If you spend enough time in the vicinity of San Francisco Bay, you will eventually notice the quiet. Not quiet and serene. the alternative type. The kind that causes you to notice a gap in your surroundings. For those who have spent years walking those tidal marshes, that something is frequently a bird, or more accurately, the lack of many birds where there once were dozens.
Morgan Tingley has devoted his professional life to closely observing precisely this type of absence. Tingley, an ecology and evolutionary biology professor, has focused a large portion of his work on figuring out why bird populations change, decline, and occasionally just disappear. His work on the San Francisco Bay Index, a framework for monitoring environmental stress throughout the estuary system, links the steady decline of songbirds from Bay Area shorelines to rising water levels, a connection that most people hadn’t considered.
Once the reasoning is clear, it is difficult to refute. For a variety of small bird species, tidal marshes along the estuary provide vital habitat. These are not spectacular migrants traveling across hemispheres. Marsh wrens, song sparrows, common yellowthroats, and other species that nest low, feed in dense vegetation, and depend on the unique structure of wetland edges that form where land meets estuary water are examples of relatively local birds. That habitat doesn’t simply change when those boundaries shift and tidal flooding pushes higher and more frequently inland. It vanishes.
The relationship between water and bird populations isn’t gradual as people often think, as Tingley’s research has shown more and more. It’s not a gradual fade. Up to a certain point, seemingly stable bird populations can withstand small changes. However, if breeding seasons consistently fail or if nesting sites flood too frequently or too early, numbers can decline unexpectedly quickly. Instead of treating rising water as a stand-alone variable, the San Francisco Bay Index tracks several stressors collectively. Although the precise threshold for each species is still unknown, the directional trend is clear.
This relates to a more general pattern that Tingley has previously identified. His well-known research on bird communities in the Sierra Nevada showed that precipitation, not just temperature, is a significant, sometimes conflicting factor in determining where species can survive. The Bay has the same level of complexity.
In addition to sea level rise, other factors contributing to rising estuary waters include changed precipitation patterns, decisions made about upstream water management, and the diminished ability of nearby wetlands to absorb and buffer these changes. Determining which factor is most important for each species is a truly challenging task.
As this research progresses, it seems as though scientists like Tingley are fighting against the public’s inclination to see wetland birds as a luxury rather than a necessity. His work uses data to challenge that, but not in a dramatic way. In these marsh systems, birds play a role in seed dispersal, insect populations, and ecological processes that go far beyond the shoreline. When their numbers decline, it usually indicates a more serious issue.
Timing is another important consideration. According to Tingley’s research on migratory species, birds that migrate from South America are particularly at a disadvantage because they frequently arrive too late to take advantage of the window of peak food availability and lack local seasonal cues. The same mismatch affects marshland species in the Bay Area in a different way. The conditions that define their habitat are changing, but their habitat itself is not. The landscape that these birds have adapted to over many generations is changing as a result of the water arriving earlier and sitting longer.
The more difficult question is whether conservation efforts can effectively respond in a timely manner. Protection of land is beneficial. A portion of the damage can be mitigated by managed wetland restoration. However, the underlying pressure caused by rising estuary waters is not going away, and the San Francisco Bay Index at least gives managers something to do by making that pressure visible and quantifiable. Sometimes the most significant thing science can do is make a problem unavoidable. This work appears to be accomplishing that.
