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Black Rail, cont.

From 1986-1988 we surveyed 78 discrete marshes in the Delta, Suisun Marsh, San Pablo Bay, Central Bay and South Bay during the breeding season (March-June). Because the birds are so covert, we played tape recordings of their vocalizations - an emphatic, cricket-like "kic-kik-kerr," or a mechanical "grrring" sound - to elicit responses. Estimating numbers of birds by sound presents many confounding problems and it is difficult to come up with an exact number of birds present. But, we did derive an index to estimate relative abundance among marshes. (We are still working on this thorny problem.) We discovered, to our surprise, that almost all territorial black rails were confined to the North Bay, with a majority in the San Pablo Bay system. The Suisun marshes also held a fair share of birds, but black rails were virtually absent as a breeding species in the Central and South bays. The one site where black rails were found within the South Bay was in the Don Edwards San Francisco Bay National Wildlife Refuge's large Dumbarton marsh.

Having figured out where they were (and where they weren't), the next logical question was: What are the habitat characteristics of those marshes that support black rails and how do they differ from sites where the birds do not breed?

Old records indicate that black rails formerly nested in the South Bay, as well as the coastal marshes of Southern California. But for all intents and purposes, they have disappeared as a breeding species. Black rails do occur in the South Bay during the winter months, however. For many years, birders have seen black rails clinging to emergent marsh vegetation at Palo Alto Baylands during the winter flood tides. Often the observers report that the rail is picked off by a hawk, heron or egret. We assume that most of these winter birds are juveniles that have dispersed to the South Bay after being reared in the North. If they don't survive into the breeding season because of high mortality (predation, drowning, or other factors) the South Bay would be regarded as a "population sink," an area where more birds die than survive. There is also a possibility that rails winter in the South Bay and breed in the