Home Range, Habitat Selection, and Movements of California Black Rails at Tidal Marshes at San Francisco Bay, California

Abstract.

Little is known about the movements and habitat selection of California Black Rails (Laterallus jamaicensis coturniculus) in coastal California. We captured 130 Black Rails, of which we radio-marked 48, in tidal marshes in San Francisco Bay during 2005 and 2006. Our objective was to examine their home ranges, movements, and habitat selection to improve the species' conservation. The mean fixed-kernel home range was 0.59 ha, the mean core area was 0.14 ha. Home ranges and core areas did not differ by year or site. Males had significantly larger home ranges and core areas than did females. All sites combined, Black Rails used areas with ≥94% total vegetative cover, with perennial pickleweed (Sarcocornia pacifica) the dominant plant. The rails' habitat selection varied by year and site but not by sex. A multivariate analysis of variance indicated that Black Rails selected areas with pickleweed taller and denser than average, greater cover and height of alkali bulrush (Bolboschoenus maritimus) and common saltgrass (Distichlis spicata), more stems between 20 and 30 cm above the ground, maximum vegetation height, and shorter distance to refugia. On average, Black Rails moved 27.6 ±1.8 (SE) m daily and 38.4 ± 5.5 m during extreme high tides. Understanding the California Black Rail's movements, home range, and habitat use is critical for management to benefit the species.

Resumen.

Se conoce poco sobre los movimientos y la selección de habitat de Laterallus jamaicensis coturniculus en la costa de California. Durante 2005 y 2006, capturamos 130 individuos, de los cuales marcamos 40 con radiotransmisores, en las marismas en la Bahía de San Francisco. Nuestro objetivo fue examinar sus ámbitos de hogar, movimientos y selección de habitat para mejorar la conservatión de la especie. El ámbito de hogar promedio de kernel fijo fue de 0.59 ha y el área núcleo promedio fue de 0.14 ha. Los ámbitos de hogar y las áreas núcleo no difirieron entre años ïsitios. Los machos tuvieron ámbitos de hogar y áreas núcleo significativamente mayores que las hembras. Considerando todos los sitios combinados, L. j. coturniculus usó areas con >94% de cobertura de vegetación, siendo Sarcocornia pacifica la planta dominante. La selección de hábitat de L. J. coturniculus varió entre años y sitios, pero no entre sexos. Un análisis multivariado de varianza indicó que L. J. coturniculus seleccionó áreas con plantas de la especie S. pacifica más altas y densas, con mayor cobertura y altura de las plantas Bolboschoenus maritimus y Distichlis spicata, con más tallos entre 20 y 30 cm sobre el suelo, con máxima altura de la vegetación y menor distancia a los refugios que el promedio. En promedio, los individuos de L. J. coturniculus se movieron 27.6 ± 1.8 (EE) m diariamente y 38.4 ± 5.5 m durante las mareas altas extremas. El conocimiento de los movimientos, ámbito de hogar y uso de hábitat de L. J. coturniculus es crucial para el manejo en beneficio de esta especie.

Introduction

The California Black Rail (Laterallus jamaicensis coturniculus) is one of the least observed bird species in North America. Its secretive nature has contributed to inadequate understanding of its ecology. More than 80% of the Black Rail population in the western United States resides year-round in the San Francisco Bay area (Manolis 1978, Evens et al. 1991) in the high-elevation zones of tidal salt marshes, defined as the area between mean higher high water and the highest margin of the marsh (Goals Project 1999). The population is estimated at 7100 individuals in northern section of the bay area and 7200 in the western Sacramento—San Joaquin River delta (Evens and Nur 2002). The remaining populations on the outer coast, Sierra Nevada foothills, and southeastern California and western Arizona are small, isolated groups that may be vulnerable to extirpation (Evens et al. 1991). Historically, Black Rails were found throughout the bay area (Wilbur 1974), but recent surveys show breeding populations are largely confined to the northern region and nearly absent from the southern reaches of the San Francisco Bay estuary, likely because of the lack of adequate high-elevation tidal marshes (Evens et al. 1989, Nur et al. 1997, Spautz et al. 2005). The remaining breeding populations in the northern bay area are in decline and confined to contiguous, relatively undisturbed mature tidal marshes (Evens et al. 1991, Nur et al. 1997); therefore, it is critical for managers to understand how Black Rails use these habitats.

The San Francisco Bay estuary encompasses 88% of all tidal marshlands in California (Nichols et al. 1986). However, it has lost nearly 80% of the historic tidal marshes that provide critical habitat for species protected by federal or state laws (Goals Project 1999). This habitat loss has substantially reduced the distribution and abundance of several tidal-marsh vertebrates including the Black Rail, listed as threatened under California state law (Evens et al. 1991, California Department of Fish and Game 2007). A consensus of scientists and managers recommended increasing tidal marsh habitat in the bay area by restoring at least 22 000 ha for the benefit of native wildlife (Goals Project 1999).

Previous studies of the Black Rail on the west coast have used call—response methods to estimate its distribution, abundance, and habitat use (Evens et al. 1991, Evens and Nur 2002, Spautz et al. 2005), but little is known about the movements of individuals. The purpose of our study was to examine the Black Rail's movements and habitat selection to better understand its habitat requirements. Our objectives were to estimate the rails' home ranges (defined as the area an individual covers in its daily activity), habitat selection, and movements.

Study Area and Methods

Study Area

We studied Black Rails at three high-elevation tidal-marsh sites 3–7 km apart along the Petaluma River in the northwestern portion of the San Francisco Bay estuary (Fig. 1). We assumed these sites were independent of each other because the home range estimated for the Black Rail in Florida is small (0.6–1.3 ha; Legare and Eddleman 2001). Gambinini Marsh (38° 12′ N, 122° 35′ W) is located 5.3 km south of the city of Petaluma; its salinity is the lowest of the three sites. Mid-Petaluma Marsh (38° 11′ N, 122° 33′ W) is located in the middle stretch of the river and is part of the largest contiguous tidal marsh in the bay area (Balling and Resh 1991). Both Gambinini Marsh and Mid-Petaluma are ancient, high-elevation tidal marshes (Atwater 1979, San Francisco Estuary Institute 2000). Black John Slough (38° 08′ N, 122° 31′ W) is located closest (3.5 km) to the mouth of the river, is a historically younger high-elevation marsh, is slightly lower in elevation than the other sites, and is thus flooded more frequently during high tides than the other two sites.

Capture and Radio Marking

We captured 130 Black Rails in spring (10 March—25 April 2005 and 6 March—13 April 2006) and summer (12–28 July 2005 and 10–25 July 2006). Our efforts at capture began approximately 30 min after sunset. An outstretched rope lightly weighted with gravel in 0.5-L plastic containers was dragged over the vegetation to flush Black Rails without damaging the vegetation, while personnel with high-intensity spotlights followed behind. When a rail was detected, a long-handled dip net was carefully placed over the vegetation where the bird was illuminated (K. Popper, The Nature Conservancy, pers. comm.). We marked each captured Black Rail with a uniquely numbered metal U.S. Geological Survey (USGS) leg band and then measured the bird's mass (g), wing chord, culmen, and tarsus length (mm). We determined sex and age from plumage characteristics (Eddleman et al. 1994, Pyle and Howell 2008). When plumage was not definitive, we collected a small blood sample for DNA-based sex determination (Zoogen Inc., Davis, CA).

In spring 2005 and 2006, we fitted 48 adult Black Rails with 0.9-g radio transmitters with anterior and posterior suture channels (model BD-2, Holohil Systems, Ltd., Carp, Ontario, Canada). The average mass of an adult Black Rail was 29.3 g, so a radio transmitter constituted on average 3.07% of an individual's body mass (Warnock and Warnock 1993); we did not mark gravid females, identified by swelling of the lower abdomen and around the cloaca. We attached transmitters with cyanoacrylic glue (Loctite 224, Henkel Corp., CT) and absorbable sutures (4–0 PDS II, Ethicon, Somerville, NJ) anchored at the anterior and posterior ends of the transmitter according to methods described by Martin and Bider (1978), Wheeler (1991), and Robert and Laporte (1999). To ensure ease of movement after the transmitter was attached, we monitored radio-marked individuals briefly then released them at the site of capture.

We captured and marked Black Rails under a California Department of Fish and Game memorandum of understanding with USGS scientific collection permit SC-801158-03, U.S. Fish and Wildlife Service permit 22911, and with approval from the USGS Western Ecological Research Center Animal Care and Use Committee.

Radio Telemetry

After a 24-hr adjustment period, we located radio-marked Black Rails with a hand-held three-element Yagi antenna and receiver (Model R4000, Advanced Telemetry Systems, Inc., Isanti, MN) by following the homing method described by White and Garrott (1990). Observers used triangulation to approach within 3–5 m of a radio-marked individual and recorded a GPS reading to estimate its location without flushing it (Garmin GPS 60, WAAS enabled, accuracy ≤3 m, Garmin, Ltd., Olathe, KS). We located birds twice daily between dawn and dusk until transmitters failed ( battery life = 42 days) or fell off the bird ( days marked = 17 days). Tides rarely rose high enough to flood the marsh-plain vegetation, so we did not time tracking sessions to coincide with tides. Instead, we categorized radio-tracking sessions as crepuscular (0.5 hr before to 1 hr after sunrise and 1 hr before to 0.5 hr after sunset) or daytime. Flores and Eddleman (1995) reported Black Rails to be inactive at night, so we rarely tracked after dark. We used only locations separated by >1 hr to reduce any potential autocorrelation among locations (White and Garrott 1990), and 78% of the locations were separated by >3 hr. For each observation, we recorded tide height on the Petaluma River at Lakeville. At each location where we detected a rail, we used a 0.25-m² quadrat frame to visually estimate the total percent vegetative cover as well as the percent cover of each plant species present (Elzinga et al. 1998) and measured maximum height of each species with a pole 1 cm in diameter with 1-cm graduations. At each of these locations we also recorded water depth measured with a pole with 1-cm graduations. Distances to nearest refugia (levee or vegetation >1 m in height) were measured with aerial digital ortho-quarter quadrangle imagery, resolution 1 m, in a geographic information system (GIS; National Agricultural Imagery Program 2005, U.S. Department of Agriculture; ArcGIS 9.1, ESRI, Redlands, CA).

Statistical Analyses

Home ranges and movements. A home range is defined as “that area traversed by the individual in its normal activities of food gathering, mating, and caring for young” (Burt 1943). We used the fixed-kernel method to calculate home ranges (95% of the utilization distribution) and core areas (50%) with the likelihood-cross-validation (CVh) smoothing parameter (Horne and Garton 2006). The fixed-kernel method is preferred because it is a measurement of the intensity of use, excluding areas used minimally (White and Garrott 1990, Seaman et al. 1999). The CVh parameter is more appropriate than other smoothing parameters for estimating home ranges from sample sizes of ≤50 locations (Horne and Garton 2006). We calculated CVh with Animal Space Use 1.1 Beta (Home and Garton 2007) and estimated the size of each radio-marked bird's home range and core area in ArcGIS version 9.1 (ESRI, Inc., Redlands, CA) with Home Range Tools for ArcGIS (Rodgers et al. 2005). To determine the minimum number of locations necessary to calculate a home range, we examined the effect of sample size on home-range size. We calculated home ranges with an increasing number of randomly selected observations and plotted home-range size by number of observations. We found that 10 observations was the minimum sample size beneath which fewer locations affected the estimate, so our analyses include home-range estimates only for individuals with >10 observations. We also calculated home ranges as 95% minimum convex polygons with Home Range Tools and report mean home-range size so our results can be compared to those of other studies of the Black Rail (Flores and Eddleman 1995, Legare and Eddleman 2001).

We estimated the average distance (m) rails moved between daily observations and compared movement distance during normal water levels to those during extreme high tides in the spring of 2006 (observed water level at Mare Island >1.8 m, NAVD 88; NOAA 2007). We used three-way ANOVA to compare home-range and core-area overlap and movement distance by site, sex, and water level (normal or extreme).

Habitat Selection. To examine habitat selection, we compared habitat measurements at randomly selected locations within Black Rails' home ranges (hereafter, “used locations”) with randomly selected locations outside of Black Rail home ranges within the same study site (hereafter, “unused locations”). Unused locations were >100 m from home ranges, in areas where no individuals were detected during efforts at capture. Used and unused locations were determined by GIS (Spatial Analyst, ArcGIS 9.1, ESRI, Inc., Redlands, CA). In 2005, we sampled one pair of used and unused locations per individual. In 2006, to improve spatial resolution, we sampled three paired locations per individual. The three used locations were randomly selected from within home ranges; the three unused locations were located within a 50-m radius of a central point ≥100 m from home ranges. We used a 0.25-m² quadrat at each location to estimate percent cover of each plant species, total percent cover of all vegetation, maximum height of each plant species (cm), maximum height of all vegetation (cm), depth of dead pickleweed lying horizontally above the ground (cm), and count of rooted stems of each plant species. In the center of the quadrat, we inserted a graduated 1-cm-diameter pole vertically from ground level to the top of the vegetation and counted the number of times that stems intercepted the pole within 10-cm vertical intervals (0–10, 11–20, 21–30, 31–40, and ≥41 cm; Spautz and Nur 2002). We measured distance from each location to the nearest refugium (levee or taller vegetation >1 m) in GIS.

Prior to the habitat-selection analyses, we used correlation and principal-component analyses to identify pairs or clusters of highly correlated variables that could contribute to multicollinearity, which would cause difficulty in assessing the effects of specific variables (PROC CORR and PROC PRINCOMP; SAS Institute 2004, Khattree and Naik 2000). We found that the percent cover of a plant tended to correlate strongly with that species' height. Therefore, for four plants (alkali bulrush, Bolboschoenus maritimus; saltgrass, Distichlis spicata; alkali heath, Frankenia salina; and marsh gumplant, Grindelia stricta), we combined maximum height and percent cover by multiplying the two variables to obtain a single variable representing the species' volume. We tested 13 additional variables in our habitat-selection analyses: percent cover, maximum height, count of rooted stems of live pickleweed, depth of the dead pickleweed layer, count of all plant stems contacting a 1-cm-diameter pole in five 10-cm vertical intervals above the ground, maximum height (cm), total percent cover of all vegetation, water depth (cm), and distance to nearest refugium (m). The remaining plant species encountered were generally uncommon and categorized as “other.”

The multivariate nature of the response variables for habitat selection and the replication of three paired samples per individual required that we use a combination of MANOVA and univariate ANOVA with repeated measurements to determine evidence of selection by year, site, or sex. We calculated the differences in measurements for each pair of used and unused locations. If the differences were significantly different from zero, we assumed a selection effect. If the differences varied by year, site, or sex, then this suggested that the selection effect varied by those factors. We used MANOVA to test overall selection on the basis of the average of the three paired differences for each individual Black Rail (PROC GLM; SAS Institute 2004, Khattree and Naik 1999). If MANOVA results were significant, we tested each habitat-selection variable univariately with ANOVA. Since there were three replicates per individual, we applied a repeated-measures ANOVA by including the band number as a random effect (PROC MIXED; SAS Institute 2004, Littell et al., 1996). All results are presented as means () ± standard error (SE).

Results

We obtained on average 24.5 ± 1.2 radio-telemetry locations from each radio-marked Black Rail. Daily movement averaged 27.6 ± 1.8 m (Table 1). Daily movement did not differ by year (F_1,41 = 1.37, P = 0.248), site (F_2,40 = 1.41, P = 0.257) or sex (F_1,41 = 0.21, P = 0.646). Black Rails moved farther between successive observations during times of high water in the spring of 2006 than during normal tidal fluctuations (F_1,76 = 4.66, P = 0.034).

Common vegetation at Black Rail locations included pickleweed, alkali bulrush, saltgrass, marsh gumplant, and alkali heath. Coyote brush (Baccharis pilularis), marsh jaumea (Jaumea carnosa), perennial pepperweed (Lepidium latifolium), curly dock (Rumex crispus), seaside arrowgrass (Triglochin maritima), and saltmarsh dodder (Cuscuta salina) occurred at low frequency and are pooled as “other forbs” in Table 2. Total percent cover was 94.0 ± 1.1%, and at all sites Black Rails used areas dominated by pickleweed ( = 69.5 ± 0.8%). Overall use of habitat differed by year (F_17,21= 5.28, P = 0.002) and site (F_34,42 = 8.04, P < 0.001) but not by sex (F_17,21 = 1.54, P = 0.173).

Home Range, Core Area, and Overlap

Forty radio-marked Black Rails for which we had >10 observations were included in our home-range analyses. Overall, kernel home ranges averaged 0.59 ± 0.05 ha, core areas 0.14 ± 0.01 ha. Males had larger home ranges than did females (F_1,39 = 3.96, P = 0.05), but core-area size did not differ (F_1,39 = 3.82, P = 0.06; Table 3). Home ranges and core areas did not differ by year (F_1,39 = 0.49, P = 0.49) or site (F_2,38 = 0.74, P = 0.48). Overall, minimum-convex-polygon home ranges averaged 0.26 ± 0.03 ha.

Habitat Selection

Results of the full MANOVA model, which included all dependent and independent variables, indicated that Black Rails generally selected areas closer to refugia (F_3,74 = 10.20, P < 0.0001), with greater volume of alkali bulrush (F_3,74 = 4.91, P = 0.004) and alkali heath (F_3,74 = 2.93, P = 0.039), deeper layers of dead pickleweed (F_3,74 = 5.64, P = 0.002), and greater cover of pickleweed (F_3,74 = 3.08, P = 0.033; Table 4). Habitat selection was not affected by year (Wilks' λ = 0.93, F_11,84 = 0.54, P = 0.92) or sex (Wilks' λ = 0.81, F_11,84 = 1.65, P = 0.15), but it was affected by site (Wilks' λ = 0.42, F_26,164 = 3.38, P < 0.0001).

Black Rails selected areas with greater depth of dead pickleweed (t = -3.90, P = 0.0002) and greater volume of alkali heath (t = -2.66, P = 0.01) at Mid-Petaluma than at other sites. At Black John Slough the birds selected for deeper water and greater volume of alkali bulrush more than at Gambinini Marsh (water depth: t = 2.63, P = 0.010; alkali bulrush: t = 2.17, P = 0.03) and Mid-Petaluma (water depth: t = 3.10, P = 0.003; alkali bulrush: t = 3.17, P = 0.002). At Gambinini Marsh the birds selected areas closer to refugia more strongly than those at Mid-Petaluma (t = -2.39, P = 0.019) or Black John Slough (t = 5.90, P < 0.0001), and at Mid-Petaluma the birds selected areas closer to refugia more strongly than did those at Black John Slough (t = 4.63, P < 0.0001). Compared to males, females selected areas with shallower water (t = -3.05, P = 0.003) and shorter maximum vegetation height (t = -2.64, P = 0.0098).

In general, Black Rails selected for several habitat variables (Table 4). Pickleweed height was significantly greater at used locations than at unused locations (t = 3.33, P = 0.0012). Used locations had greater volume of alkali bulrush (t = 4.07, P = 0.0002) and saltgrass (t = 4.11, P < 0.0001), maximum height of vegetation (t = 2.08, P = 0.04), and stem count between 20 and 30 cm (t = 2.63, P = 0.01) than unused locations. Black Rails also selected sites with notably (but not significantly) greater counts of rooted stems of pickleweed (t = 1.93, P = 0.064) and shorter distances to refugia (t = -1.99, P = 0.084).

Discussion

At San Francisco Bay, Black Rails had remarkably small home ranges relative to those found in previous studies of the Black Rail and other Rallidae in North America. Average estimated home ranges of Black Rails were 1.7× larger in Arizona and 3.6× larger in Florida than those we measured (Flores and Eddleman 1991, Legare and Eddleman 2001). This marked difference in home-range size within the same species has several plausible explanations. The eastern subspecies of the Black Rail (Laterallus j. jamaicensis) is larger than the California Black Rail (Eddleman et al. 1994), and the greater energetic demands of a larger body may require the eastern subspecies to travel farther when foraging. At San Francisco Bay the risk of predation may constrain Black Rails' movements to small areas near refugia (Evens and Page 1986), or perhaps the nutritive value of rail's food in San Francisco Bay's tidal salt marshes is high, allowing distances traveled for foraging to be shorter.

Another explanation is that at San Francisco Bay the home ranges of Black Rails are small because of the limited availability of preferred habitat. At San Francisco Bay, Black Rails reside in wetlands with greater tidal flooding, whereas the other populations studied occupy areas with little to no tidal flooding, including freshwater marshes (Eddleman et al. 1994). Within tidal marshes, suitable nesting habitat free from regular flooding is often much less extensive than the habitat adult birds use during daily foraging, which may lead to more individuals aggregating in a small area (Post 1974, Greenberg et al. 2006). Black Rails' home ranges may be larger outside of the breeding season. Studies of other Rallidae have demonstrated larger home ranges when adults are not tied to nest locations (Bookhout and Stenzel 1987, Conway 1990); however, in Arizona, where water levels remain steady throughout the year, Black Rails' home ranges did not vary by season (Flores and Eddleman 1991). Further study of the Black Rail's space use at San Francisco Bay through the year is warranted.

Sex Differences

The home ranges of males were 46% larger than those of females, but the size of the core area did not differ, indicating that males range farther from the nest than do females. Both sexes spend the majority of their time in a restricted area, presumably near nest sites. However, the mean distance the birds moved daily did not differ by sex. In the Black Rail the sexes share in incubation and chick rearing, but females spend more time carrying out these duties (Eddleman et al. 1994). Previous studies of the Rallidae have documented sexual differences in home-range size (Bookhout and Stenzel 1987, Flores and Eddleman 1991, Legare and Eddleman 2001). Legare and Eddleman (2001) found that in Florida male Black Rails had home ranges 91% larger than those of females. For the Yellow Rail, Bookhout and Stenzel (1987) reported that breeding males had home ranges 5.5 times larger than those of females. For Black Rails in the Petaluma River the difference in homerange size between the sexes is much smaller than reported by other studies, possibly because abundant food allows males to travel shorter distances when foraging.

Implications for Wetland Restoration

All home ranges of the Black Rails we located in high-elevation tidal marshes. Yet 84% of the San Francisco Bay area's historic high-elevation tidal marshes have been lost; this habitat currently constitutes only 6% of the area's wetlands (San Francisco Estuary Institute 2000). Black Rails' home ranges were very small and their site fidelity was strong (unpubl. data), suggesting that availability of their preferred habitats may be limited. All Black Rails located were on the marsh plain or on channel or levee vegetation, but never within channels (Fig. 2). They used areas dominated by native vegetation, primarily pickleweed, mixed with alkali bulrush and saltgrass. Black Rails selected areas near potential high-tide refugia with taller perennial pickleweed, greater density of stems midcanopy, maximum height of vegetation, volume of alkali bulrush and saltgrass, and, although not quite significantly, density of rooted pickleweed stems.

Our results indicate that the Black Rail's habitat selection is best explained by habitat structure (height, count of vertical and rooted stems, percent cover) rather than species composition of plants. Previous studies of the Black Rail (Rundle and Fredrickson 1981, Conway 1990, Flores and Eddleman 1995) and other birds that breed in tidal marshes (Gjerdrum et al. 2005) also suggested that structural characteristics are better predictors of habitat selection for these species. Since Black Rails may select nesting sites on the basis of their suitability for avoiding tidal flooding and escaping extreme high tides, vegetation structure that allows nests to be built high enough to avoid flooding by high tides is likely a critical habitat element.

Differences among sites in habitat selection are probably responses to the salinity gradient, marsh elevation, and hydrological effects on plant species composition and structure. Gambinini Marsh and Mid-Petaluma Marsh are ancient highelevation tidal marshes (Atwater et al. 1979, San Francisco Estuary Institute 2000). Most individuals detected at these sites were found adjacent to channel edges with taller vegetation (coyote brush and marsh gumplant) or levees. Black John Slough is lower in elevation, so the marsh plain is flooded more regularly, has a well-developed network of channels, and is bounded by a levee on the landward edge of the site. Most Black Rails were detected within 100 m of this levee. Breeding Black Rails were more concentrated at Black John Slough than at the other sites (Fig. 2). Home ranges overlapped more there than at Mid-Petaluma or Gambinini Marsh, but corearea overlap did not vary by site, indicating that regardless of population density, Black Rails may defend small areas surrounding their nests.

When capturing Black Rails, we observed that they appear to use various tidal marshes differently at different seasons (Fig. 3). All recaptures were at the same site as initial captures, and the 2005 and 2006 home ranges overlapped greatly, indicating that adult Black Rails may maintain strong site fidelity within the breeding season as well as from season to season and year to year. At Gambinini Marsh we detected very few Black Rails in the spring, but in the summer we observed an increase in numbers of both adults and postfledgling juveniles, indicating that although this site may not be used for breeding, it could be a site of post-fledging dispersal or perhaps a population sink (Pulliam 1988). At Mid-Petaluma high detection rates in both the spring and summer suggest that this site is important throughout the breeding and post-breeding period. Several pre-fledgling juveniles were captured at this site in late July of both years. At Black John Slough capture rates in the spring were high, indicating that this site was important for breeding, but in the summer capture rates were lower than at the other two sites, and few juveniles were captured. Juvenile Black Rails may disperse away from Black John Slough to avoid the midsummer floodtide cycle at this lower-elevation site, or they may go to potentially less desirable habitat where competition is less. Since Black Rails appear to use different marshes at different seasons, wetlands should be managed on a regional scale to provide a matrix of habitat types throughout the year.

At San Francisco Bay, previous studies using call-response protocols and nest searches to detect birds and examine habitat preferences found the Black Rail's presence was positively correlated with the alkali heath (an indicator of high elevation within a marsh) and negatively correlated with the amphipod, Traskorchestia traskiana (an indicator of low elevation; Evens and Nur 2002). Although we did not use these same measures, our radio-telemetry data similarly indicate that Black Rails use high-elevation regions of tidal marshes. Spautz et al. (2005) also found that larger marshes, higher proportion of pickleweed, denser vegetation at ground level, and a low degree of urbanization in the surrounding area were associated with presence and nests of the Black Rail. We did not examine marsh size or adjacent land use, but we found that Black Rails selected areas with a notably greater density of rooted pickleweed stems, and all three study sites were part of large, intact marshes without any adjacent urban areas.

Importance of High-Tide Refugia

We found that Black Rails tended to select areas closer to refugia such as levees with upland vegetation or taller vegetation along channel edges. Movements during extreme high tides further demonstrate the value of these refugia to Black Rails. During the study, in 2006 rainfall as measured at nearby Mare Island (23.9 cm) was more than twice that in 2005 (11.2 cm; California Department of Water Resources 2007). The region experienced extensive flooding in the winter and spring of 2006. During extreme high tides in the spring of 2006, when the marsh was often flooded by >1 m of water, Black Rails moved significantly farther between successive observations than they had in 2005. During these extreme tides, Black Rails were observed more often in upland or taller vegetation, such as coyote brush, marsh gumplant, and vegetation along the levees, probably taking refuge from predators. Depredations of radio-marked birds (n = 4) were observed only during extreme high tides when the marsh vegetation was completely inundated.

We suggest that upland vegetation adjacent to tidal marshes and tall vegetation >1 m within marshes (i.e., marsh gumplant and coyote bush) are important habitat elements providing refuge when high tides inundate the marsh. Previous studies around San Francisco Bay also have documented movements of Black Rails and other salt-marsh birds toward upland refugia and increased predation during extremely high tides (Sibley 1955, Evens and Page 1986). This movement, along with the Black Rail's occupancy of very different types of wetlands across North America, indicates that although this species has some plasticity in the specific wetland plants it uses, factors such as well-developed habitat structure, adjacent upland refugia, and wetland inundation are critical to Black Rail populations.

Contaminants and Other Threats

Around San Francisco Bay, the Black Rail population is threatened by increased isolation of tidal marshes as commercial and residential developments expand around the bay's periphery (Nur et al. 1997). Consequently, upland transitional environments used by Black Rails as high tide refugia have been reduced (Evens et al. 1991), potentially increasing the birds' exposure to predation (Evens and Page 1986, Eddleman et al. 1994). The threat of sea-level rise may further reduce the habitat this species requires (Simas et al. 2001, Takekawa et al. 2007) since most of San Francisco Bay's tidal marshes are restricted by levees and urbanized areas, which will impede tidal marshes from transgressing into uplands as water levels rise. In addition, exposure to contaminants such as methylmercury may degrade existing habitat of tidal marsh vertebrates by reducing reproductive success (Schwarzbach et al. 2006). At San Francisco Bay sediments in tidal marsh plain have higher loads of methylmercury than do other habitats within the estuary (Marvin-DiPasquale et al. 2003). Small home ranges and site fidelity make the Black Rail a good indicator species for studying the effects of contaminants such as methylmercury in San Francisco Bay's tidal marshes because its exposure is at a local scale.

The cumulative effects of these factors may put the remnant Black Rail populations at San Francisco Bay at risk of further declines or hinder population recovery as planned restoration of tidal marshes increases available habitat. Understanding the movements and habitat selection of the Black Rail will better inform management and conservation efforts for this species.

Acknowledgments

This study was supported by a Calfed Bay—Delta Program grant (ERP02D-P62) to the San Francisco Estuary Institute (D. Yee, J. Collins, J. Hunt), the USGS Student Career Experience Program, the U.S. Fish and Wildlife Service's San Francisco Estuary Program (R. Morat), the USGS Western Ecological Research Center and its Davis and San Francisco Bay Estuary field stations, and the University of California at Davis John Muir Institute for the Environment. We thank S. Schwarzbach for initial conception of the project and advice on study design. We thank C. Wilcox and L. Wyckoff (California Department of Fish and Game), B. Salzman (Marin Audubon), and R. Phelan and S. Brand (Gambinini Marsh) for permission to access study sites. We thank the California Department of Fish and Game (D. Steele) for assistance with permits. We especially thank field technicians K. Spragens, K. Thorne, O. Bernstein, and L. Dembosz. We thank A. K. Miles, M. Marvin-DiPasquale, K. Popper, M. Ricca, S. Spring, J. Ackerman, J. Bluso, S. Demers, R. Melcer Jr., and several volunteers for valuable assistance or input. The manuscript was improved by comments from A. K. Miles, M. Mueller, D. Van Vuren, and two anonymous reviewers. Mention of trade names is for descriptive purposes only and does not imply endorsement by the U.S. government.

Literature Cited