Distribution of California Black Rails in the Sierra Nevada foothills

Abstract

California Black Rails (Laterallus jamaicensis coturniculus) have a disjunct and poorly understood distribution. After a new population was discovered in Yuba County in 1994, we conducted call playback surveys from 1994 to 2006 in the Sierra foothills and Sacramento Valley region to determine the distribution and residency of Black Rails, estimate densities, and obtain estimates of site occupancy and detection probability. We found Black Rails in 164 small, widely scattered marshes distributed along the lower western slopes of the Sierra Nevada foothills from just northeast of Chico (Butte County) to Rocklin (Placer County). Marshes were surrounded by unsuitable habitat, creating a patchy or metapopulation structure. We observed Black Rails nesting and found that they are year‐round residents. Assuming perfect detectability, we estimated a mean density of 1.78 rails/ha. Assuming a detection probability of 0.5, this estimate increases to a mean density of 3.55 rails/ha. The probability of detecting occupancy with a single call playback survey at a marsh was high (= 0.84), and the estimated proportion of marshes occupied (across all years) was 0.58. Irrigation ditches were the primary water source for 75% of the marshes with Black Rails. Our results indicate that Black Rails are more widespread in the Sierra foothills than previously known, and the foothills distribution appears to be discontinuous with populations in the San Francisco Bay‐Delta Estuary. Occupancy surveys may be an improved method for monitoring population trends of this secretive marsh bird where habitat patches are highly fragmented.

Conservation Objectives for Wintering and Breeding Waterbirds in California's Central Valley

“…The mean size of sites where these rails occur in the Delta are larger (12-17 ha [30-42 acres]) than those where they are absent (6-8 ha [15-20 acres] (Aigner et al 1995;Richmond et al 2008). The rails' habitat in this area consists of discrete, persistent emergent marshes, particularly larger ones (> 0.25 acres [0.1 ha]) with gentle slopes, flowing water (<1.2 inches [3 cm] in depth), dense vegetation (> 60% cover), grazing at low to moderate intensities, and irrigation as the primary water source (Richmond et al 2008(Richmond et al , 2010(Richmond et al , 2012. Conservation recommendations focus on these habitat needs in these respective breeding areas in the Sacramento Valley, Sierra foothills, and the Delta (Table 7).…”

California black rails depend on irrigation-fed wetlands in the Sierra Nevada foothills

“…Concurrent with the habitat assessments, we conducted call-playback surveys between June 1 and Aug. 31 from 2002 to 2008, to determine black rail occupancy. We played recorded rail vocalizations at stations spaced 44 to 55 yards (40 to 50 meters) apart at each wetland (details in Richmond et al 2008). Sites were visited up to five times in 2002 and up to three times from 2003 to 2008 using a removal design: in each year, we did not revisit a site after we detected black rails.…”

“…Sites were visited up to five times in 2002 and up to three times from 2003 to 2008 using a removal design: in each year, we did not revisit a site after we detected black rails. The average probability of detecting occupancy at a site after multiple visits (three or five) was extremely high (0.99) from 2002 to 2006 (Richmond et al 2008).…”

“…It is the smallest rail in North America and has a patchy and poorly understood distribution. In western North America, it is found in saltwater, brackish and freshwater marshes along the Pacific coast from Bodega Bay to northwest Baja California, in the the San Francisco Bay-Delta Estuary (where it is most abundant), inland in small numbers in the Salton Trough and along the lower Colorado River, and in the northern Sierra foothills of Butte, Nevada, Placer and Yuba counties, where it was recently discovered (Aigner et al 1995;Conway and Sulzman 2007;Eddleman et al 1994;Evens et al 1991;Richmond et al 2008).…”

“…While information on the foothill population's distribution has been published (Richmond et al 2008), a detailed description of the habitats occupied by black rails in the foothills is lacking.…”