EFFECTS OF PREDATION, FLOODING, AND CONTAMINATION ON REPRODUCTIVE SUCCESS OF CALIFORNIA CLAPPER RAILS (RALLUS LONGIROSTRIS OBSOLETUS) IN SAN FRANCISCO BAY

Auk, The, Jan 2006 by Schwarzbach, Steven E, Albertson, Joy D, Thomas, Carmen M

ABSTRACT.-

We assessed the reproductive success of the California Clapper Rail (Rallus longirostris obsoletus), an endangered subspecies restricted to San Francisco Bay, and the relative importance of predation, flooding, and contaminants as factors affecting that success. Our study was conducted in six tidal marshes in the northern and southern reaches of San Francisco Bay. This assessment, conducted in four breeding seasons (1991, 1992, 1998, 1999), determined that productivity of California Clapper Rails was much reduced over the natural potential. Only 69% 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%, respectively. Only 45% 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%. Predation on eggs was a major factor affecting nest success, reducing productivity by a third. Failed eggs were examined for abnormal development and contaminant concentrations. Contamination appeared to adversely influence California Clapper Rail reproductive success, as evidenced by deformities; embryo hemorrhaging; embryo malpositions; a depressed rate of hatchability; excess concentrations of mercury, barium, and chromium over 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. Received 19 November 2003, accepted 28 April 2005.

Key words: California Clapper Rail, contaminants, mercury, nest predation, Rallus longirostris obsoletus, San Francisco Bay.

Efectos de la Depredación, las Inundaciones y la Contaminación sobre el Éxito Reproductivo de Rallus longirostris obsoletus en la Bahía de San Francisco

RESUMEN.-Evaluamos el éxito reproductive de Rallus longirostris obsoletus, una subespecie amenazada restringida a la bahia de San Francisco, y la importancia relativa de la depredación, las inundaciones y los contaminantes como factores que afectan su éxito reproductivo. Nuestro estudio fue realizado en seis pantanos mareales en los extremes norte y sur de la bahía de San Francisco. Esta evaluación, realizada en cuatro épocas reproductivas (1991, 1992, 1998, 1999), determinó que la productividad de R. l. obsoletus fue muy reducida con respecto a su potencial natural. Sólo el 69% de los huevos cuya viabilidad pudo ser determinada fueron viables. La capacidad de eclosionar de los huevos en los pantanos del norte y el sur de la bahía fue del 65% y 70%, respectivamente. Aunque los tamaños medios de las nidadas fueron 6.7 y 6.9 en el norte y el sur, respectivamente, las aves sólo produjeron 1.9 y 2.5 pichones por intento de nidificación. Las inundaciones fueron un factor poco importante, pues redujeron el número de huevos disponibles para eclosionar sólo en un 2.3%. La depredación de los huevos fue un factor importante que afectó el éxito de nidificación, reduciendo la productividad en un tercio. Los huevos que fracasaron fueron examinados en busca de evidencia de desarrollo anormal y para medir las concentraciones de contaminantes. La contaminación pareció influenciar negativamente el éxito reproductivo, como lo evidencian las deformidades, hemorragias en los embriones, posiciones inadecuadas de los embriones, disminución en la habilidad de eclosionar, concentraciones en exceso de mercurio, bario y cromo por encima de los umbrales tóxicos conocidos para los embriones de las aves y una correlación de las deformidades con concentraciones elevadas de algunos elementos halladas en los huevos que no eclosionaron. El mercurio fue el único contaminante significativo que estuvo presente en todos los pantanos.

THE CALIFORNIA CLAPPER Rail (Rallus longirostris obsoletus) is an obligate salt-marsh inhabitant of the tidal marshes of San Francisco Bay. The only breeding population of this subspecies is now located solely in the intertidal margins of San Francisco Bay. At its nadir, the population in 1991 was estimated to have declined to between 300 and 500 individuals (Albertson and Evens 2000). More recent population surveys estimated that between 1,040 and 1,264 individuals remained in all of the San Francisco Bay area (Collins et al. 1994, Albertson and Evens 2000) with ~60% located in the South Bay.

Predation by introduced predators, including sport and commercial hunting by humans, and habitat loss are the proximate causes for the historical decline of this subspecies. Present-day tidal-marsh habitat in San Francisco Bay is ~15% of historical (1850s) acreage (Dedrick 1989). Remaining California Clapper Rail habitat in San Francisco Bay is extremely fragmented (Gill 1979, Collins et al. 1994, Albertson 1995), with the Central Bay and North Bay having less habitat and more isolated fragments than the South Bay (Albertson and Evens 2000). Collins et al. (1994) found that much ostensibly suitable habitat in the North and Central bays was unoccupied by California Clapper Rails and hypothesized that particularly low fecundity might be one explanation for their apparent absence in these fragmented tidal wetlands.