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Thrasher Surveys at the Thrasher Spot 12 Holderer & Riegner 2021 Thrasher Surveys at the Thrasher Spot Section 3: Results All field survey data can be found tabled in the appendices. GIS data is available upon request. Thrasher Sightings During the span of two surveys we located desert thrashers 130 times; broken down by species, we located 71 Bendire’s, 7 Crissal, 16 LeConte’s, and 36 Sage. Unsurprisingly, the highest diversity and abundance of thrashers was detected along and radiating away from Thrasher Wash, in the vicinity of the famous Thrasher Spot. Another location of significant diversity and abundance was Luke Wash. While there were biodiversity hotspots, thrashers were found throughout a large majority of the study area. The study area was surveyed twice and therefore many of these birds may have been Fig. 8: This map depicts all 130 thrasher observations from both surveys. Points indicate anywhere from 1 – 6 birds, but most often indicate 1 – 2 birds. Sage Thrashers were often seen in larger groups during the second survey. On one occasion 6 Bendire’s Thrashers were seen in one mesquite tree. 13 Holderer & Riegner 2021 Thrasher Surveys at the Thrasher Spot counted twice (once the first survey and again the second survey), as thrashers are territorial. During Survey 1 (Jan 02 – 06, 2021), we located 46 thrashers: 25 Bendire’s, 4 Crissal, 8 LeConte’s, and 9 Sage. On several occasions two birds were observed together, possibly indicating mated pairs. We observed 3 Bendire’s, 1 Crissal, and 3 LeConte’s pairs. During Survey 2 (Feb 04 – 06, 2021), we located 84 thrashers: 46 Bendire’s, 3 Crissal, 8 LeConte’s, and 27 Sage. We observed 8 Bendire’s, 1 Crissal, and 1 LeConte’s pairs. Species Survey 1 (Jan 02 – 06, 2021) Survey 2 (Feb 04 – 06, 2021) Sightings/Possible Pair Bonds by Total Birds Possible Pair Total Birds Possible Pair Survey Sighted Bonds Sighted Bonds Bendire’s 25 3 46 8 Crissal 4 1 3 1 LeConte’s 8 3 8 1 Sage 9 - 27 - Fig. 9: A table displaying sightings and possible pair bonds for each thrasher species per survey. Sage Thrashers do not breed in the area. Bendire’s Thrasher The most abundant thrasher species across the study area was the Bendire’s Thrasher, often seen perched and singing in mesquite trees. Bendire’s were often located in areas that did not commonly support other thrashers species- an observation that exemplifies Bendire’s adaptability to various vegetation compositions and habitat types. Bendire’s were more frequently associated with landscape mosaics of taller shrubs (mesquite, palo verde), lower growing shrubs (creosote, saltbush), and plenty of openings (see Fig. 11). It was not uncommon to find Bendire’s perched in an isolated stand of mesquite along a minor ephemeral waterway otherwise completely dominated by homogenous creosote. During the first and second surveys, 25 and 46 Bendire’s Thrashers individuals were located, respectively. The 86% increase in sightings from the first to the second survey can be accounted for by an expected increase in vocal activity during the months of February and March26, which in turn increases the likelihood of detection by surveyors. Figure 9 displays all Bendire’s sightings across the study area by survey number. Data clusters within 1.67ha27 from both surveys 1 & 2, which were a month apart, point to the likelihood of established territories. 26 Corman and Wise-Gervais 2005 27 Desmund and Sutton 2020 14 Holderer & Riegner 2021 Thrasher Surveys at the Thrasher Spot Fig. 10: All located Bendire’s Thrashers in the study area by survey number. Data point sizes are weighted to indicate number of birds per sighting. Bendire's Thrasher by Dominant Vegetation (71 Total Point Count) Saltbush 8% 10% Creosote/saltbush 10% Mesquite/palo verde terraces 28% Mesquite/saltbush terraces 14% Creosote uplands 14% Mesquite/creosote patches 15% Other Fig. 11: Bendire’s Thrasher sightings (total number of birds/point count) by dominant vegetation types. 15 Holderer & Riegner 2021 Thrasher Surveys at the Thrasher Spot LeConte’s Thrasher Most of the LeConte’s sightings were either along Thrasher Wash (the ephemeral waterway due adjacent to the parking area), although we observed one at the edge of Luke Wash. One individual was consistently singing and foraging at the interface of Saltbush and Creosote uplands near S 339th Ave where we found what appeared to be an old nest in an isolated mesquite tree. LeConte’s Thrashers were most frequently located in heterogeneous vegetation structures comprised of mesquite and saltbush and/or creosote. Regardless of structure, our findings indicate an affinity for saltbush. We found 8 LeConte’s Thrashers on each survey. Data clusters within 7.34ha28 from both surveys indicate site fidelity and the fact that we saw pairs on several occasions suggests established breeding territories. Fig. 12: All located LeConte’s Thrashers in the study area by survey number. Data point sizes are weighted to indicate number of birds per sighting (1 or 2). Polygons indicate observed movements of individuals or pairs. 28 Sheppard 2018 16 Holderer & Riegner 2021 Thrasher Surveys at the Thrasher Spot Le Conte's Thrasher by Dominant Vegetation (16 Total Point Count) Creosote/saltbush 6% 19% Mesquite/palo verde terraces 13% Mesquite/saltbush terraces 63% Mesquite/creosote patches Fig. 13: LeConte’s Thrasher sightings (total number of birds/point count) by dominant vegetation types. Crissal Thrasher by Dominant Vegetation (7 Total Point Count) Creosote/saltbush 29% Mesquite/palo verde 57% terraces 14% Mesquite/saltbush terraces Fig. 14: Crissal Thrasher sightings (total number of birds/point count) by dominant vegetation types. Crissal Thrasher Crissal Thrasher sightings were few and far between, making them the least commonly detected thrashers in our study. During the first and second surveys, 4 and 3 Crissal Thrasher individuals were located, respectively. In the center of the study area, close to the Thrasher Spot, there is a data cluster with observations from both surveys 1 & 2, which suggest an established breeding territory. Another possible pair was observed in the vicinity of Luke Wash. 17 Holderer & Riegner 2021 Thrasher Surveys at the Thrasher Spot Crissal Thrashers were most frequently found associated with thicker vegetation characterized by mesquite, saltbush, and creosote along ephemeral waterways. Fig. 15: All located Crissal Thrashers in the study area by survey number. Data point sizes are weighted to indicate number of birds per sighting. All points indicate 1 – 2 birds. Polygons indicate the observed movements of individuals and possible pairs. Sage Thrasher Like Bendire’s, Sage Thrashers were located throughout the study area, in a diversity of vegetation composition and structures, although they were more commonly associated with saltbush (Atriplex polycarpa; see fig. 17). During the first and second surveys, 9 and 36 unique Sage Thrashers were located, respectively. The 300% increase in sightings from the first to the second survey is likely attributed to birds migrating. Another interesting find was that Sage Thrashers were seen alone on our first visit and in small flocks on our second visit. 18 Holderer & Riegner 2021 Thrasher Surveys at the Thrasher Spot Fig. 16: All located Sage Thrashers in the study area by survey number. Data point sizes are weighted to indicate number of birds per sighting. Points indicate 1 – 4 birds. Sage Thrasher by Dominant Vegetation (36 Total Point Count) Saltbush 3% Creosote/saltbush 14% 22% Mesquite/palo verde terraces Mesquite/saltbush terraces 17% Creosote uplands 33% Mesquite/creosote patches 11% Other Fig. 17: Sage Thrasher sightings (total number of birds/point count) by dominant vegetation types. 19 Holderer & Riegner 2021 Thrasher Surveys at the Thrasher Spot Section 4: Discussion & Recommendations Our study documented high thrasher species richness and abundance throughout the study area addressing the deficit of avian data beyond the immediate vicinity of the Thrasher Spot parking area by providing a more comprehensive look at the local distribution of desert thrasher species. Our findings indicate diversity hotspots, habitat preferences, and possible breeding territories; along with a comprehensive literature review, these findings are the bases of our recommendations seen hereafter. Site plans (as of 02/28/21) for the Sun Streams Expansion Solar Project (SSE) by First Solar, Inc. will result in landscape-scale habitat destruction and fragmentation. A side-by- side analysis of thrasher survey data and these site plans indicate pending widespread destruction of habitat being directly used by LeConte’s, Bendire’s, Crissal, and Sage thrashers. Habitats that are not directly destroyed will become increasingly fragmented. Numerous studies have investigated the effects of habitat fragmentation on avian communities and diversity generally declines as habitats become increasingly fragmented29. First Solar, Inc. has proposed numerous wildlife corridors, which follow washes and floodplains through the project area. These proposed wildlife corridors are a step in the right direction, and in many instances overlap with survey data, however, they are not enough to preserve the ecological integrity and composition of the Thrasher Spot. Connectivity contributes to both the structure and function of ecosystems. Structural connectivity is the physical relationship between patches of habitat, while functional connectivity is the degree to which landscapes actually facilitate or impede the movement of organisms and processes of ecosystems30. Widening proposed corridors and connecting more patches could improve structural connectivity in the project area, while conserving more areas of preferred thrasher habitat within the corridors would improve functionality. However, connectivity is merely a dimension of landscape-scale ecological integrity. Preserving connectivity (i.e. corridors) alone cannot safeguard the diversity of desert thrashers at the Thrasher Spot from solar development. This frame of thought needs to shift from connectivity to also protecting core habitat. To protect the ecological integrity of the Thrasher Spot, homes need to be protected, not just hallways. Desert thrashers are territorial and occupy fairly large territories and therefore are often quite spread out from their interspecific relationships. For example, LeConte’s breeding territories were found to be 7.34ha, but they can expand to be 10-18 hectares during the nonbreeding season31. As a result, local populations need a lot of space. All of our recommendations aim to widen proposed wildlife corridors and protect larger swathes of thrasher habitat in areas where our findings indicated high thrasher diversity. 29 Pickett and Cadenasso 1995, Kreuper 1992, Wegner and Merriam 1979 30 Ament et al. 2014 31 Sheppard 2018 20 Holderer & Riegner 2021 Thrasher Surveys at the Thrasher Spot The maps on the following pages display four areas that we recommend for protection and they are titled REC #1-4. An overview map helps orient the reader. Areas outlined in yellow depict the recommended areas. These recommendations are prioritized in the following numerical order: 1. REC #1: Thrasher Wash Vicinity This recommendation entails preserving the entire delineated area (see map) north of Salome Highway by abandoning plans to develop an entire solar array and instead protecting the ~118 acres it would have occupied. Heeding this recommendation will improve structural connectivity by connecting the Thrasher Wash corridor to protected corridors adjacent to 339th Ave to the west. Functional connectivity will improve by protecting the saltbush/mesquite vegetation type found here, which supports a high diversity of thrasher. We found a territory of LeConte’s Thrashers in this area and we had numerous other sightings of this species adjacent to it. Additionally, an active Burrowing Owl colony was detected here; ideally, these owls would not be disturbed and relocated. 2. REC #2: Thrasher Spot Parking Area This recommendation entails widening the proposed corridor in the vicinity of the Thrasher Spot parking area to improve structural connectivity. In the event that REC#1 is not actualized, the edges of three separate solar arrays are considerably close together here and should be spread out to improve corridor structure. Additionally, this would increase habitat availability for the high diversity of thrashers recorded in the area. The more land that is protected in the vicinity of the Thrasher Spot the better off the birds and the birders will be. 3. REC #3: Luke Wash Vicinity This recommendation entails widening the eastern boundary of the Luke Wash wildlife corridor by approximately 800m to protect this excellent thrasher habitat. Expanding this boundary would improve the connectivity of this corridor in addition to protecting habitat that is being utilized by multiple species of desert thrashers. Luke Wash has many of the same structural and ecological elements as the Thrasher Spot and thus should be protected. We found a singing LeConte’s here, not to mention numerous Bendire’s, Crissal and Sage thrashers here as well. High diversity was found in the protected area of Luke Wash, but also in upland washes where First Solar plans on developing. 4. REC #4: 331 Avenue Washes This recommendation entails expanding the wildlife corridor to fill the areas outlined in yellow. The overarching goal of this recommendation is to increase habitat availability by protecting an area that is evidently favored by desert thrashers. Minor, ephemeral tributaries extend NE from Thrasher Wash, cross S 331 Ave, and provide excellent thrasher 21 Holderer & Riegner 2021 Thrasher Surveys at the Thrasher Spot habitat characterized by mesquite and creosote. Here we found LeConte’s, Bendire’s and Sage thrashers. Other recommendations include: • Avoid disturbing the birds during nesting season when they are most sensitive. We suggest avoiding construction Feb-June, with March-May being the most critical time for nesting for LeConte’s Thrasher32. • First Solar should repeat these surveys 5 years post construction in order to ascertain a temporal dataset of thrasher diversity before and after construction. • The integrity of wildlife corridors must be upheld during construction (i.e. not trammeled by adjacent construction activities putting in extra roads through undisturbed habitat). RECOMMENDATION MAPS BEGIN ON NEXT PAGE . 32 Corman and Wise-Gerais 2005 22 Holderer & Riegner 2021 Thrasher Surveys at the Thrasher Spot 23 Holderer & Riegner 2021 Thrasher Surveys at the Thrasher Spot 24 Holderer & Riegner 2021 Thrasher Surveys at the Thrasher Spot 25 Holderer & Riegner 2021 Thrasher Surveys at the Thrasher Spot 26 Holderer & Riegner 2021