Nesting activities of the California clapper rail (Rallus longirostris obsoletus) were observed during April through August 1980 in three marshes in south San Francisco Bay. Fifty nests were found, primarily using rope drags, from 11 April through 14 July. Clutch size averaged 7.27 (SD =+0.96) and hatching success was 38 percent for 26 nests. Predation by Norway rats (Rattus norvegicus) accounted for the failure of 24 percent of the nests. Based on the composition of their canopies, five nest types occurred: pickleweed (Salicornia virginica). Pacific cordgrass (Spartina foliosa), gurnplant (Grindelia humilis), salt grass (Distichlis spicata) and wrack (drifting dead vegetation). Pickleweed and wrack provided the primary nesting cover at 58 and 20 percent, respectively, of the nests examined. Nests were found in greater numbers in the lower marsh zones where Pacific cordgrass and tidal sloughs were more abundant. In optimal habitat such as Dumbarton Point and Mowry Slough, breeding densities ranged from 0.90 to 1.60 rails/ha.
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