Greater Sage-Grouse Nest Predators in the Virginia Mountains of Northwestern Nevada

Range-wide declines in greater sage-grouse Centrocercus urophasianus, hereafter sage-grouse, populations (U.S. Fish and Wildlife Service 2010) point to a need to better understand sage-grouse reproduction and factors that influence reproductive rates. Nest survival is a central component of reproduction, and nest failure may limit sage-grouse populations (Bergerud and Gratson 1988; Schroeder 1997; Schroeder and Baydack 2001). Nest survival explains more variation in sage-grouse population growth rates than any other vital rate (Taylor et al. 2012). Nest depredation represents approximately 94% of sage-grouse nest failures (Moynahan et al. 2007), suggesting that variation in abundance and species of nest predators among areas influences sage-grouse population size (Bergerud and Gratson 1988; Schroeder and Baydack 2001; Beck et al. 2006).

Identification of sage-grouse nest predators based on diagnostic remains at the nest (Holloran and Anderson 2003; Moynahan et al. 2007) and direct identification (Coates et al. 2008) indicates that sage-grouse nests are subject to a wide range of nest predators. Unfortunately, predator identification based on nest and egg remains after nest depredation is subject to considerable error (Marini and Melo 1998; Lariviére 1999; Coates et al. 2008). The use of continuous video monitoring (Coates et al. 2008; Bell 2011) and remote digital cameras (Holloran and Anderson 2003) has increased our understanding of sage-grouse nest predators. Video recordings of sage-grouse nest depredation indicate that female sage-grouse do not defend nests successfully upon discovery by meso-predators (i.e., American badgers Taxidea taxus, striped skunks Mephitis mephitis, common ravens Corvus corax), the only type of predator so far unambiguously identified depredating sage-grouse nests (Coates et al. 2008; Bell 2011). Video recordings of sage-grouse nest depredations also have clarified previous hypotheses regarding identity of sage-grouse nest predators originally formed from observations of nest remains. Research that identifies sage-grouse nest predators and estimates the timing and occurrence of nest depredation could contribute substantially to management and conservation decisions for sage-grouse populations. For example, the probability of a predator detecting a sage-grouse nest is often influenced by the quantity and quality of concealment cover around the nest (Schroeder and Baydack 2001; Coates and Delehanty 2010; Hagen 2011). Implementing targeted habitat management to improve concealment cover for nesting sage-grouse will be significantly more effective if managers know what the predator types are, when depredations occur, and at what frequency they occur.

Range-wide sage-grouse populations are exposed to a suite of predator communities, the composition of which varies among regions. Our goal was to use video monitoring to identify sage-grouse nest predators on the western edge of sage-grouse distribution where western Great Basin and eastern Sierra Nevada ecosystems meet and where habitat features and predator communities differ from the interior of the Great Basin. We deployed continuous video-recording systems at sage-grouse nests from 2009 to 2011 in the Virginia Mountains of northwestern Nevada, USA, an area with a sage-grouse population that breeds at relatively high elevation and occupies the eastern flank of the Sierra Nevada Mountains on the western edge of historic sage-grouse range.

This study area consisted of a topographically complex sagebrush–steppe ecosystem in the Virginia Mountains of northwestern Nevada (Figure 1), an area encompassing approximately 676 km² with elevations ranging from 1,218 to 2,683 m. Mean annual precipitation was 18.8 cm, and temperatures ranged from 6.8 to 18.2°C from 2009 to 2011 (Western Regional Climate Center). The U.S. Department of Interior, Bureau of Land Management administered the majority of land (588 km²) in the study area, with the remaining portion owned privately (88 km²). The Pyramid Lake Reservation borders the eastern portion of the Virginia Mountains and California borders to the west. A sage-grouse hunting season existed until 2005, after which the season was discontinued by the Nevada Department of Wildlife due to declining sage-grouse numbers in the region. Cattle grazing occurred within sage-grouse nesting areas during the latter part of the nesting season each year.

The vegetation community within the study area reflected a response to a fire (Fish Fire) that occurred in 1999 and resulted in reduced shrub abundance and increased stands of cheatgrass Bromus tectorum. Lower elevation shrub communities were dominated by sagebrush Artemisia spp., with overstory primarily consisting of big sagebrush A. tridentata spp., Bailey's greasewood Sarcobatus baileyi, horsebrush Tetradymia spp., and several species of rabbitbrush Chrysothamnus spp. Higher elevation communities consisted of montane shrub complexes with big sagebrush, Saskatoon serviceberry Amelanchier alnifolia, snowberry Symphoricarpos albus, and antelope bitterbrush Purshia tridentata making up the common woody overstory species. Woolly mule's ear Wyethia mollis, lupine Lupinus spp., and arrowleaf balsamroot Balsamorhiza sagittata dominated the forb communities. Dominant grass species included bluebunch wheatgrass Pseudorogeneria cristatum, crested wheatgrass Agropyron cristatum, basin wildrye Leymus cinereus, needle-and-thread grass Hesperostipa comata, Indian ricegrass Achnatherum hymenoides, and cheat grass. Scattered stands of pinyon–juniper woodlands consisting of singleleaf pinyon Pinus monophylla and Utah juniper Juniperus osteosperma were found throughout the study area.

Over the course of this study, we observed several potential sage-grouse nest predators, including common ravens, American crows Corvus brachyrhynchos, black-billed magpies Pica hudsonia, American badgers, Great Basin gopher snakes Pituophis catenifer deserticola, coyotes Canis latrans, bobcats Lynx rufus, kit foxes Vulpes macrotis, striped skunks, and long-tailed weasels Mephitis frenata.

We captured female sage-grouse (n  =  72) at nocturnal roosting locations by using spotlights in concert with handheld nets attached to 3-m extension handles (Giesen et al. 1982; Wakkinen et al. 1992) and with handheld net launching devices (SuperTalon®, Advanced Weapons Technology, La Quinta, CA) during spring and fall 2008 to 2011. We equipped captured sage-grouse with 18–22 g (<3% body mass; Schroeder et al. 1999) necklace-style, battery-powered radiotransmitters with 22-cm antennas bent back along the contour of the body to reduce interference with flight (Advanced Telemetry Systems, Isanti, Minnesota). Capture, handling, and marking procedures were approved by the US Geological Survey Western Ecological Research Center's Animal Care and Use Committee. We classified captured sage-grouse as adult or yearling based on plumage characteristics of the 9th and 10th primaries (Eng 1955; Dalke et al. 1963). Sage-grouse were held for less than 30 min and were released at point of capture.

We relocated sage-grouse via telemetry using 3-element Yagi antennas and handheld receivers (Communication Specialist Inc., Orange, CA; Advanced Telemetry Systems). We circled sage-grouse while maintaining a 30- to 50-m buffer distance to minimize disturbance except when female sage-grouse were approached more closely during our efforts to confirm nesting status. We recorded sage-grouse locations as Universal Transverse Mercator data derived from handheld GPS devices. We attempted to relocate all female sage-grouse two or more times per week. Nests were located by visual searches after females were found in the same location on two consecutive relocation observations. Subsequent nest visits occurred every 3–4 d for the duration of that nest. Upon completion of a nest, we classified them as successful if one or more eggs hatched (Rearden 1951) as determined by visual assessment of eggshell remains or observing one or more chicks in the nest bowl (Table S1, Supplemental Material). Nests were considered to be unsuccessful when the entire clutch failed to hatch. We recorded depredated nests as partial depredation when one or more intact whole eggs remained in the nest bowl or as complete depredation when all eggs were destroyed or missing from the nest bowl. After depredation, we recorded scene characteristics such as nest bowl disturbance, vegetation disturbance, eggshell and egg membrane remains, and any other pertinent evidence potentially implicating predator type.

Sage-grouse nesting behavior was monitored and nest predators were identified through the use of continuous video-recording systems and camouflaged day/night micro bullet true color cameras (model ENC-100, EZ-Spy Cam, Los Angeles, CA). The cameras were equipped with eight light-emitting diodes producing 950-nm wavelength infrared illumination that is beyond the visible light spectrum for most vertebrates and sufficient for infrared-sensitive digital recording. Cameras were placed 0.5–1.0 m from the nest bowl and attached to existing vegetation when available or a camouflaged steel stake when vegetation was insufficient. Care was taken during camera placement to ensure that the entire nest was visible in the camera's field of view while avoiding disturbance to the nest and surrounding vegetation. Cameras were connected to single-channel micro digital video recorders (model MDVR14, SuperCircuits, Austin, TX) placed approximately 30 m from the nest. Cables were buried 3–5 cm in the ground. The camera and recorder were powered by two marine grade deep cycle 12-V batteries. Batteries, digital video recorders, and associated components were housed in weatherproof camouflaged boxes concealed under the canopy of a nearby shrub, approximately 30 m from the nest. Continuous images were recorded onto memory cards (16–32 GB) via digital video recorders that were set to record 3–4 frames/s. Frequency of our visits to nests was limited by battery life, not data storage. We approached each video-monitored nest every 3–4 d to replace batteries before depletion and also replaced memory cards. Nests that were not monitored with videography also were visited every 3–4 d (control) from approximately 30 m away to document nesting status and reduce bias in nest failure rate that could have resulted from a disparity between the number of nest visits for video and non-video-monitored nests. Because the frequency of nest visits by researchers was every 3–4 d, the time between nest depredation and nest visits varied from a few hours to as much as 4 d. During camera installations and nest visits, we wore rubber gloves, rubber boots, and used scent masking sprays to reduce the possibility of attracting or deterring predators (Whelan et al. 1994). We used vegetation mimicking that of the associated shrub–steppe microhabitat to camouflage camera and the storage box containing the digital video recorder, batteries, and other components. Researchers diligently watched for any potential predators during camera installations and nest visits. If any predators were detected, we postponed approaching nests to avoid drawing attention to sage-grouse nests that may influence probability of depredation (Vander Haegen et al. 2002).

We placed video systems at nests (n  =  39; Table S1, Supplemental Material) based on fewest estimated days of incubation from the nest initiation date, postponing installation until ≥3 days of incubation to reduce risk of female abandonment (Renfrew and Ribic 2003). Nest initiation date was estimated based on radiotelemetry monitoring. We installed cameras at all qualifying nests until all camera systems were deployed. Camera systems were moved to the next qualifying nest after nest cessation due to hatch or failure. Nests receiving cameras were randomly chosen and not selected based on nest accessibility. We were unable to install camera systems quickly enough during early dawn when females take a brief recess from incubation. Sage-grouse were incubating when we approached to install cameras, and we usually caused sage-grouse to flush. To reduce risks of abandonment and egg mortality, we refrained from camera installations during inclement weather (i.e., extreme ambient temperatures, precipitation, high winds). On average, we spent 25–30 min completing camera installations before vacating the nest site. After nest fate (i.e., successful, abandoned, or depredated), we continued to video monitor nests for up to 24 h to document any additional female behaviors or animal encounters at the nest site.

We estimated daily survival rate (DSR) and cumulative survival rate using the RMark package (R version 2.13, www.r-project.org; Laake and Rexstad 2007; Table S1, Supplemental Material) that implements Program MARK (White and Burnham 1999). We conducted the data analysis in three steps. First, we examined variation in DSR explained by year. We compared a model that included year as a group level factor to an intercept-only model. The most parsimonious model was used as a base model for subsequent analysis. If these data supported year as a group level factor, then we included this factor as an additive effect in successive models that also included other factors of interest. Second, we compared a model that consisted of a factor variable for first and second nests against the base model. The rationale for this step was to pool nest attempts if we did not find evidence of a difference or restrict the data set to first attempts only if a difference was supported. Third, we estimated differences between nests with and without cameras. In this analysis, we compared a model with group-level factor of camera to the base model. Because we postponed camera installation until ≥3 d of incubation to reduce risk of female abandonment, we similarly excluded non-video-monitored nests (n  =  15) under the same criterion until ≥3 d of incubation were achieved (Table S1, Supplemental Material). Thus, nests that failed between first and second nest visits (3–4 d) did not meet the standard for camera installation, and we did not include these nests relative to measuring any camera effect. To do so would have imposed bias because video-monitored nests, by design, could not have failed during early incubation. Nests without cameras that met the same criteria for nests with cameras (n  =  17; Table S1, Supplemental Material) served as controls. We calculated Akaike's Information Criterion ([AIC]; Akaike 1973) with second-order bias correction for small sample size (c; Anderson 2008) to evaluate support for each model. Model uncertainty was quantified by calculating differences between model AIC_c values (ΔAIC_c) and by comparing model weights (w_i).

Video monitoring identified ravens, American badgers, coyotes, long-tailed weasels, Great Basin gopher snakes, multiple rodent species, and a bobcat visiting sage-grouse nests, although not all of these species consumed eggs. Video monitoring also allowed us to observe total clutch depredation, partial clutch depredation, as well as successful hatches.

We monitored a total of 71 nests (n  =  18, 2009; n  =  20, 2010; n  =  33, 2011; Table S1, Supplemental Material) from 2009 to 2011. A total of 61 (n  =  15, 2009; n  =  18, 2010; n  =  28, 2011; Table S1, Supplemental Material) nests were first nesting attempts, and 10 nests (n  =  3, 2009; n  =  2, 2010; n  =  5, 2011; Table S1, Supplemental Material) were second nesting attempts. Cameras were installed on 39 nests (n  =  6, 2009; n  =  16, 2010; n  =  17, 2011; Table S1, Supplemental Material). Of these nests, 30 were first nest attempts (n  =  3, 2009; n  =  14, 2010; n  =  13, 2011; Table S1, Supplemental Material) and 9 were second attempts (n  =  3, 2009; n  =  2, 2010; n  =  4; 2011; Table S1, Supplemental Material). Nest abandonment occurred on seven (9.9%) occasions. Nest survival across all nests was 22.4% (95% CI, 13.0–33.4%) as follows: 2009, 7.4% (95% CI, 1.2–21.6%); 2010, 13.2% (95% CI, 3.1–31.1%); and 2011, 41.8% (95% CI, 22.3–60.3%). Nest initiation rate across all radio-marked females and years was 88.8 ± 0.10%. Mean clutch size was 7.19 ± 0.95, with mean clutch size for first and second nest attempts 7.13 ± 1.02 and 7.11 ± 2.37, respectively.

We recorded approximately 11,800 h of female incubation, an average of 12.6 (SE  =  2.02) days of video monitoring for each video-monitored nest. Predators were recorded at 17 nests. Fifteen (88.2%) of these nests were depredated and failed, whereas two (11.8%) nests were partially depredated and one or more eggs hatched after partial depredation. Successful hatching was recorded at 21 nests. Equipment failure occurred on three occasions, and nest fate was not recorded. Camera installation at nests did not cause nest abandonment insofar as recorded females returned to nests and resumed incubation in all cases after camera placement.

In step I of the analysis, we found year accounted for more variation in DSR (Table 1; AIC_c ω  =  0.93) compared with the intercept only survival model (Table 1; AIC_c ω  =  0.07). Therefore, year was included in all models as a fixed effect to account for interannual variation (Table 1). Also, the base model for steps 2 and 3 consisted of the factor year. In step II, model analysis did not support a difference in DSR between first and second nest attempts (Table 1; ΔAIC_c  =  1.90); thus, we pooled first and second nest attempts in our analysis to evaluate camera effects. In step III, we did not find support for an effect of camera presence (ΔAIC_c  =  1.79). The base model (ω  =  0.71) was 2.4 times more likely to describe DSR compared with the model including camera presence (AIC_c ω  =  0.29). Estimated cumulative nest survival for nests with cameras was 38.2% (95% CI, 21.7–54.6%) and without cameras was 36.3% (95% CI, 12.1–61.8%). The difference in variability between nest survival estimates for nests with and without cameras results from the added precision obtained from videography on exactly when a hatch or depredation occurred. Conversely, we were unable to determine the exact day that a hatch or depredation took place for nests without cameras, and we therefore selected the midpoint between nest visits (3–4 d) that increased variation in survival estimates. Estimated cumulative nest survival for all nests, which included 15 nests not available for camera analysis, was 22.4% (95% CI, 13.0–33.4%).

Common ravens (n  =  7 incidents of common ravens at sage-grouse nests) were the most frequent nest predator identified by video monitoring in our study and caused partial (n  =  3) and full (n  =  4) nest depredation. Common ravens were the only nest predator for which we observed complete egg removal, with no eggshell fragments or other remains left in the nest. In these cases, common ravens carried away whole eggs. After partial clutch depredations by common ravens, sage-grouse returned to their nests and on one occasion resumed incubation. Ultimately, all females abandoned the remaining eggs after partial depredation by common ravens. We did not observe female sage-grouse defending nests after discovery by common ravens, although the camera view was limited to the nest bowl and areas immediately adjacent to it. One common raven depredation occurred while the female was absent from the nest. The remaining depredations involved common ravens flushing the incubating female from the nest. In one situation, a common raven violently struck an incubating female and continued to harass the female beyond the nest bowl before removing eggs (Figure 2; Video S1, Supplemental Material). We could not determine conclusively whether common raven depredations occurred from one or multiple common ravens, but the rate of egg removal in some cases suggested that more than one common raven was involved in the depredation. Timing of common raven depredation occurred from 0706 to 1831 hours (i.e., during daylight hours).

Depredations by coyotes (Figure 3A) occurred on three occasions, each resulting in complete nest failure. All coyote depredations were nocturnal, taking place from 2131 to 2350 hours. In each case, incubating sage-grouse females flushed from the nest, escaping capture by coyotes, and did not attempt to defend nests. In two coyote depredations, eggshells were left mostly intact except for large holes in the sides of the shells, and the shells were scattered within a 10-m radius of the nest bowl. The third coyote depredation left two empty eggshells with holes in the sides, and the fragments of crushed eggs were within 5 m of the nest. Based on remains, it appeared that a few eggs were either consumed entirely or were carried away from the nest site. Egg contents were removed in all cases where egg remains were located.

We documented two American badger nest depredations (Figure 3B), and both resulted in complete nest clutch loss. Incubating sage-grouse females flushed from the nests at 0445 and 0544 hours, respectively, did not attempt to defend nests, and they were not captured by the badger. One American badger depredation left three crushed eggshells partially buried in the nest bowl and five eggshells with large holes in the sides or tips, and shells were scattered within 5 m of the nest bowl (Video S2, Supplemental Material). In the other American badger depredation, the predator consumed all but one egg during the night and then returned at 0804 hours and removed the remaining whole egg from the nest bowl. One empty eggshell with a large hole in the side was found within 1 m of the nest in addition to a crushed eggshell and eggshell fragments from other eggs. In both cases, numerous American badger digs were located around the periphery of the nest bowl, but no cached eggs were located.

One nest was depredated by a bobcat (Figure 3C; Video S3, Supplemental Material). At 0204 hours, the incubating sage-grouse flushed from the nest. The sage-grouse did not defend the nest and was not captured by the bobcat. The bobcat cautiously entered the view of the camera shortly after the sage-grouse flushed and meticulously consumed the contents of all eggs (n  =  8). After approximately 21 min, the bobcat left a neat, clean pile of crushed eggshell fragments inside the nest bowl. The nest bowl and surrounding vegetation were negligibly disturbed.

Long-tailed weasels were recorded at two sage-grouse nests sites, both of which led to partial depredations. At 0751 hours, a long-tailed weasel entered the camera view of one nest (Figure 4), and the incubating sage-grouse stood but did not leave the nest bowl area. The female sage-grouse appeared to be defending her nest, but during the encounter one egg from the clutch was moved beyond the camera's field of view. We could not determine whether the egg rolled out during the interaction or whether the weasel removed the egg. No egg remains were located near the nest site. The female sage-grouse resumed incubation after the encounter and continued to incubate for 18 more days before the nest failed due to depredation by an unknown predator.

The second long-tailed weasel depredation occurred at 0506 hours as eggs were hatching. The sage-grouse stood but did not flush and appeared to defend her nest. During the encounter, the long-tailed weasel was clearly visible, but we could not determine what, if anything, the predator took from the nest. Ultimately, the female sage-grouse left the nest, and our subsequent examination of nest remains identified one eggshell from a hatched egg and eggshell fragments from crushed eggshells. Subsequently, we located the female and found her brooding one chick. The remaining unhatched eggs in the nest were destroyed, perhaps trampled by the female sage-grouse during the encounter between the grouse and the long-tailed weasel. This was a successful nest because one or more eggs hatched (Rearden 1951) despite the partial depredation.

On two occasions, Great Basin gopher snakes entered sage-grouse nest bowls. On the first occasion (Figure 5A), during an incubation recess, a snake of approximately 1 m in length entered the nest bowl at 1320 hours and attempted to consume eggs (Figures 5B and 5C) for approximately 1 h, repeatedly mouthing eggs but not extending its gape over the eggs. Ultimately, the snake did not consume any eggs. After the snake left the nest, the sage-grouse returned 2 h later and resumed incubation. Ultimately, the female sage-grouse abandoned the nest approximately 7 h after the initial encounter and no eggs hatched. The second Great Basin gopher snake encounter occurred at 1111 hours after the hatching of four chicks. The female sage-grouse was incubating the remaining single egg before the arrival of a snake of approximately 1 m in length (Figure 6A; Video S4, Supplemental Material). During the interaction, the snake captured a chick (Figures 6B and 6C; Video S4, Supplemental Material), constricting the chick while fighting with the defending female sage-grouse (Figures 6B and 6C; Video S4, Supplemental Material). The female struck and pecked at the snake numerous times. The snake made strikes directed at the sage-grouse and the snake did not retreat. Eventually, the female left the nest bowl with the remaining three chicks (Figure 6D; Video S4, Supplemental Material). The snake consumed the constricted chick (Figure 6D) in the nest bowl and then attempted to consume the unhatched egg. The remaining three chicks left the nest bowl area with the female. The snake was unsuccessful in consuming the unhatched egg, seemingly due to insufficient gape width.

Many small rodents were documented visiting sage-grouse nests, including California ground squirrels Spermophilus beecheyi, least chipmunks Tamias minimus, Great Basin pocket mice Perognathus parvus, kangaroo rats Dipodomys spp., and other encounters with mice and voles that could not be identified to species via videography. Rodents were recorded at nest locations only while the female sage-grouse was absent from the nest during an incubation recess or after nest termination. Most encounters involved a quick dash through the nest bowl. Occasionally, small rodents fed on broken eggshells that remained in nest bowls after depredation or hatch. On two occasions, California ground squirrels visited nests after partial depredations where whole eggs were left in the nest bowl. These California ground squirrels were adept at manipulating sage-grouse eggs (Figure 7A; Video S5, Supplemental Material), but they were unable to bite into whole eggs (Figures 7B and 7C; Video S5, Supplemental Material), presumably due to a limited gape width. On rare occasion, these California ground squirrels appeared capable of removing eggs from the nest bowl. One California ground squirrel did access an egg after dropping the egg and breaking the shell. We did not document any complete destruction of nest remains by a rodent after a hatch or depredation that would have caused researchers to misclassify the fate of the nest. In all cases of successful nests, we were still able to find egg remains that clearly indicated a successful hatch, even after rodents had visited the nest post hatch. However, for nests without cameras we did not always know the precise number of hatched vs. depredated eggs if some of the egg remains were crushed or destroyed. No rodents were documented flushing female sage-grouse from sage-grouse nests.

Depredation was the primary cause of sage-grouse nest failure, and we observed avian, mammalian, and reptilian predators taking eggs or chicks at the nest. Common ravens were the most frequent sage-grouse nest predator in the Virginia Mountains, accounting for 46.7% of nest depredations. Common raven population size, density, and distribution have increased substantially across the western United States as a result of habitat conversion and human activities that act to subsidize common ravens with food and nesting opportunities (Sauer et al. 2004; Kristan and Boarman 2007; Bui et al. 2010; Howe 2012). For example, historically, the sagebrush–steppe ecosystem likely had relatively low common raven population densities (Leu et al. 2008); but currently, this ecosystem supports higher numbers of common ravens because of increased vertical perching and nesting substrates (e.g., electrical power line towers and other structures), as well as human-related food sources (e.g., roadkill and refuse; Boarman 1993; Sauer et al. 2004). The increase in common raven numbers within the sagebrush–steppe ecosystem is an important change because sage-grouse rely on visual concealment for nesting and common ravens rely on visual detection for hunting (Gregg et al. 1994; Conover et al. 2010). Common ravens are common in the Virginia Mountains, and our findings indicate that common ravens regularly are detecting and depredating sage-grouse nests.

The Virginia Mountains have been subject to disturbances from fire, agricultural practices, and renewable energy exploration that have led to a reduction in extent and quality of sagebrush habitat for nesting sage-grouse. The impacts of predators on prey populations may be elevated when the quality or quantity of habitat, or both, are degraded (Hagen 2011). This habitat degradation coupled with the presence of common ravens may explain why common ravens were the most frequent sage-grouse nest predator and thus the low overall nest survival (22.4%) in this area. In Wyoming, common raven densities were highest near sage-grouse nesting areas and areas with human activity (Bui et al. 2010). In northeastern Nevada, the probability of a sage-grouse nest being depredated by a common raven increased with less shrub canopy cover in the vicinity of the nest (Coates and Delehanty 2010). Furthermore, an increase in one common raven per 10 km was associated with a 7.4% increase in probability of nest failure (Coates and Delehanty 2010). In the Arco Desert of southeastern Idaho, USA, common raven occurrence and common raven nesting were strongly associated with the presence of artificial structures such as power line towers (Howe 2012).

Common ravens are not universally implicated as a major predator of sage-grouse nests. Some studies using direct identification of nest predators have not found common ravens to be a significant factor (Holloran and Anderson 2003; Bell 2011). Differences in common raven effects among sage-grouse populations could be the result of geographic location, behavioral plasticity of common ravens or sage-grouse, prey abundance, habitat characteristics, or monitoring techniques. Further research is needed to understand variation in sage-grouse nest depredation rates by common ravens, but the variation that has been documented helps to understand local dynamics when considering management intervention.

Coyotes (20.0%) and American badgers (13.3%) also were nest predators, occurring at frequencies similar to other published reports (Holloran and Anderson 2003; Coates et al. 2008; Bell 2011). Sage-grouse have been hypothesized to select nest sites with greater concealment from visual predators (birds) and not from olfactory predators (mammals), although rates of nest depredation by visual and olfactory predators were equal (Conover et al. 2010). Coyotes and American badgers consistently are identified as sage-grouse nest predators across studies but at rates lower than other nest predators, which may not warrant management concern.

This study represents the first confirmed bobcat depredation of sage-grouse nests. Bobcat depredations of sage-grouse nests likely occur at low frequencies, although bobcats are known to take sage-grouse chicks and adults (Nelson 1955; Hartzler 1974) and may leave diagnostic signs at nest sites (Holloran et al. 2005). During our study, we also documented mortality of a nesting adult sale-grouse adjacent to her nest bowl. Conspicuous bobcat tracks in the snow near the nest suggested that a bobcat killed the adult sage-grouse, and in this way, was indirectly associated with clutch loss.

Long-tailed weasel interactions differed from interactions with other predatory mammals in that incubating sage-grouse females actively defended their nests against weasel intrusion. One female was able to resume incubation and the other female departed with at least one hatched chick after taking initial defensive actions against the long-tailed weasel. These results, coupled with aggression directed toward long-tailed weasels at the nest, indicate that female sage-grouse can actively defend nests against some nest predators. There is little doubt that long-tailed weasels are adept at taking young sage-grouse chicks, but these may be opportunistic depredations considering long-tailed weasels' primary prey consists of voles and mice (DeVan 1982).

Although multiple rodent species were observed visiting sage-grouse nests, we did not observe a rodent flush an incubating sage-grouse nor did we observe a rodent capable of biting open an intact sage-grouse egg. These results are consistent with previous findings from camera or video recordings involving rodents at sage-grouse nests (Holloran and Anderson 2003; Coates et al. 2008; Bell 2011). Rodents appeared to be unable to access intact sage-grouse eggs through biting, probably limited by their gape width (Michener 2005). On this basis, a rodent sign at sage-grouse nests does not demonstrate that rodents caused nest failure, especially given the propensity of rodents to scavenge at previously depredated nests. California ground squirrels are relatively large with forelimb dexterity that allowed them to lift sage-grouse eggs, but even the California ground squirrels appeared to be unable to bite into intact eggs. Similar to rodents, Great Basin gopher snakes were unsuccessful at consuming intact sage-grouse eggs, seemingly because of inadequate gape width. Inability of gopher snakes to consume sage-grouse eggs has been observed previously in two other sage-grouse populations within the Great Basin (Coates et al. 2008; Bell 2011).

We did not detect an effect of camera presence on DSR for sage-grouse nests in the Virginia Mountains. These results closely follow the results found by Coates et al. (2008) in northeastern Nevada, USA, using similar techniques. Cumulative nest survival was higher for monitored nests (video-monitored nests, 38.2%; non-video-monitored nests, 36.3%) considered in this analysis compared with cumulative nest survival for all nests (22.4%). But to be a monitored nest meant that the nest had to survive ≥3 d of incubation. Fifteen nests were located but did not survive to 3 d of incubation, the starting point for comparing video-monitored and non-video-monitored nests.

In summary, we positively identified a suite of sage-grouse nest predators within a high elevation population of sage-grouse occupying the Virginia Mountains on the eastern flank of the Sierra Nevada by using continuous videography over a 3-y period. These results were the first to confirm bobcats and long-tailed weasels as sage-grouse nest predators as previously suspected (Schroeder et al. 1999; Holloran and Anderson 2003; Hagen 2011; Kaczor et al. 2011). Rodent and snake species appear to be limited by gape width, and evidence of these species as predators remains unsubstantiated. Besides unambiguous predator identification, we were able to determine the relative frequency at which depredations by predator type occur within our study area, thereby providing reasonable and valuable insight to which predator species are effective. Undoubtedly, our estimates are subject to some degree of unintended bias; yet, they provide a basis for future comparisons as our understanding of sage-grouse nest failure grows. Unequivocal documentation of the predator identity is especially useful given that the population under study experienced an estimated cumulative nest survival rate of 22.4%, a rate lower than published maximum likelihood estimates within the Great Basin (43%, Kolada et al. 2009; 36%, Rebholz et al. 2009; 42%, Coates and Delehanty 2010). Of the 40 nests that failed in our study, 33 (82.5%) were confirmed to have been caused by predators. Efforts to curb high rates of nest depredation may be desirable, but one potentially effective practice of predator management might be to restore and manage vegetation cover and reduce anthropogenic resource subsidies (i.e., roadkill and tall structures) that support predators such as common ravens. Further research that identifies the circumstances in which depredation occurs will best guide these types of management decisions.

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Table S1. Data table containing the encounter history of sage-grouse Centrocercus urophasianus nests in the Virginia Mountains, Nevada, from 2009 to 2011 that was analyzed with the RMark package (R version 2.13, www.r-project.org) that implements Program MARK for estimating daily survival rate (DSR) and cumulative survival rate for nests. nest  =  unique nest identification number, FirstFound  =  day nest was first detected, LastPresent  =  last day the nest was known to be present, LastChecked  =  last day the nest was checked, Fate  =  fate of the nest (0 means nest was successful; 1 means nest was unsuccessful), Freq  =  number of nests that had this history, yr  =  calendar year that the nest existed, camera  =  whether a nest was monitored with a camera or not (0 means a camera was present; 1 means no camera was present), n1  =  whether a nest was a first nest attempt or a second nest attempt (0 means the nest was a first attempt; 1 means the nest was a re-nest attempt). Individual covariates for year, presence of a camera, and nest attempt were included in addition to encounter history to test for effects of these factors on DSR and cumulative survival rate for sage-grouse nests.

Found at DOI: http://dx.doi.org/10.3996/122012-JFWM-110R1.S1 (15 KB XLSX).

Video S1. Video recording of a common raven Corvus corax attacking an incubating female greater sage-grouse Centrocercus urophasianus and then depredating the eggs within an 8-s period in the Virginia Mountains, Nevada, 2010.

Found at DOI: http://dx.doi.org/10.3996/122012-JFWM-110R1.S2 (1,710 KB WMV)

Video S2. Video recording of an American badger Taxidea taxus entering a greater sage-grouse Centrocercus urophasianus nest in the Virginia Mountains, Nevada, in 2010 and removing the single remaining sage-grouse egg from the nest bowl.

Found at DOI: http://dx.doi.org/10.3996/122012-JFWM-110R1.S3. (2,157 KB WMV)

Video S3. Video recording of a bobcat Lynx rufus consuming eggs at a greater sage-grouse Centrocercus urophasianus nest in the Virginia Mountains, Nevada, in 2010.

Found at DOI: http://dx.doi.org/10.3996/122012-JFWM-110R1.S4. (1,648 KB WMV)

Video S4. Video recording of a Great Basin gopher snake Pituophis catenifer deserticola entering a greater sage-grouse Centrocercus urophasianus nest during hatch in the Virginia Mountains, Nevada, 2010. The video recording depicts the female sage-grouse incubating moments before a Great Basin gopher snake enters the nest, the snake entering the nest, and the ensuing struggle as the female sage-grouse attempts to defend the nest while the Great Basin gopher snakes constricts a hatched chick that it has captured.

Found at DOI: http://dx.doi.org/10.3996/122012-JFWM-110R1.S5. (4,445 KB WMV)

Video S5. Video recording of a California ground squirrel Spermophilus beecheyi at a greater sage-grouse Centrocercus urophasianus nest in the Virginia Mountains, Nevada, 2010. The video recording depicts the ground squirrel manipulating eggs and attempting, unsuccessfully, to bite an egg at a sage-grouse nest following a partial nest depredation by a Common Raven Corvus corax.

Found at DOI: http://dx.doi.org/10.3996/122012-JFWM-110R1.S6. (1,504 KB WMV)

Reference S1. Bell CB. 2011. Nest site characteristics and nest success of translocated and resident greater sage grouse at Clear Lake National Wildlife Refuge. M.S. thesis. Arcata, California: Humboldt State University.

Found at DOI: http://dx.doi.org/10.3996/122012-JFWM-110R1.S7; also available at http://humboldtdspace.calstate.edu/bitstream/handle/2148/862/CBELL_Thesis_Final_Submitted.pdf (335 KB PDF).

Reference S2. Laake J, Rexstad E. 2007. RMark—an alternative approach to building linear models. In Cooch E, White G, editors. Appendix C, Program MARK: A Gentle Introduction.

Found at DOI: http://dx.doi.org/10.3996/122012-JFWM-110R1.S8; also available at http://www.phidot.org/software/mark/docs/book/ (30 MB PDF).

Reference S3. Nelson OC. 1955. A field study of the sage-grouse in southeastern Oregon with special reference to reproduction and survival. M.S. thesis. Corvallis: Oregon State University.

Found at DOI: http://dx.doi.org/10.3996/122012-JFWM-110R1.S9; also available at http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/9218/Nelson_Otto_C_1955.pdf (2.2 MB PDF).

This research was part of a cooperative effort with U.S. Geological Survey, Nevada Department of Wildlife, Idaho State University, and the U.S. Fish & Wildlife Service. We thank C. Hampson with Nevada Department of Wildlife for expertise, logistical support, and assistance with data collection efforts. We thank T. Kimball, P. Gore, M. Meshiry, J. Sweeney, and V. Johnson for entering data, performing analyses, producing reports, and managing logistics. Winnemucca Ranch, Big Canyon Ranch, and Fish Springs Ranch provided access onto private land as well as housing for field crews. We are extremely grateful to J. Dudko, S. Lockwood, K. Buckles, and N. Kelly for diligence collecting data in the field. Comments from the Subject Editor and anonymous reviewers greatly enhanced the quality of this manuscript.

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