BIRD POPULATION AND VEGETATION TRENDS AT THE LAS VEGAS WASH,

Transcription

1 BIRD POPULATION AND VEGETATION TRENDS AT THE LAS VEGAS WASH, (SNWA Contract No ) Prepared by: Great Basin Bird Observatory 1755 E. Plumb Lane #256 Reno, NV Prepared for: Las Vegas Wash Coordination Committee Southern Nevada Water Authority P.O. Box Las Vegas, NV Final Report, 30 December 2016

2 Table of Contents Abstract... 2 Acknowledgments... 3 Introduction... 3 Methods... 4 Study Area... 4 Bird Data and Analyses... 4 Data Collection... 4 Data Analysis... 5 Vegetation Measurements... 7 Data Collection... 7 Data Analysis... 7 Results... 8 Species List... 8 Species Richness and Total Abundance Species-Specific Abundances Overall Abundance Patterns Breeding Season Species Abundances in Relation to Regional Abundances Non-Breeding Season Species Abundances Trends Trends in Species Richness and Total Abundance Trends in Species Abundances Vegetation Measurements Tree Cover Perennial Plant Height Cover Types Vertical Perennial Plant Structure Horizontal Perennial Plant Structure Discussion Literature Cited Figures Appendices

3 Abstract After a three-year break in landbird monitoring at the Las Vegas Wash (Wash), the Great Basin Bird Observatory completed another year of bird surveys and vegetation assessments along an 8.7 km stretch of the Wash between September 2014 and August 2015, which marked the tenth year after bird monitoring was begun at this site. The Wash has undergone major construction and habitat restoration, resulting in 19 weirs and impoundments, a major reduction of tamarisk cover and over 400 acres of revegetation with native plants. In this report, we summarize bird species occurrence, trends in richness and abundance, species-specific trends and changes in vegetation over the ten years of the study. In each of the seven years of surveys in that period, bird surveys were completed year-round. Visits occurred every two weeks at 29 (later amended to 31) permanently established points. Once a year, in the fall, vegetation assessments were conducted at each survey point. A total of 199 bird species were recorded over the ten-year period, constituting about two-thirds of the number of birds regularly recorded in Nevada. Of these, 39 were conservation priority species according to regional bird initiatives. Average bird species richness significantly increased over the ten years, as did total bird abundance. Species-specific trends were mixed, with six species showing significant declines and six showing significant increases over the study period. The vegetation data collected revealed that tamarisk cover has decreased significantly while the proportion of native tree cover has increased. Several of the declining birds are riparian shrub-associated and are known to also use the non-native tamarisk, and several of the species with increasing trends are associated with marshes or native shrub habitats. We expect that with further recovery of native vegetation, even the species currently in decline will recover at the Wash in the long term. 2

4 Acknowledgments This study was funded by the Southern Nevada Water Authority through a grant from the Bureau of Reclamation. Thanks to all of our field surveyors of the Nevada Bird Count program who participated in the surveys, particularly David Henderson, Dorothy Crowe, and Kelly Colegrove. Many thanks to Debbie Van Dooremolen of the Southern Nevada Water Authority for project administration, coordination, and support, and to Gerald Braden and Aaron Miller of the San Bernardino County Museum for their assistance. We also thank Debbie Van Dooremolen, Keiba Crear, and the members of the Research and Environmental Monitoring Study Team for reviewing this document and the Las Vegas Wash Coordination Committee for their support of this project. Introduction The Las Vegas Wash (hereafter: Wash) is the primary drainage of the Las Vegas Valley Hydrographic Basin and is located in the southeastern portion of Las Vegas Valley. The lower Wash extends approximately 20 km, flowing through the Clark County Wetlands Park (Figure 1) and terminating in Las Vegas Bay of Lake Mead. While the Wash was historically ephemeral, it has become a perennial riverine and wetland complex due to treated wastewater, urban runoff and shallow groundwater. From the increasing Las Vegas Valley discharge, the Wash began to degrade through channel down-cutting, which led in 1998 to the formation of the Las Vegas Wash Coordination Committee (LVWCC), a stakeholder group that includes local, state, and federal agencies, citizens, businesses, a university and an environmental group. The LVWCC generated the Las Vegas Wash Comprehensive Adaptive Management Plan that recommended measures to halt the channel degradation, restore riparian and wetland habitats, and to conduct wildlife monitoring (LVWCC 2000). In 2000, implementation of this plan began and continues to the present. Plan activities include installing weirs and bank protection to halt erosion, and extensive vegetation improvements through tamarisk (Tamarix ramosissima) control, revegetation with native woodland species, and other plantings. For more details on the history of these efforts, see Braden et al. (2009). The plan also led to the creation and implementation of the Las Vegas Wash Wildlife Management Plan, which contains 31 recommended actions designed to conserve native species, protect and enhance their habitats and increase community awareness of these resources (Shanahan et al. 2008). In 2005, the San Bernardino County Museum, in conjunction with the Southern Nevada Water Authority (the lead agency of the LVWCC), began point count bird surveys along an 8.7 km reach of the Wash (Braden et al. 2007, 2009). The purpose of these bird surveys was to (1) inventory bird populations and bird habitat parameters to provide a baseline dataset, (2) set up a long-term monitoring program that is designed to document the benefits of project activities, and (3) assist in the adaptive management process by providing valuable insight to which activities are effective and in what time frame. The museum conducted four years of surveys which are summarized in Braden et al. (2009). Our report summarizes seven years of surveys: the four years conducted by the museum, and three years conducted by the Great Basin Bird Observatory 3

5 (GBBO; and , respectively), and we analyzed all data together to summarize bird community structure, early population trends, and habitat conditions. Birdhabitat changes and bird community changes based on the first 5-6 survey years were discussed in more detail in GBBO (2011a and b). Methods Study Area The study area encompasses 8.7 km of the Wash between the Upper Diversion and Powerline Crossing weirs (Figure 1). The upland areas are covered by Mojave scrub (dominated by creosote bush, Larrea tridentata). Through Year 6 of this study, the riparian areas were still dominated by the invasive, non-native tamarisk and common reed (Phragmites australis). By the end of Year 10, cover by these species was greatly reduced from treatments. Native vegetation present includes Goodding s willow (Salix gooddingii), sandbar willow (S. exigua), seep willow (Baccharis salicifolia), Fremont cottonwood (Populus fremontii), honey and screwbean mesquite (Prosopis glandulosa and P. pubescens), arrowweed (Pluchea sericea), cattail (Typha domingensis), and bulrush (Schoenoplectus spp.). Active channel stabilization and revegetation activities occurred along the length of the study area throughout the study period. By the end of the first year, nine weirs and 75 acres of revegetation had been installed. Mid-way through the study period (Year 6), 12 weirs and approximately 280 acres of revegetation were in place. By 2015, 19 weirs and over 400 acres of revegetation had been established. Since project activities have been ongoing throughout this study, our monitoring data are likely directly or indirectly affected by them. Bird Data and Analyses Data Collection Birds were surveyed using standardized five-minute point counts (Ralph and Scott 1981). The study began with 29 points in 2005 and with later additions (3 points) and deletions (1 point) now comprises 31 points, arranged along both sides of the Wash (Figure 1). The survey points were established at regular intervals to monitor the bird community and vegetation where channel modification and revegetation has occurred or will occur, as well as in areas where project activities are unlikely to occur (Braden et al. 2007), providing a broad cross-section of the different habitat types found at the Wash. For more details on the point count protocol, see Braden et al. (2007). Each survey event occurred approximately every two weeks over a two-day period. The order in which points were sampled was rotated among surveys. Surveys were conducted from sunrise to approximately five hours post-sunrise, to capture the period of greatest bird activity and vocalization. Nesting evidence was collected using standard breeding bird atlas methods (e.g., 4

6 Floyd et al. 2007), which consider nesting to be confirmed if active nests, dependent young, food/nest material/fecal sac carrying, or nest building is observed. Twenty-six surveys of the points were completed per year through 2011 (except in 2010, when some sites were occasionally inaccessible due to construction), with each survey year typically running from mid-february through January (Year 5 surveys began on January 31). In 2014, the survey year began in September 2014 and continued through August The majority of this report covers the full six years between 12 February 2005 and 30 January 2011, and the tenth year between 1 September 2014 and 31 August The seventh-year surveys conducted in 2011 (through 24 April 2011) were not included in most of our analyses, but they were included in the comprehensive species list. While surveyors were rotated to minimize observer effects, a review of the data suggested that some observer effects remained, particularly with regard to distance measurements. Prior to 2009, rangefinders were not used to determine distances to birds, which adds to the difficulty in interpreting distance data in the full dataset. Since then, the use of rangefinders has been required. Data Analysis Species List A comprehensive species list of all birds recorded at the Wash was generated based on all visits between 12 February 2005 and 31 August 2015, including all survey points (the total of which varied among years between 29 and 31 over the study period), all detection distances, and all birds detected incidentally at the Wash outside of formal surveys. Several surveys were conducted in 2011, the results of which were not used for this report s analyses, but were included in the species list. The list also includes fly-over sightings (e.g., Red-tailed Hawks flying high overhead) of birds that were in the Wash area but may not have been closely tied to the vegetation present at a survey point. This species list was generated to characterize the bird community of the Wash as comprehensively as possible, and because no quantitative comparisons are necessary to do that, all survey results and incidental detections were included regardless of survey effort. Species Richness and Abundance Unlike our approach for the comprehensive species list, species richness and abundance patterns were analyzed using standardized comparisons that included only bird detections that occurred within a 100 m radius of each survey point. Fly-overs were also excluded, even if they occurred directly above a survey point, because these birds were generally not assumed to actively use the surveyed area. Limiting the sample to detections within 100 m of the point allowed us to compare bird abundances among survey points and treatment areas, but it precluded analyses for species with a primarily aerial lifestyle, such as swallows, swifts, and nighthawks. For these species, a separate analysis that includes fly-overs would be necessary, if determining their trends and habitat associations is desired. 5

7 We included survey data from all survey points for which data were available, which varied slightly over the first few years of the project. Although species richness (i.e., the number of species detected) can be sensitive to survey effort, we considered the variation to be relatively insignificant, particularly given that the added and deleted survey points were representative of the rest of the study area in habitat types and species composition present. To make the abundance results useful for comparing data from this project to other regional and national abundance data (and to help control for small variations in survey effort), we used the standardized estimate of density of the number of bird detections per 40 ha, converted from the fixed radius of 100 m around each point (3.14 ha). Using this approach, we plotted the number of bird species and the number of bird detections per 40 ha by survey visit in order to illustrate temporal variation in species richness and abundance and to show differences among treatment areas. Juvenile birds were excluded from analyses of breeding season data. We performed linear regression analyses to determine whether population trends over time were significant. The dataset used was trimmed to exclude periods where species were not, or were not likely to be, present. To determine this, the full dataset was reviewed for the presence of each species, including all distance categories and incidental records; if the species was present during a survey visit, that survey visit was included in the analyses. Periods with zero detections that consisted of less than six weeks were also included within the analysis. Data were then transformed, as needed, to achieve a normal distribution. For all statistical analysis results in this report, P < 0.05 was used as the significance level. Species richness and abundance were also examined for the breeding and non-breeding seasons. In previous analyses (Braden et al. 2009), the breeding season was defined as the period between 15 March through 31 August to encompass the breeding of the majority of both year-round resident and migrant species. In this report, we used the same definitions for consistency with previous analyses. This time period also encompasses spring and early fall migration of several mid and long-distance migrants so, inevitably, at least some non-breeding birds are included in the breeding season estimates. The non-breeding season was defined as 1 October through 31 January to include overwintering birds, but to exclude nesting of most resident species. Average richness and estimated density per survey were calculated for these seasons. Transitional periods, where included in the results, cover the dates not included in either the breeding or non-breeding seasons. Species-Specific Abundances For each species, estimated bird density (birds per 40 ha) was calculated for each year overall, and for its breeding and non-breeding seasons. Relative abundance (proportion of total bird abundance contributed by a species) was calculated for the same periods, but only for species representing at least 1% of the overall bird community in one or more periods. Only species that were detected during at least 40 survey visits between February 2005 and August 2015 were originally included in the trend analyses. The arbitrarily chosen 40-event threshold was necessary because meaningful population trends over the ten years can only be established for at least moderately common species using linear regression. This criterion resulted in an extremely long species list, so to narrow it further, we followed the recommendation in GBBO (2011b) to 6

8 focus on select indicator and conservation priority species, particularly those for which the point count method is well-suited for detection. We further ensured that the list included species with different seasonal uses of the Wash, which resulted in a final list of 20 species for which analysis results are reported here. Breeding season abundances from the Wash surveys were also compared to data collected as a part of GBBO s Nevada Bird Count (NBC) in other Clark County lowland riparian areas. This was done to provide a reference point for the estimated densities at the Wash from regional data collected in similar habitat types. The NBC data were collected on 190 survey visits of 46 transects, generating surveys on approximately 1,900 point-visits (since each transect typically consists of ten points) in roughly the same ten-year period ( ) as the Wash surveys (5,399 point-visits). Both datasets included only detections from within 100 m of the survey point for the purpose of comparisons. However, NBC data were collected over a ten-minute survey period per point, rather than the five-minute period used at the Wash. Further, the breeding season periods differed in that the Wash data were for the period between 15 March through 31 August, with surveys evenly distributed throughout that time period, while the NBC data were collected from mid-april through 30 June, with surveys primarily occurring in May and early June. Regardless of these differences in methods, we consider the regional comparisons of estimates of breeding densities reported here informative for most breeding landbird species. Vegetation Measurements Data Collection Vegetation data were collected at each bird survey point along the Wash during each fall in and in 2014, using a vegetation protocol developed by Braden et al. (2009). Three 100 m vegetation transects were laid out radially, at 120 o angles from the point. Data were collected every 2 m along each transect at the distances of m and m from the survey point. Along each of these six 20 m segments, plant species occurrence, height, and vertical structure were recorded at 2 m intervals, yielding 60 sampling stations per point. Using an m survey rod, perennial plant height was measured to within 0.1 m; any vegetation exceeding the height of the survey rod was estimated to within 0.5 m. Perennial plant structure was measured as the number of vegetation contacts at 1 m vertical intervals along the survey rod. Data Analysis Percent cover type composition was calculated for each survey point, using averages from the point s vegetation transects, and also across the Wash, using averages from the 28 points surveyed consistently over the study period. Cover types included in the analyses were categorized as trees, shrubs, grasses, forbs, and cattails. Because some vegetation transects ended in the channel, emergent vegetation may be underestimated overall due to its naturally-clustered distribution along the channel shores. Analysis of Variance (Anova) was used to investigate vegetation cover type differences. 7

9 Percent cover of live trees by species was also calculated for each survey point and across the Wash. The following species were included in this analysis: catclaw (Acacia greggii), Fremont cottonwood, honey mesquite, screwbean mesquite, Goodding s willow, sandbar willow, and tamarisk. Perennial vegetation height for each point was calculated as the average perennial height of the 60 sampling stations, and was analyzed for differences among years using the point as the sample unit. Vertical structural diversity was assessed by plotting the mean number of contacts vegetation made with the survey rod against the height categories 0-2 m, 2-4 m, 4-6 m, 6-8 m, and 8-10 m. These data were also analyzed for between-year differences. Horizontal perennial heterogeneity was evaluated using the Hill (1973) proportional diversity measure [1/ pi 2 ], where pi is the proportion of vegetation-rod hits at each of the six vegetationtransect segments at a survey point. The horizontal diversity measure was calculated for each point over the seven years of data collected, with values increasing as vegetation becomes more evenly distributed. Results Species List Between 12 February 2005 and 31 August 2015, 199 bird species were observed during visits to the Wash (Appendix 1). During the last year of surveys (2014/2015), fourteen new species were detected at the Wash, including Bonaparte s Gull, Brown Creeper, Cattle Egret, Dunlin, Hammond s Flycatcher, Hooded Merganser, Hooded Warbler, Horned Grebe, Lesser Blackbacked Gull, Lesser Scaup, Neotropic Cormorant, Whimbrel, White-tailed Kite, and Willet (all scientific bird names in Appendix 1). Forty-one species that had been recorded at least once during the first six years were not found in this tenth year, including for example, Brewer s Blackbird, Bushtit, Golden-crowned Kinglet, Pine Siskin, Rock Pigeon, Snow Goose, and (unidentified) Solitary Vireo. No new species were detected in the few seventh-year surveys that were conducted between 1 February and 24 April Of the 199 species observed during the ten-year period, 175 were recorded during the breeding season (15 March - 31 August), and 148 were recorded during the non-breeding season (1 October - 31 January). Only four species were detected exclusively during the transitional seasons between the designated breeding and non-breeding seasons, suggesting that most migrants actually passed through during the breeding or non-breeding seasons. Thirty-nine species recorded at the Wash are conservation priorities according to the Nevada Comprehensive Bird Conservation Plan (GBBO 2010), the Clark County Multiple Species Habitat Conservation Plan (Clark County 2000), or the Lower Colorado River Multi-Species Conservation Program (Bureau of Reclamation 2006; Appendix 1). Thirty-five of these priority species were recorded during the breeding season, and 27 were recorded during the non-breeding season, while two priority species were only detected in the transitional seasons. Six of the 8

10 priority species were confirmed to nest at the Wash, including Abert s Towhee, Bell s Vireo, Blue Grosbeak, Gambel s Quail, Lucy s Warbler, and Yellow Warbler. Ten species were confirmed as breeders for the first time in the tenth year, including Bell s Vireo, Black Phoebe, Canada Goose, Costa s Hummingbird, House Finch, Indigo Bunting, Killdeer, Northern Mockingbird, Red-winged Blackbird, and Yellow-headed Blackbird, resulting in a total of 34 species confirmed to be nesting at the Wash throughout the project area (Table 1). Table 1. List of species confirmed as breeding in the Las Vegas Wash project area, Abert's Towhee American Kestrel Bell s Vireo Bewick's Wren Black Phoebe Black-crowned Night-Heron Black-tailed Gnatcatcher Blue Grosbeak Brown-headed Cowbird Canada Goose Common Yellowthroat Cooper's Hawk Costa s Hummingbird Crissal Thrasher Gambel's Quail Great-tailed Grackle Greater Roadrunner House Finch Indigo Bunting Killdeer Loggerhead Shrike Lucy's Warbler Mallard Mourning Dove Northern Rough-winged Swallow Northern Harrier Red-tailed Hawk Red-winged Blackbird Say's Phoebe Song Sparrow Verdin Yellow Warbler Yellow-breasted Chat Yellow-headed Blackbird 9

11 Species Richness and Total Abundance Seasonal patterns of species richness were similar among years, with richness being lowest in January and peaking during August-September due to migration and juvenile dispersal (Table 2, Figure 2). Species richness remained similar to slightly increasing through Year 6, but was substantially higher in Year 10 (P < 0.001; Figure 3). Total bird abundance (birds per 40 ha) also varied seasonally (Table 3). Overall, peaks were observed from May to July in the breeding season and during October and November of the nonbreeding season (i.e., late fall migration/early winter; Figure 4). While numbers during January/February have overall been low, there are numerous outliers due to the irregular presence of large winter flocks of birds such as the American Pipit. Total bird abundance increased significantly over the study period (P < 0.001; Figure 5), particularly in Year 10 when average abundance rose by %, depending on the season (Table 3). These increases were largely driven by aquatic species, such as American Coot, Gadwall and American Wigeon (Appendices 2a and b). Table 2. Average number of species recorded per survey visit by year and season. Only species recorded within 100 m of the point were included. Breeding Fall Non- Winter Transition Breeding Transition Combined Year Year Year Year Year Year Year Combined Table 3. Average bird abundance recorded per survey visit by year and season (number of detections per 40 ha). Only species recorded within 100 m of the point were included. Breeding Fall Non- Winter Transition Breeding Transition Combined Year Year Year Year Year Year Year Combined

12 Species-Specific Abundances Overall Abundance Patterns For the whole study period ( ), the species with the greatest abundances included Abert s Towhee, Yellow-rumped Warbler, American Coot, Red-winged Blackbird, Song Sparrow, Marsh Wren, Mallard, Bewick s Wren, White-crowned Sparrow, American Pipit, Black-tailed Gnatcatcher, Verdin, and Common Yellowthroat. Density estimates by species, averaged for years and seasons, are provided in Appendices 2a and b. The species with the highest abundances also contributed the greatest proportions to total bird abundance (i.e., they had the highest relative abundances). Relative abundances (% of total bird abundance by species) are reported in Appendices 3a and b for all species that represented at least 1% of the total bird community in any season or year. The relative frequency of each species, i.e., the percentage of total survey points at which the species was detected at least once, is listed in Appendix 4. Breeding Season Species Abundances in Relation to Regional Abundances During the breeding season, the ten most abundant species along the Wash included Red-winged Blackbird, Abert s Towhee, Song Sparrow, Common Yellowthroat, Brown-headed Cowbird, Bewick s Wren, Lucy s Warbler, Great-tailed Grackle, Verdin, and Marsh Wren (Appendix 2a). In comparison, the ten most abundant species recorded during seven years of NBC surveys in other Clark County lowland riparian areas included Gambel s Quail, Abert s Towhee, Lucy s Warbler, Mourning Dove, Brown-headed Cowbird, Yellow Warbler, Verdin, House Finch, Redwinged Blackbird, and Song Sparrow (Appendix 5). Of the ten most abundant species in each dataset, six were the same. Several species had greater estimated densities along the Wash than at the NBC transects used for comparisons, with 11 species with at least five detections per 40 ha recorded on the Wash compared with only three species with that many records in the NBC dataset. The number of species with lower estimated densities were more similar among the datasets, with 24 species with at least one detection per 40 ha recorded along the Wash compared with 23 species with similar estimated densities in the NBC dataset. When comparing the Wash s breeding abundances to the NBC dataset, however, the difference in survey periods for the breeding season needs to be considered. Even so, the overlap in species between the two monitoring efforts indicate an overall large degree of similarity between the Wash and other lowland riparian areas of the region. However, many of the abundant species in the NBC dataset were even more abundant at the Wash including riparian shrub and wetland-associated species, such as Redwinged Blackbird, Abert s Towhee, Song Sparrow, Common Yellowthroat, Yellow-breasted Chat, and Marsh Wren (Appendix 5). Exceptions to this pattern included Gambel s Quail, House Finch, Black-throated Sparrow, Phainopepla, and Ash-throated Flycatcher. Thirty-eight conservation priority species have been recorded during the breeding season in either the Wash or other NBC lowland riparian transects (Appendix 5). Of the common priority species, Lucy s and Yellow warblers had similar abundance estimates among the two datasets, while Abert s Towhees and Blue Grosbeaks were more abundant at the Wash and Gambel s 11

13 Quail, Phainopepla, and Bell s Vireos were more abundant at the NBC transects in comparison (Appendix 5). Non-Breeding Season Species Abundances The ten most abundant species of the Wash included Yellow-rumped Warbler, American Coot, White-crowned Sparrow, Abert s Towhee, Mallard, American Pipit, Marsh Wren, Song Sparrow, Red-winged Blackbird, and Ruby-crowned Kinglet for the non-breeding season (Appendix 2a). No comparable datasets were available from elsewhere that would allow us to make easy regional comparisons as we could with the NBC dataset for the breeding season. Regardless, the most abundant birds at the Wash during the non-breeding season were a mix of year-round residents (e.g., Abert s Towhee and Red-winged Blackbird) and migrant and wintering species of the Mojave Desert (e.g., Yellow-rumped Warbler, White-crowned Sparrow, and American Pipit; Appendix 2a). Trends Trends in Species Richness and Total Abundance Linear regression analysis showed a significant increase in species richness at the Wash over the ten-year period of this study, with an Adjusted R 2 = and P < (Figure 3). Total bird abundance also increased significantly over this time period, with an Adjusted R 2 = and P < (Figure 5). Trends in Species Abundances Trends in abundance over the ten-year survey period were examined for 20 focal species found at the Wash. Six of these species showed significant declines including Abert s Towhee, Song Sparrow, Bewick s Wren, Lucy s Warbler, Yellow-breasted Chat, and Blue Grosbeak (Figures 6a-j, Table 4). These are species associated with riparian shrublands and woodlands, and further, Lucy s Warbler and Bewick s Wren are associated with cavities and crevices typically located in large-diameter trees. Both species have been recorded nesting in steep wash banks, though we did not formally document this as part of the study reported here. Six focal species showed significant increases over the study period, including Yellow-rumped Warbler, Marsh Wren, American Pipit, Black-tailed Gnatcatcher, Verdin, and Gambel s Quail (Figures 6a-j, Table 4). These species are associated with several different habitat types, including mesquite, quailbush, open uplands, and wetlands. 12

14 Table 4. Summary regression statistics for 20 species that were detected within 100 m of survey points at the Las Vegas Wash during at least 40 (of a total of 182) survey events between February 2005 and August Species listed in descending order of detections. Dark gray shading indicates significantly negative trends (six species), and light gray shading indicates significantly positive trends (six species). Species No. of Visits with Detections Total No. of Birds Detected Overall Density Estimate (Birds/40 ha) Transformation (Detections/40 ha) Adj R 2 Regression Coefficient Abert's Towhee Ln <0.001 Yellow-rumped Warbler Ln Song Sparrow Square Root <0.001 Marsh Wren Square Root White-crowned Sparrow Square Root Bewick's Wren Square Root <0.001 American Pipit Ln <0.001 Black-tailed Gnatcatcher Square Root Verdin Square Root <0.001 Common Yellowthroat Square Root * Gambel's Quail Square Root <0.001 Ruby-crowned Kinglet Square Root Lucy's Warbler Square Root <0.001 Yellow-breasted Chat Square Root Orange-crowned Warbler Ln * Yellow Warbler Square Root Blue Grosbeak Square Root Lesser Goldfinch N/A * Wilson's Warbler Square Root * Black-chinned N/A * Hummingbird * Deviated from normal distribution. P 13

15 Vegetation Measurements Tree Cover Tree cover provided approximately 20-40% of total vegetative cover at the Wash bird survey points (Figure 7). Based on our measurements, tree cover significantly declined over the ten-year period (Figure 7, Table 5), averaging 41% cover in 2005 and 20% cover in Native riparian tree cover showed a slight increase over this time period, but this change was not statistically significant (Figure 8). Tamarisk cover declined significantly between 2005 and 2014 (Figure 9, Table 5). Tamarisk originally formed the majority of the tree cover of the Wash, averaging 36% of total vegetative cover in 2005 compared with 6% in 2014 (Figures 7 and 9). Cover estimates for cottonwood, Goodding s willow, and sandbar willow were not statistically different over the ten-year period (Figures 10-12, Table 5). Mesquite cover showed an apparent increase from 1 to 7% over the ten years, but the increase was not statistically significant (Figure 13, Table 5). The proportion of tree cover provided by natives did increase significantly, however, from 16% in 2005 to 61% in 2014 (Figure 14, Table 5). Perennial Plant Height Mean perennial plant height showed a slight decrease across the ten-year period, from 2.9 m in 2005 to 2.4 m in 2014 (Figure 15, Table 5). This decrease was not statistically significant. Cover Types Beyond tree cover, four other cover types were also evaluated for changes at the Wash over the ten-year study period, including shrubs, forbs, grasses, and cattails (Figures 16-19, Table 5). Shrub cover (including Phragmites) significantly increased over the study period, from 23% in 2005 to 32% in 2014 (Figure 16). Forb cover significantly declined over this time period, from 9% to 1% (Figure 17). Grass cover also significantly changed over the ten years, ranging from 0 to 6% cover (Figure 18). Cattail cover did not significantly change over this time period (Figure 19). Vertical Perennial Plant Structure There was no overall difference in vertical perennial plant structure over the ten-year period based on the number of hits along a vertical rod (Figure 20, Table 5). A significantly increasing trend was, however, observed in perennial vegetation in the 0-2 m height interval (Figure 21), particularly in the years 2005 through An apparent decrease was also found in the 2-4 m height interval (Figure 22), but this decrease was not statistically significant. There were no significant changes in the 4-6 m height interval (Figure 23), the 6-8 m height interval (Figure 24), and the 8-10 m height interval (Figure 25) over the time period of the study. Horizontal Perennial Plant Structure Horizontal structure of perennials was evaluated using a proportional plant diversity measure based on rod hits along the vegetation transect line. The horizontal heterogeneity index 14

16 significantly declined over the ten-year survey period (Figure 26, Table 5), indicating that vegetation was becoming less evenly-distributed over time. Table 5. Results of Anova analysis evaluating differences in vegetation measurements among years of riparian bird monitoring along the Las Vegas Wash. Habitat Variable F-statistic P DF Overall Tree Cover , 195 Native Riparian Trees , 195 Tamarisk < , 195 Cottonwood Trees , 195 Goodding s Willows , 195 Sandbar Willows , 195 Mesquite Trees , 195 Perennial Height , 191 Shrub Cover , 195 Forb Cover , 195 Grass Cover < , 195 Cattail Cover , 195 Native Tree Proportion , 195 Perennial Structure , m , m , m , m , m , 195 Horizontal Heterogeneity ,

17 Discussion As discussed in previous reports (GBBO 2011a and b), the bird community of the Wash is rich, featuring about two-thirds of the total number of species regularly recorded in the state of Nevada (GBBO 2010). Our regional comparisons show that the Wash functions much like other riparian areas in Clark County, although it features higher estimated densities of several riparian and wetland-associated species than normally encountered. Bird species richness was highest during the breeding and post-breeding season, as is typical for temperate riparian areas that support a diverse bird community during the seasons of insect productivity. Total bird abundance was also high during the breeding season, but was at its highest in October and November, which is mostly attributable to regularly-recorded winter birds that flock. The site not only provides important habitat for 39 conservation priority species, some of which were confirmed to be nesting in the site, but also appears to be particularly important as a stopover site for migrants and post-breeding vagrant populations of species that nest in other habitat types. Over the life of the Wash stabilization and enhancement project, 19 weirs have been constructed, non-native tamarisk has been removed, and over 400 acres have been revegetated with native plants. Recent construction has resulted in multiple survey areas lacking significant vegetation, which explains overall decreases in mean perennial plant height and forb and grass cover. Cover of non-native tamarisk has decreased while shrub cover and the proportion of native tree cover have increased, which we directly attribute to the habitat restoration efforts that are ongoing at the site. The resulting impounded water and habitat changes have in turn resulted in increased richness and abundance of birds, particularly those associated with aquatic and wetland habitat types. Species that increased significantly in abundance include Yellow-rumped Warbler, Marsh Wren, Verdin, and Gambel s Quail, while others, such as Abert s Towhee, Song Sparrow, Yellow-breasted Chat and Lucy s Warbler have decreased (Figures 6a-j, Table 4). We attribute the decreases primarily to the temporary loss of riparian thickets, which are expected to recover and increase beyond the pre-construction levels once sufficient time has passed for the site to recover. Among the changes that were most evident in the bird community was the large increase in species richness present in the tenth year of the project. Much of that increase can be attributed to the impoundments and the consequent increase in shorebirds and waterfowl. Similarly, the increase in total bird abundance recorded in the tenth year can be attributed in large measure to these impoundments. For example, in the first two years of the project, only two species had more than ten detections per 40 ha, Abert s Towhee and Song Sparrow. In the third year, four species were found in similarly high abundance, Yellow-rumped Warbler, Song Sparrow, Abert s Towhee, and American Coot. In the fifth year, three species were this abundance, Yellow-rumped Warbler, Red-winged Blackbird, and Abert s Towhee, and in the tenth year, seven species with at least ten detections per 40 ha were recorded, American Coot, Mallard, Redwinged Blackbird, Yellow-rumped Warbler, Gadwall, American Pipit, and American Wigeon. The increases in aquatic birds demonstrate the increased availability of suitable wetland and open water habitat. 16

18 The Wash supported larger estimated densities of many riparian shrub and wetland bird species compared with similar areas in Clark County, including Red-winged Blackbird, Abert s Towhee, Song Sparrow, Common Yellowthroat, Yellow-breasted Chat, and Marsh Wren. Species found to be more common at the regional level included Gambel s Quail, House Finch, Black-throated Sparrow, Phainopepla, and Ash-throated Flycatcher. For further details on the natural history and habitat use of these species, see GBBO (2010, 2011a, b). Literature Cited Braden, G.T., L. Crew, and A. Miller Avian diversity, vegetation composition, and vegetation structure of the Las Vegas Wash: Year One Final Report. Unpubl. Report, Prepared by San Bernardino County Museum for the Las Vegas Wash Coordination Committee, August Braden, G.T., L. Crew, and A. Miller Avian diversity, vegetation composition, and vegetation structure of the Las Vegas Wash: 2005 to Unpubl. Report, Prepared by San Bernardino County Museum for the Las Vegas Wash Coordination Committee, November Bureau of Reclamation Lower Colorado River Multi-Species Conservation Program. Clark County Clark County Multi-Species Habitat Conservation Plan: Final Environmental Impact Statement. iplespecieshabitatconservationplan.htm Floyd, T., C.S. Elphick, G. Chisholm, K. Mack, R.G. Elston, E.M. Ammon, and J.D. Boone Atlas of the Breeding Birds of Nevada. University of Nevada Press, Reno. 581 pp. (GBBO) Great Basin Bird Observatory Nevada Comprehensive Bird Conservation Plan, Version (GBBO) Great Basin Bird Observatory. 2011a. Bird community and vegetation of the Las Vegas Wash, Unpubl. Report Submitted to the Southern Nevada Water Authority, Las Vegas, Nevada. (GBBO) Great Basin Bird Observatory. 2011b. Bird population trends and habitat treatment effects at the Las Vegas Wash, Unpubl. Report Submitted to the Southern Nevada Water Authority, Las Vegas, Nevada. 17

19 (LVWCC) Las Vegas Wash Coordination Committee Las Vegas Wash Comprehensive Adaptive Management Plan. Las Vegas Wash Project Coordination Team, Southern Nevada Water Authority, Las Vegas, Nevada. Ralph, C.J., and J.M. Scott Estimating the numbers of terrestrial birds. C.J. Ralph and J.M. Scott (eds). Studies in Avian Biology No. 6. Shanahan, S.A., D.M. Van Dooremolen, T. Sharp, S. Martin, and B. Brown Las Vegas Wash Wildlife Management Plan. Prepared by the Southern Nevada Water Authority, Las Vegas, NV, and SWCA Environmental Consultants, Salt Lake City, UT. Prepared for the Las Vegas Wash Coordination Committee. 18

20 Figures 19

21 Figure 1. Distribution of points across Las Vegas Wash. Map courtesy of Southern Nevada Water Authority. 20

22 Figure 2. Seasonal species richness per survey visit from 182 surveys of the Las Vegas Wash ( ). Curve represents a LOWESS smoothed fit for variation in richness among survey visits. 21

23 Figure 3. Trend in species richness from 182 surveys of the Las Vegas Wash ( ). Adjusted Multiple R 2 = 0.254; Regression Coefficient = ; P <

24 Figure 4. Seasonal total bird abundance per survey visit from 182 surveys of the Las Vegas Wash ( ). Curve represents a LOESS smoothed fit for variation in abundance among survey events. 23

25 Figure 5. Trend in estimated total bird abundance from 182 surveys of the Las Vegas Wash ( ). Adjusted Multiple R 2 = 0.249; Regression Coefficient = ; P <

26 Figure 6a. Detections per 40 hectares of two common species along the Las Vegas Wash ( ). See also Table 4 for statistical analysis results. 25

27 Figure 6b. Detections per 40 hectares of two common species along the Las Vegas Wash ( ). See also Table 4 for statistical analysis results. 26

28 Figure 6c. Detections per 40 hectares of two common species along the Las Vegas Wash ( ). See also Table 4 for statistical analysis results. 27

29 Figure 6d. Detections per 40 hectares of two common species along the Las Vegas Wash ( ). See also Table 4 for statistical analysis results. 28

30 Figure 6e. Detections per 40 hectares of two common species along the Las Vegas Wash ( ). See also Table 4 for statistical analysis results. 29

31 Figure 6f. Detections per 40 hectares of two common species along the Las Vegas Wash ( ). See also Table 4 for statistical analysis results. 30

32 Figure 6g. Detections per 40 hectares of two common species along the Las Vegas Wash ( ). See also Table 4 for statistical analysis results. 31

33 Figure 6h. Detections per 40 hectares of two common species along the Las Vegas Wash ( ). See also Table 4 for statistical analysis results. 32

34 Figure 6i. Detections per 40 hectares of two common species along the Las Vegas Wash ( ). See also Table 4 for statistical analysis results. 33

35 Figure 6j. Detections per 40 hectares of two common species along the Las Vegas Wash ( ). See also Table 4 for statistical analysis results. 34

36 Figure 7. Tree cover (% cover of all trees) in the Las Vegas Wash by year, Multiple R 2 = 0.115; P = Figure 8. Native riparian tree cover (%) in the Las Vegas Wash by year, Multiple R 2 = 0.043; P =

37 Figure 9. Tamarisk tree cover (%) in the Las Vegas Wash by year, Multiple R 2 = 0.194; P < Figure 10. Cottonwood tree cover (%) in the Las Vegas Wash by year, Multiple R 2 = 0.007; P =

38 Figure 11. Goodding s willow tree cover (%) in the Las Vegas Wash by year, Multiple R 2 = 0.011; P = Figure 12. Sandbar willow cover (%) in the Las Vegas Wash by year, Multiple R 2 = 0.018; P =

39 Figure 13. Mesquite tree cover (%) in the Las Vegas Wash by year, Multiple R 2 = 0.046; P = Figure 14. Mean native proportion of tree cover in the Las Vegas Wash by year, Multiple R 2 = 0.105; P =

40 Figure 15. Mean maximum perennial plant height (m) in the Las Vegas Wash by year, Multiple R 2 = 0.028; P = Figure 16. Mean shrub cover (%) in the Las Vegas Wash by year, These results include Phragmites. Multiple R 2 = 0.065; P =

41 Figure 17. Mean forb cover (%) in the Las Vegas Wash by year, Multiple R 2 = 0.113; P = Figure 18. Mean grass cover (%) in the Las Vegas Wash by year, Multiple R 2 = 0.170; P <

42 Figure 19. Mean cattail cover (%) in the Las Vegas Wash by year, Multiple R 2 = 0.021; P = Figure 20. Perennial plant structure (number of rod hits) in the Las Vegas Wash by year, Multiple R 2 = 0.041, P =

43 Figure 21. Perennial plant structure in the 0-2 m height category (number of rod hits) in the Las Vegas Wash by year, Multiple R 2 = 0.092; P = Figure 22. Perennial plant structure in the 2-4 m height category (number of rod hits) in the Las Vegas Wash by year, Multiple R 2 = 0.063; P =

44 Figure 23. Perennial plant structure in the 4-6 m height category (number of rod hits) in the Las Vegas Wash by year, Multiple R 2 = 0.061; P = Figure 24. Perennial plant structure in the 6-8 m height category (number of rod hits) in the Las Vegas Wash by year, Multiple R 2 = 0.019; P =

45 Figure 25. Perennial plant structure in the 8-10 m height category (number of rod hits) in the Las Vegas Wash by year, Multiple R 2 = 0.035; P = Figure 26. Horizontal vegetation heterogeneity based on the proportional number of hits for each of six vegetation transects per survey point, in the Las Vegas Wash by year, Multiple R 2 = 0.089, P =

46 Appendices Appendix 1. All bird species detected at Las Vegas Wash, February 2005 through August 2015, during breeding, non-breeding, and transitional seasons. Bolded species are conservation priorities (Clark County 2000, Bureau of Reclamation 2006, GBBO 2010). X denotes at least one detection in that season (for season definitions, see Methods). Asterisks indicate species that were not recorded within 100 m during a survey, which are not included in the rest of the analyses. Species listed in taxonomic order. Bird Species Scientific Name Breeding Non-Breeding Transi- Season Season tional Greater White-fronted Goose * Anser albifrons X Snow Goose Chen caerulescens X X X Ross's Goose * Chen rossii X Canada Goose Branta canadensis X X X Wood Duck Aix sponsa X X Gadwall Anas strepera X X X American Wigeon Anas americana X X X Mallard Anas platyrhynchos X X X Blue-winged Teal * Anas discors X X X Cinnamon Teal Anas cyanoptera X X X Northern Shoveler Anas clypeata X X X Northern Pintail Anas acuta X X X Green-winged Teal Anas crecca X X X Redhead Aythya americana X X X Ring-necked Duck Aythya collaris X X X Lesser Scaup Aythya affinis X X X Bufflehead Bucephala albeola X X X Common Goldeneye Bucephala clangula X X X Hooded Merganser Lophodytes cucullatus X X X Common Merganser Mergus merganser X X X Ruddy Duck Oxyura jamaicensis X X X Gambel's Quail Callipepla gambelii X X X Pied-billed Grebe Podilymbus podiceps X X X Horned Grebe Podiceps auritus X X X Eared Grebe Podiceps nigricollis X X X Western Grebe Aechmophorus occidentalis X X Clark's Grebe * Aechmophorus clarkii X X American White Pelican Pelecanus erythrorhynchos X X X Neotropic Cormorant Phalacrocorax brasilianus X X 45

47 Bird Species Scientific Name Breeding Non-Breeding Transi- Season Season tional Double-crested Cormorant Phalacrocorax auritus X X X American Bittern Botaurus lentiginosus X X Least Bittern Ixobrychus exilis X X X Great Blue Heron Ardea herodias X X X Great Egret Ardea alba X X X Snowy Egret Egretta thula X X X Cattle Egret * Bubulcus ibis X Green Heron Butorides virescens X X X Black-crowned Night-Heron Nycticorax nycticorax X X X White-faced Ibis Plegadis chihi X X X Turkey Vulture Cathartes aura X X Osprey Pandion haliaetus X X X White-tailed Kite * Elanus leucurus X Northern Harrier Circus cyaneus X X X Sharp-shinned Hawk Accipiter striatus X X X Cooper's Hawk Accipiter cooperii X X X Northern Goshawk * Accipiter gentilis X Red-shouldered Hawk Buteo lineatus X X X Swainson's Hawk * Buteo swainsoni X Red-tailed Hawk Buteo jamaicensis X X X American Kestrel Falco sparverius X X X Merlin Falco columbarius X Peregrine Falcon Falco peregrinus X X X Prairie Falcon Falco mexicanus X X X Virginia Rail Rallus limicola X X X Sora Porzana carolina X X X Common Gallinule Gallinula galeata X X X American Coot Fulica americana X X X Sandhill Crane * Grus canadensis X Semipalmated Plover Charadrius semipalmatus X Killdeer Charadrius vociferus X X X Black-necked Stilt Himantopus mexicanus X X American Avocet Recurvirostra americana X X X Spotted Sandpiper Actitis macularius X X X Greater Yellowlegs Tringa melanoleuca X X X Willet * Tringa semipalmata X 46