Ridgway's rail

Ridgway's rail (Rallus obsoletus) is a near-threatened species of bird. It is found principally in California's San Francisco Bay to southern Baja California. A member of the rail family, Rallidae, it is a chicken-sized bird that rarely flies.

Ridgway's rail
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Gruiformes
Family: Rallidae
Genus: Rallus
Species:
R. obsoletus
Binomial name
Rallus obsoletus
Ridgway, 1874
Synonyms

Rallus longirostris obsoletus
Rallus crepitans obsoletus

This species is closely related to the clapper rail, and until recently was considered a subspecies.[2] It has a long, downward curving bill and is grayish brown with a pale chestnut breast and conspicuous whitish rump patch. The population levels of Ridgway's rail are precariously low due to destruction of its coastal and estuarine marshland habitat by prior land development and shoreline fill. It has year-long, circadian activity and is most vocal nocturnally and crepuscularly.[3]

Habitat
Pickleweed Creek, the upper arm of Richardson Bay looking toward Bothin Marsh

Ridgway's rail forages at the upper end of, along the ecotone between mudflat and higher vegetated zones, and in tidal sloughs. Mussels, clams, arthropods, snails, worms and small fish are its preferred foods, which it retrieves by probing and scavenging the surface while walking. The bird will only forage on mudflats or very shallow water where there is taller plant material nearby to provide protection at high tide. At such high tides it may also prey upon mice, and has been known to scavenge dead fish.[4]

One of the largest population of Ridgway's rails is in San Francisco Bay, where a total of about 1100 are resident.[5] The photo above right was taken of a Ridgway's rail at the San Francisco Bay perimeter near the Dumbarton Bridge; the Don Edwards San Francisco Bay National Wildlife Refuge in the area provides protected habitats for this and other native species. Other frequent sightings of this species around the San Francisco Bay include the Napa Sonoma Marsh, Bothin Marsh in Mill Valley, Gallinas Creek in San Rafael, Arrowhead Marsh and Damon Marsh in Oakland, the Palo Alto baylands,[6] Charleston Slough in Mountain View, Seal Slough in San Mateo and Belmont Slough.

For cover, Ridgway's rail seeks out emergent wetland dominated by pickleweed and cordgrass, or brackish emergent wetland with those two plants plus bulrush. It is not clear whether it requires any source of fresh water. Although not migratory in coastal wetlands, this species disperses juveniles into freshwater wetlands in late August through October. Ridgway's rail has been observed to forage in or near relatively disturbed areas, leading one to deduce the importance of protecting even numeral marsh areas; for example this species was seen foraging in a small mudflat area within Seal Slough in San Mateo, three miles from the nearest known breeding area in Belmont.[7]

Breeding

Ridgway's rail breeds (California rail subspecies) in the San Francisco Bay from mid-March through August, with peak activity in late June. During this breeding season the bird density was approximately 0.1 to 0.6 individuals per acre; outside of breeding season densities decline to 0.04 to 0.40 individuals per acre.[8] The twig nest is placed low, sometimes among plant roots, and purple-spotted buff eggs are laid. Eggs are produced in clutches of four to fourteen, with an average yield of 7.6. The incubation period is 18 to 29 days, and the hatching success is 38%, notably less than the similar light-footed rail indigenous to southern California.

Subspecies
  • Rallus o. obsoletus, formerly California clapper rail, nominate subspecies [9]
  • Ridgway's Rail was officially named by ornithologist James Maley, Los Angeles, after the renowned ornithologist Robert Ridgway.
  • Rallus o. levipes, light-footed rail, a U.S. federal and California state listed endangered subspecies that ranges from Santa Barbara County to the extreme north of the Mexican coast of the Pacific Ocean.[10]
  • Rallus o. yumanensis, Yuma rail, southeastern California and southern Arizona, to northwestern Mexico[11][12]
  • Rallus o. beldingi, Belding's rail, southern Baja California

References
  1. BirdLife International (2016). "Rallus obsoletus". IUCN Red List of Threatened Species. 2016. Retrieved 15 September 2020.CS1 maint: ref=harv (link)
  2. Chesser, R. Terry; Banks, Richard C.; Cicero, Carla; Dunn, Jon L.; Kratter, Andrew W.; Lovette, Irby J.; Navarro-Sigüenza, Adolfo G.; Rasmussen, Pamela C.; Remsen, J. V.; Rising, James D.; Stotz, Douglas F.; Winker, Kevin (October 2014). "Fifty-Fifth Supplement to the American Ornithologists' Union". The Auk. 131 (4): CSi–CSxv. doi:10.1642/AUK-14-124.1. S2CID 198151282.
  3. David C. Zeiner, William F. Laudenslayer and Kenneth E. Meyer, California’s Wildlife Volume II Birds, State of California Department of Fish and Game (990)
  4. R.L. Zembal and B.W. Massey, The light-footed clapper rail, distribution, nesting strategies and management, Cal-Neva Wildl. Manage. 36:631–634 (1983)
  5. L. Liu et al., "California Clapper Rail (Rallus longirostris obsoletus) Population monitoring: 2005–2011" PRBO Technical Report to the California Department of Fish and Game. (2012)
  6. John Kemper, Birding in California Globe Pequot Press (2001) ISBN 1-56044-832-6
  7. T.E. Harvey, H.S. Goosehead, C.M.Hogan, K.Wilson, G.W.Ball, V. Strifle et al., Section 7 endangered species biological assessment for the proposed East Third Avenue widening project, city of San Mateo, San Mateo County, prepared by Earth Metrics Inc. for the city of San Mateo, California (1980)
  8. R. E. Gill Jr., The breeding birds of south San Francisco Bay estuary, master's thesis, San Jose State University, San Jose, Ca. (1973)
  9. US-FWS: Species Profile for California Clapper rail (Rallus longirostris obsoletus)
  10. Seaworld.org: Light-Footed Clapper Rail (Rallus longirostris levipes)
  11. US-FWS: Species Profile for Yuma Clapper rail (Rallus longirostris yumanensis) . accessed 6.4.2014
  12. BLM: Yuma Clapper Rai (Rallus longirostris yumanensis)
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