Censusing by vocalizations of the known populations of light-footed clapper rails (Rallus longirostris levipes) in California revealed 203 pairs in 11 coastal marshes from Carpenteria to Tijuana Marsh in the spring of 1980, 173 pairs in 15 marshes in 1981, and 221 pairs in 18 marshes in 1982. The rails were detected, in order of decreasing abundance, in marshes with abundant, dense cordgrass (Spartina foliosa), in pickleweed (Salicornia virginica) dominated marshes with little or no cordgrass, and in brackish to freshwater marshes with dominant reeds (Typha spp. and Scirpus spp.). Dense cordgrass provides a highly utilized habitat, but all of a marsh and its' environs are used to some degree. A most productive situation is apparently provided by a large marsh comprised of numerous habitats as exemplified by Upper Newport Bay, where nearly half of the nesting pairs in the state were detected in 1980 and 1982. Birds do occur, however, in very small marshes with nearly monotypic vegetational cover. In contrast, populations have disappeared from several marshes that were, and in most cases still are, periodically subjected to closed ocean entrances and subsequent prolonged flooding. Additionally, viable populations have not been detected in those coastal brackish to freshwater marshes comprised totally of open water and emergent reeds. A combined total of 221 nesting sites were examined at Upper Newport Bay, Tijuana Marsh, and Anaheim Bay during 1979 through 1981. Incubation nests were placed, in order of decreasing frequency, in dense cordgrass, in higher marsh plants and usually in stands isolated by low marsh or mudflat, in tumbleweeds or wrack lodged mostly in lower marsh plants, and in stands of freshwater reeds. About 95% of the incubation nests examined were built directly in or were isolated by lower marsh habitats. Known agents of nest destruction were high tides in the lower marsh and upland predators in the upper marsh. Optimally, therefore, nesting sites must be densely vegetated and high enough to afford protection from high tides, yet isolated enough in the marsh to be effectively protected from upland predators. The hatching success of 57% for poorly isolated upper marsh nests was significantly lower than that of about 81% for all other nests. Habitat quality, as affected in part by weather, appeared to influence nest placement. Upper marsh sites were most heavily utilized where and when tall, dense cordgrass was least abundant. The scarcity of suitable nesting habitat appears to be a major limiting factor for several clapper rail populations. Methods are suggested for providing additional nesting habitat and for improving the quality of existing habitat.
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