Ridgway's rail (Rallus obsoletus) is a species of bird found principally along the Pacific coast of North America from the San Francisco Bay Area to southern Baja California, as well as in some regions of the Gulf of California. A member of the rail family, Rallidae, it is a chicken-sized bird that lives brackish tidal marshes and rarely flies. Its name commemorates American ornithologist Robert Ridgway.
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. It is most active nocturnally and crepuscularly.[3]
Habitat
Pickleweed Creek, the upper arm of Richardson Bay looking toward Bothin Marsh
Ridgway's rail forages along the ecotone between mudflat and higher vegetated zones and in tidalsloughs. 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]
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.[8]
Feeding and ecology
The omnivorous Ridgway's rail eats many things, including clams, crabs, mussels, and occasionally small rodents and birds.[9]
Breeding
By mid-February, nest building has begun. Ridgway's rail then breeds (California rail subspecies) in the San Francisco Bay from mid-March through August, with peak activity in late June.[6] 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.[10] 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. Incubation is shared between both the male and female Ridgway's rail.[6]
Subspecies
R. o. obsoletus, formerly California clapper rail, nominate subspecies [9]
R. 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.[11]
R. o. yumanensis, Yuma rail, southeastern California and southern Arizona, to northwestern Mexico[12][13]
^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. S2CID198151282.
^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)
^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)
^John Kemper, Birding in California Globe Pequot Press (2001) ISBN1-56044-832-6
^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)
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