The California clapper rail (Rallus longirostris obsoletus) was one of 43 avian species included in the 1971 breeding bird survey of south San Francisco Bay. Data on nesting and populations were obtained from direct observation and rope drag techniques from March to December, 1971. Nesting was studied from April to July with peak nesting found in May. Eighty-seven nests were located during the study; 58 of which were active when discovered. Mean clutch size was 6.83 eggs with a range between 3-11. All but one nest were made of dried cord grass stalks with 69 (79%) of the nests located in 2-3' tall stands of cord grass. No data were taken on hatching or fledging success. Population densities were assessed from April to December. A modified rope drag was used during the nesting season while direct observation during flood tides was used during winter months. Densities with rope drags varied with tide conditions and habitat type surveyed. Medium high tides (4.5 1+) produced high densities of 1.08 rails per acre on primary rail habitat and 0.70 rails per acre on secondary rail habitat. Censusing primary habitat marshes inundated by high winter tides produced a density of 1.42 rails per acre. Clapper rail habitat within the study area was figured at 4,250 acres with 1,950 acres classified as primary rail habitat, 1,120 acres as secondary habitat and 1,180 acres as bare surface in the form of sloughs, creeks, and mudflats or brackish sloughs which were considered poor rail habitat. Population projections based on density figures and habitat acreage put the number of California clapper rails in the study area at 2,750 with a range of 2,420 to 2,880. Population projections based on direct observation at flood tide conditions need further investigation.
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