The San Francisco Bay has been recognized as a Western Hemisphere Shorebird Reserve Network site of international importance due to the habitat it provides for resident and migratory birds. San Francisco Bay provides the most important complex of wetland habitat for migratory and wintering waterbirds on the Pacific Coast, including wintering habitat for more than 50% of the diving ducks on the Pacific Flyway and more than 500,000 individuals annually.
Early accounts of the San Francisco Bay Area frequently mention the abundance of birds, but the population has decreased due to a combination of many stressors such as habitat loss, pollution, and invasive species. This habitat loss happens in both the summer and wintering areas, resulting in more adverse impacts. Although there are variations, many bird species are sensitive to human disturbance, including disturbance by non-motorized small boats – remember to be considerate of the wildlife you do encounter and avoid disturbing them by maintaining recommended buffer distances.
Many waterfowl use San Francisco Bay in winter months for flocking, feeding, and forming pair bonds in preparation for spring migration to their breeding grounds in Alaska, Canada, and the northern United States. The peak season is November through mid-March, but this can vary year to year. By the time birds stop to rest, their fat reserves are nearly exhausted and San Francisco Bay provides food for them to refuel and continue migration.
Threatened and Endangered Species
The Ridgway’s rail (Rallus longirostris obsoletus) is a large, hen-like bird with a long, slightly down-curved bill that is restricted to the marshes around San Francisco Bay. Once abundant, it is currently a federal endangered species and California state fully protected species and is very sensitive to disturbance. Thousands of Ridgway’s rails were eliminated by market hunters from the gold rush until passage of the Migratory Bird Treaty Act. Since then, the loss of over 85% of marsh habitat in the Bay has reduced population numbers.
Populations of Ridgway’s rails are found in remnant salt marshes such as Bair and Greco Islands, along Coyote Creek, and within the Don Edwards San Francisco Bay National Wildlife Refuge. Smaller populations can be found in western Contra Costa, eastern Marin, and northern Alameda Counties; northern San Pablo Bay; along the Petaluma River; and along major creeks and marshes in Sonoma and Napa Counties.
Ridgway’s rails inhabit a range of salt and brackish water marshes. They use the network of small tidal sloughs for foraging and quick escape. The Ridgway’s rail breeding season usually begins in February and can last as long as August. Both male and female share in incubation and chick rearing. Their nests are often found immediately adjacent to a channel, many of which are navigable by shallow-draft non-motorized small boats, making nest sites particularly vulnerable to disturbance. Human disturbance into Ridgway’s rail habitat would likely disturb incubating or brooding birds, potentially reducing reproductive success by breeding pairs (possibly to the point of abandonment of nests, eggs, or young) and possibly destroying active nests. A lost nesting effort, even by a single pair, may have population-level implications for this critically-endangered species.
During very high winter tides, Ridgway’s rails are often concentrated in limited high-tide areas, referred to as “refugia.” Marshes used to have vegetation zones with low, middle, and high marshlands. The high marsh zones act as refuge for many salt marsh animals escaping high tides. These refuge areas have been largely destroyed, resulting in small patches with high concentrations of animals. During high tide events, rail species are particularly vulnerable to predation by mammals and raptors, or other larger birds. If non-motorized small boat users approach high-tide refugia during such extreme tides, they may flush rails into areas where the likelihood of predation would increase. During extremely high winter tides, when much of the marsh plain is inundated, determining the locations of channels, or marsh edge, may be difficult. Boaters should remain at least 50 feet from the edge of the marsh, as demarcated by emergent or inundated vegetation, or other features that may be visible, and not approach vegetation that is not inundated during those times. This means non-motorized small boat users should avoid channels less than 100 feet wide where rails may be present.
Environmental Protection Agency Endangered Species Fact Sheets
Western snowy plover
The Pacific Coast western snowy plover (Charadrius nivosus nivosus) is a small, threatened shorebird that forages for small invertebrates. Its population has declined due to poor reproductive success, which is likely due to habitat loss and alteration, human disturbance, and predation.
Although snowy plovers often nest on beaches, they also nest on salt ponds and levees in the South Bay. Nesting season is typically March through September. Plover nests are camouflaged and are barely visible to even the well-trained eye. Plovers are polyandrous (the female may breed with more than one male), and the female typically deserts the brood shortly after hatching, leaving the chick-rearing duties to the male.
Human activities and predation are key factors in the decline of breeding sites and populations. Western snowy plovers can be disturbed if boaters disembark near salt ponds or levees – either flushing the adults or stepping on the eggs. If the adult leaves the nest, predation by California gulls, common ravens, northern harriers, or red-tailed hawks may occur. To help protect snowy plovers, please keep a distance of 656 feet (200 meters) from plovers and do not land near salt ponds and levees.
Environmental Protection Agency Endangered Species Fact Sheet
For many wintering waterfowl, the amount of energy reserves built up during the winter months directly impacts migration, survival, and reproductive success. Although there are variations, many waterfowl are sensitive to human disturbance, including disturbance by non-motorized small boats.
Disturbance includes human activity that is an intrusion or interruption in the natural, daily activity of an animal. Usually there is an alert response first, such as the bird stops its normal activity and watches you. This usually escalates to “flushing” in which a bird or group of birds moves away from or flees an approaching threat. In waterbirds this looks like swimming, diving, or flying. Because birds concentrate their activities where there is the best opportunity to maximize energy gain, flushing may reduce the time waterbirds spend feeding or resting and may cause them to be displaced to less than optimal feeding and resting areas, or even abandon foraging habitat. Repeated flushing costs energy, and may have cumulative effects on migratory energy and reproductive success or in a local decline in abundance. Numerous small disturbances may be more damaging than fewer, larger disturbances because birds may not have time to recover between disturbances.
To help waterfowl, maintain a buffer distance of 250 meters, or approximately 800 feet, from congregations of waterbirds to minimize disturbance. If this buffer distance is not possible, users should move steadily through, parallel to, or away from birds.
USFWS Don Edwards National Wildlife Refuge
Golden Gate Audubon
San Francisco Bay Wildlife
Sibley Guide to Bird Life and Behavior by David Allen Sibley