|A marsh at extreme high tide. Photo by Peter Baye.|
High tides occur twice a day; they are greatest each month at new moon and full moon; and they peak in winter and summer. In San Francisco Bay, high water at the wrong time can be challenging for tidal-marsh birds such as rails, Song Sparrows, and Salt Marsh Common Yellowthroats, which use low-growing vegetation for cover and nest habitat. When storms coincide with extreme high tides, rainwater and winds raise the water even higher. Birds nesting too low in the vegetation will find their eggs floating away or their nestlings drowning. Birds nesting too high will not be as well concealed from predators such as raptors, raccoons, foxes, and cats.
Not just nests but adult birds themselves can be at risk: extreme tides can push them into areas with less cover, where the same hungry predators await them. Adult mortality during winter storms is of particular concern for the endangered California Clapper Rail, which makes its home exclusively in the tidal marshes of San Francisco Bay.
|Song Sparrow. Photo by Tom Grey.|
For now, marsh birds are able to cope with extreme high-tide events by nesting early in the season and by timing their nest attempts with the tides of two lunar cycles. By initiating nests immediately after one set of exceptionally high tides, Song Sparrows have a better chance of completing a nest cycle (which takes about 24 days) before the next extreme high-tide flood arrives about a month later. This timing strategy—"saddling between the tides"—may not work as sea-level rise and increased storminess bring even higher waters.
PRBO is working to help the conservation community prepare for and adapt to climate-change effects in San Francisco Bay, by assessing the impacts of predicted changes on tidal marsh plant and bird communities. Our results will help direct conservation dollars to areas of the bay where they are most needed and where they will have a lasting effect long into the future.