San Francisco Bay is a critical stopping point for winter waterfowl migrating along the Pacific Flyway. This corridor, extending from arctic to temperate regions, represents to migratory birds what rivers represent to salmon -- a complex and interconnected passageway between habitats they depend on for survival. A broken link in the chain of habitats can lead to long-term population declines over a very large region. In many parts of the nation, long-term waterfowl population trends have been down, making the San Francisco Bay, where the trend is not as discouraging, relatively more important.
Because ducks rely on so many different habitats at different parts of their life cycle, it is difficult to draw conclusions about the specific causes of population swings from year to year. Indeed, the numbers -- particularly over the short run -- could obscure significant habitat issues if, for example, a positive trend in one habitat temporarily overcame the negative effect of habitat degradation elsewhere. From a longer perspective, however, bay waterfowl numbers are a bellwether for the overall health of the Pacific Flyway.
Factors that Influence Population Trends
Two main factors in the Bay Area influence population trends on the Pacific Flyway. The first is habitat quantity and quality, because different waterfowl rely on different habitats for feeding, roosting, and nesting. Salt ponds, the open bay, and tidal and seasonal wetlands all play important roles in meeting the habitat requirements of migratory waterfowl. Significant changes in the acreage of one or more of these types of wetlands, for example, can result in significant changes in duck numbers.
The second factor is Bay Area water quality. Waterfowl in the bay have been found to have selenium levels comparable to those that have led to disastrous reproductive problems for ducks in other areas. It is difficult, however, to isolate the effects of selenium and other bay contaminants on these birds, as many do not nest here (many nest in the Arctic) and they are exposed to a host of other environmental factors, natural and synthetic, along the flyway.
Nonetheless, the drop in the duck population tells us important things about the state of our piece of the flyway. It is clear that healthier Bay Area wetlands and water quality would benefit the ducks that use our waters as a critical stop on their long migration.