These endangered rails come out of the salt marshes when the tide is high. They are the largest subspecies R. o. obsoletus which are confined to salt marshes around San Francisco Bay. Formerly abundant, this species declined precipitously because of over-hunting and habitat loss. With protection they started to increase, but have since declined by about 50% because of a poorly implemented project to remove invasive Spartina (cordgrass) from San Francisco Bay. Many areas which formerly supported healthy breeding populations are now sterile mudflat with the most disturbing example being the destruction of the SamTrans marsh in San Mateo County where over 60 of these rails disappeared when Spartina was removed without adequate consideration for the rails. Unfortunately Arrowhead Marsh is also scheduled for destruction. The rails we saw today will have no place to go when the runaway train known as the "Invasive Spartina Project" removes the remaining hybrid Spartina which makes up this marsh.
The bottom photo shows an "exclosure" which provides protection for Ridgway's Rails during these King Tides and also offers nesting habitat.
References:
del Hoyo, J., Collar, N. & Christie, D.A. (2018). Ridgway's Rail (Rallus obsoletus). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/467247 on 24 December 2018).
Eddleman, W. R. and C. J. Conway (2018). Ridgway's Rail (Rallus obsoletus), version 2.1. In The Birds of North America (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.ridrai1.02.1
Morlan, J. 2017. California Ridgway’s Rails Imperiled by Spartina Removal. Sequoia Audubon Society Bulletin 68(1):1 [PDF]