Rallus obsoletus yumanensis

Ripley (1977) included subspecies rhizophorae and nayaritensis of western coastal Mexico (Banks and Tomlinson 1974) in subspecies yumanensis.

This bird has a small breeding range in the southwestern United States and northwestern Mexico. Reliable long-term trend data are lacking, but populations have declined substantially since the 1900s due to habitat degradation and loss. There is evidence that management and restoration efforts, along with an increase in survey efforts, may be reversing this trend in parts of the range, however, its remaining wetland habitat continues to be threatened by human-led activities.

Yuma Ridgway’s rails occur along the lower Colorado River and tributaries in Arizona, California, and Nevada, USA, the Salton Sea in Imperial County, California, and in northern Mexico at the Ciénega de Santa Clara, Sonora, and Colorado River Delta, Baja California. Although most do not migrate, individuals in parts of the breeding range have been recorded from mangrove wetlands along the coast of Sonora, Sinaloa, and Nayarit, Mexico (Harrity and Conway 2020, iNaturalist 2024, GBIF 2024).

In Arizona, rails occur as far north as Lake Mead, the Virgin River, the Bill Williams River, the lower Gila River from near Phoenix to the Colorado River, and the lower Salt and Verde Rivers. Occasional records outside this range include Picacho Reservoir, Tavasci Marsh, Roosevelt Lake, and Quitobaquito Pond (Arizona Heritage Data Management System 2021). In California, they are found in appropriate habitat along the entire length of the California portion of the Colorado River and some tributaries in the immediate vicinity of this river and in the Imperial Valley, Imperial County (California Natural Diversity Database 2023). In Nevada, they have been detected as far north as Key Pittman Wildlife Management Area (WMA) and Pahranagat National Wildlife Refuge (NWR) in Lincoln County, along the Virgin River south of Mesquite to upper end of Lake Mead, Overton WMA in Clark County, and west to Ash Meadows NWR in Nye County (A. Chaney, Nevada Division of Natural Heritage, pers. comm. 2024).

Using breeding records since 2004 (i.e., the past 20 years) from Global Biodiversity Information Facility (GBIF 2024), the calculated range extent is approximately 101,000 km².

Using a separation distance of 5 km, data provided by NatureServe Network Programs indicate there are approximately 50 extant occurrences in the US. Additional occurrences are located in the Ciénega de Santa Clara and Colorado River Delta, Mexico.

According to USFWS (2022), there are five population centers across the rail’s range of 14 watersheds within the Colorado River Basin. Of the five, three are considered core population areas. The first is along the lower Colorado River, with the highest densities at the Imperial, Cibola, and Havasu National Wildlife Refuges. The second largest population center is at the Salton Sea in California, which includes the Salton Sea National Wildlife Refuge and the Wellton Mohawk Irrigation District. The third major population center, the Ciénega de Santa Clara in Sonora, Mexico, supports the largest marsh in the rail's U.S.-Mexico range and >70% of the global population (Hinojosa-Huerta et al. 2013). The two peripheral populations, which are smaller and represent the expansion of the subspecies, occur along the lower Gila and Salt Rivers in central Arizona, and in southern Nevada along the Las Vegas Wash. In Mexico, rails were widely distributed in the Colorado River Delta, occupying almost all marshlands dominated by cattail.

The most immediate threat to Rallus obsoletus is the continued loss, degradation, and fragmentation of its wetland habitat. The conversion and deterioration of wetlands due to development for industry, agriculture, salt production, and urbanization have drastically reduced the extent of this species’ habitat (BirdLife International 2020, Eddleman and Conway 2020). Potential poisoning from selenium is likely a significant threat factor in the Imperial Valley, California, but the effects on this subspecies are unknown (Ricca et al 2022). Incidental mortalities at solar facilities have also been documented (Harrity and Conway 2020).

In southern California, loss of coastal saltmarsh habitat is estimated at 75-90% since settlement of the region (Eddleman and Conway 2020). In Arizona, this subspecies is threatened by habitat fragmentation due to human activities such as river channelization, dredging, drying, and flooding of marshes, floodplain development, and diversion of water sources (Arizona Natural Heritage Program 2021). In addition, chigger mites in populations in southwestern Arizona and Nevada could result in population declines; in southwestern Arizona upwards of 92% of some sub-populations are infested (Harrity and Conway 2019, BirdLife International 2020, Nevada Natural Heritage Program 2020). In Mexico, the biggest threat to the habitat at Ciénega de Santa Clara is the lack of a secured water supply and the possible diversion of this water to be used by the Yuma Desalting Plant in Arizona (Hinojosa-Huerta et al. 2013). In Baja California, this subspecies is threatened by resort and industrial development planned by international companies and salt evaporation pond development (Eddleman and Conway 2020), and in northern Mexico, disjunct populations of yumanensis may be vulnerable to habitat loss and degradation in their wintering areas resulting from increased development along the coast, reduced freshwater inflows, and pressures from shrimp farming (Harrity and Conway 2020).

Existing changes in climate conditions do not appear to have had a negative impact on Yuma Ridgway’s rails yet, however, rail populations may become affected by projected future water shortages in the Colorado River (USFWS 2009).