Limited Reproductive Success for California Clapper Rail in San Francisco Bay

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Limited Reproductive Success for California Clapper Rail in San Francisco Bay

By Steven E. Schwarzbach
June 2006

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The only breeding population of the federally endangered California clapper rail nests in the intertidal margins of San Francisco Bay. Present-day tidal-marsh habitat in the bay is about 15 percent of historical acreage, and remaining California clapper rail habitat is extremely fragmented. Understanding the causes of what appears to be baywide low fecundity of the clapper rail is important to support management and habitat-restoration efforts for its recovery. Contaminants and egg predation appear to be major factors limiting the reproductive success of California clapper rails in both the northern and southern reaches of the bay, according to a study in the January issue of The Auk by U.S. Geological Survey (USGS) scientist Steven Schwarzbach and coauthors Joy Albertson and Carmen Thomas of the U.S. Fish and Wildlife Service. The study's findings indicate that strategies to increase the population will need to do more than provide new tidal-marsh habitats.

Map of San Fancisco Bay.

Above left: This study determined that the productivity of the federally endangered California clapper rail, whose distribution is restricted to San Francisco Bay, was much reduced over the natural potential. Photograph by Carmen Thomas, U.S. Fish and Wildlife Service. [larger version]

Above right: In the recent report, the part of San Francisco Bay north of the Golden Gate Bridge is referred to as the North Bay, and the part south of the San Mateo Bridge is referred to as the South Bay.

The study was conducted in six tidal marshes in San Francisco Bay—two in the North Bay (defined by the authors as north of the Golden Gate Bridge) and four in the South Bay (defined by the authors as south of the San Mateo Bridge)—during four breeding seasons (1991, 1992, 1998, 1999). The authors determined that the productivity of clapper rails was much reduced over the natural potential. Only 69 percent of clapper-rail eggs whose viability could be assessed were viable. Hatchability of eggs in North Bay and South Bay marshes was 65 and 70 percent, respectively. Only 45 percent of the nests successfully hatched at least one egg. Despite mean clutch sizes of 6.7 and 6.9 in the North and South bays, respectively, clapper rails produced only 1.9 and 2.5 young per nesting attempt. Flooding was a minor factor, reducing the number of eggs available to hatch by only 2.3 percent; the loss that occurred was related to spring flood tides in the El Niño years of 1992 and 1998. Predation on eggs was a major factor affecting nest success, reducing productivity by a third.

Steven Schwarzbach looks for nests of California clapper rail in Wildcat Marsh.

Above left: Steven Schwarzbach (formerly with the U.S. Fish and Wildlife Service) looks for nests of California clapper rail in Wildcat Marsh in the North Bay in 1999. Photograph courtesy of Steve Schwarzbach.

Above right: On average, California clapper rails laid nearly 7 eggs per clutch, but only 2.4 chicks were produced per nesting attempt. Juvenile survival was not followed in this study, but the proportion of young to fledge was likely much less than 2.4 fledged per nest. Photograph by Carmen Thomas, U.S. Fish and Wildlife Service. [larger version]

Failed eggs were examined for abnormal development and contaminant concentrations. Contamination appeared to adversely influence clapper-rail reproductive success. Evidence included deformities; embryo hemorrhaging; embryo malpositions; a depressed rate of hatchability; concentrations of mercury, barium, and chromium greater than known avian embryotoxic thresholds; and a correlation of deformities with elevated concentrations of some trace elements in eggs that failed to hatch. Mercury was the only significant contaminant common to all marshes.

Above left: Overall in the study, 31 percent of the eggs were nonviable. Note the shortened wings and extra toes in this late-stage rail embryo, which was collected as a fail-to-hatch egg from a nest at Wildcat Marsh in North Bay in 1998. Elevated chromium and barium are among the most likely trace elements responsible for such deformities at this marsh, but mercury may also have contributed to the occurrence of deformities. (The red area is not a deformity, but a yolk sac.) Photograph by Steve Schwarzbach. [larger version]

Above right: Remains of eggshells tell a story of predation. In successfully hatched nests, eggshell remains are removed by parents. Overall in the study, about one-third of all eggs were lost to predators. Rodents, particularly the Norway rat (Rattus norvegicus), accounted for 90 percent of the eggs lost to predation in South Bay marshes in 1992. Photograph by Carmen Thomas, U.S. Fish and Wildlife Service. [larger version]

Predation and pollution effects might interact at many life stages. Such contaminants as mercury may slow growth or impair the ability of young to detect predators, impair the ability to fly or forage for food, and compromise the effectiveness of parental care for young. All of these contaminant-induced adverse effects would give an advantage in the wild to potential predators.

Among the authors' conclusions are the following management recommendations:

For egg hatchability to improve, sufficiently protective sediment- and water-quality objectives, particularly for mercury, must be achieved within the San Francisco Bay habitat.
To minimize flooding losses during the incubation phase, new tidal marshes from wetland-restoration efforts need to be designed to achieve appropriate elevations during spring, when clapper rails are constructing nests.
New marshes will also require significant buffers from residential areas and active predator-control efforts to address predation of both nests and adult clapper rails.

The full citation of the new report is Schwarzbach, S.E., Albertson, J.D., and Thomas, C.M., 2006, Effects of predation, flooding, and contamination on the reproductive success of California Clapper Rails (Rallus longirostris obsoletus) in San Francisco Bay: Auk, v. 123, no. 1, p. 45–60.

The Auk
American Ornithologists' Union

The Birds of North America Online: Clapper Rail
Cornell University

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June 2006

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Limited Reproductive Success for Endangered California Clapper Rail

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June 2006 Publications List