 300,000. Long-term trends are not entirely clear, but current population levels are down significantly from highs in the mid-1980s.