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Black Rail, cont. North Bay. We have no evidence of this, however; another mystery yet to be solved.
We think we understand why they don't breed in the South Bay: There is simply not enough habitat above mean higher high water. In the South Bay, subsidence has caused more and more marsh lands to be flooded on high tides. Simultaneously, levees and roads constructed on the upland edge of these sinking baylands effectively eliminate most of the higher marsh habitats. The North Bay marshes are somewhat higher in elevation. But marsh elevation alone does not explain why rails in the North Bay have clustered distributions. Rails were much more commonly encountered in fully-tidal marshes than in marshes with restricted tidal flow, in marshes along large tributaries or along the bayshore than in smaller tributaries, and in marshes located at the mouths of sloughs and creeks.
One of the primary threats to high marsh inhabitants is the condition of the habitat just above the tideline, the natural vegetative transition between the tidal marsh plane and the adjacent upland that serves as a buffer zone and provides refuge during periods of flooding. Although these flood tides are |
Prime
black rail habitat is that thin ribbon of salt marsh vegetation that occurs
between the high tideline (mean higher high water) and the upland shore, a gently
sloping plain with very little elevational rise. Since the Gold Rush, the high
tidal marsh may have been the most altered habitat in San Francisco Bay. A study
in the late 1970's determined that high salt marsh
habitat averaged only about three meters in width. Along the tideline we have
built levees and dikes, salt ponds, roads, factories, landfills, bike paths,
parking lots and sewage treatment plants. In the Central and South bays, bayfill
has been added to the shoreline to build the cities of San Francisco, Oakland,
Redwood City, and Fremont. In the North Bay conversion to agriculture has
been the primary culprit.Last updated:
May 27, 2008
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