NT
Ridgway's Rail Rallus obsoletus



Justification

Justification of Red List category
This rail is thought to have a moderately small population, which is thought to be declining owing to a variety of threats including conversion and degradation of wetlands as a result of agricultural, industrial and residential development, pollution, and predation by invasive species. It is therefore classified as Near Threatened.

Population justification

The population of obsoletus in 2009-2011 was estimated at 1,167 individuals (range 954-1,426) using both survey data and model predictions of densities for unsurveyed sites (Liu et al. 2012a,b). Application of the same methodology to data from 2005-2008 yields an average population estimate of 1,719 individuals (range 1,169 to 2,172; Liu et al. 2012a). Form levipes has been estimated to number 633 pairs in the USA (Zembal et al. 2015) and c.240 pairs in Mexico (Eddleman et al. 1988; Ehrlich et al. 1992), while form yumanensis has been estimated to number 1,700-2,000 individuals (Ehrlich et al. 1992). A more recent population estimate, however, put the population at 6,629 individuals (95% CI: 4,859-8,399) in the Colorado River delta region of Mexico (Hinojosa-Herta et al. 2001). There are apparently no published population estimates for beldingi. The total population is therefore uncertain and whilst Partners in Flight (2019) estimate that the population currently exceeds 15,000 mature individuals, it is tentatively placed here within the band 10,000-19,999 mature individuals.

Trend justification
Data suggest that the population of obsoletus was relatively stable in 2005-2007, but declined by 51% between 2007 and 2008, followed by an apparent weak recovery in 2009-2011 (Liu et al. 2012a,b). Trends for other forms are less well known, but an overall decline is inferred.

Distribution and population

Rallus obsoletus (incorporating levipes, yumanensis and beldingi) is found along the western coasts of the USA and Mexico, disjunctly from San Francisco and San Pablo bays, California, south to Baja California Sur and southern Nayarit, Mexico.
The nominate form is confined to central California, mainly in San Francisco Bay and San Pablo Bay (and Suisun Bay; Overton 2007). Formerly, it also occurred in Morro, Tomales, Humboldt and Monterey Bays (Eddleman and Conway 1998). The population of this form was estimated at 1,167 individuals (range 954-1,426) in 2009-2011 (Liu et al. 2012a,b). Population density in 2011-2013 (>1,167 individuals but likely to be <1,426 individuals [N. Nur in litt. 2016]) was higher than for the period 2009-2010.
Form levipes is found from coastal central California to northern Baja California and has been estimated to number 633 pairs in the USA (Zembal et al. 2015) and c.240 pairs in Mexico (Eddleman et al. 1988; Ehrlich et al. 1992) and has lost huge areas of habitat, while form yumanensis of southeast California, southern Arizona and northwest Mexico has been estimated to number 1,700-2,000 individuals (Ehrlich et al. 1992) and has experienced increases and decreases in habitat availability, with habitat threatened overall with conversion and high water flows. A more recent population estimate, however, put the population at 6,629 individuals (95% CI: 4,859-8,399) in the Colorado River delta region of Mexico (Hinojosa-Herta et al. 2001). There are apparently no published population estimates for beldingi, which is confined to southern Baja California.

Ecology

The species forages along the ecotone between mudflats and higher vegetated zones, and in tidal sloughs. It feeds primarily on mussels, clams, arthropods, snails, worms and small fish. Breeding in San Francisco Bay is from mid March-July, peaking in late June; the clutch size is 4-14 eggs.

Threats

This species was originally threatened by hunting, but more contemporary threats include development for industry, agriculture, saltpans and urbanisation, leading to a drastic reduction in its habitat (Taylor and van Perlo 1998). By the late 1980s, c.85% of the species’s habitat in the 1850s had been lost, with much of the remaining habitat in a degraded state (Ehrlich et al. 1992). Urban development, pollution and introduced predators continue to pose major threats (Taylor and van Perlo 1998, Schwarzbach et al. 2006). Population declines in the nominate form were accelerated (numbers dropped by nearly 50% over a nine-year period) when invasive Spartina vegetation was removed leading to habitat loss (U. S. Fish and Wildlife Service 2013; Lampert et al. 2014; Casazza et al. 2016). Changes in agricultural water supply and management of agriculture water runoff may affect habitat availability at the Salton Sea (C. T. Overton in litt. 2016). Sea level rise poses a significant future threat (Zhang and Gorelick 2014; Rosencranz et al. 2019). A recent novel and severe chigger mite infection is also threatening populations of form yumanensis in southwestern Arizona where upwards of 92% of some sub-populations are infested with the mite; such infestations may result in population declines of this form (Harrity and Conway 2019).

Conservation actions

Conservation and research actions underway
Expansion of survey protocols throughout the species's range including 61 km2 in South San Francisco Bay and over 10 km2 in San Pablo Bay (C. T. Overton in litt. 2016). Work to protect, enhance and restore coastal tidal marsh habitats in California (C. T. Overton in litt. 2016). Development of population monitoring programs in California and assessments of survival and habitat use (including use of artificial habitats. Action plans have been developed for the nominate form, levipes and yumanensis within the United States (C. T. Overton in litt. 2016). The species is included on the 'Watch List' of the State of North America's Birds as a species of high conservation concern (NABCI 2016).

Conservation and research actions proposed 
Survey wetlands throughout the range to obtain population and trend estimates and ensure that these surveys are standardised across the range (J. Wood in litt. 2016). Conduct research into the species's reproductive output, survival and dispersal rates (C. T. Overton in litt. 2016). Develop models of population change and population viability (N. Nur in litt. 2016). Clarify scale and severity of threats. Protect and restore wetland habitats within its range as well as enhancing transition ecotone habitats to ameliorate sea level rise and the effect of storms (Overton et al. 2014; Zhang and Gorelick 2014; C. T. Overton in litt. 2016). Manage alien invasive predators (C. T. Overton in litt. 2016). Manage invasive Spartina cordgrass carefully to ensure that management to remove it does not have a detrimental impact on the species (Lampert et al. 2014).

Acknowledgements

Text account compilers
Everest, J.

Contributors
Ashpole, J, Butchart, S., Casazza, M., Ekstrom, J., Nur, N., Overton, C.T., Symes, A., Taylor, J. & Wood, J.


Recommended citation
BirdLife International (2024) Species factsheet: Rallus obsoletus. Downloaded from https://datazone.birdlife.org/species/factsheet/ridgways-rail-rallus-obsoletus on 05/03/2024.
Recommended citation for factsheets for more than one species: BirdLife International (2024) IUCN Red List for birds. Downloaded from https://datazone.birdlife.org on 05/03/2024.