Le Conte’s Thrasher Surveys at Tejon Ranch, 2012

Chris McCreedy

PRBO Conservation Science

3820 Cypress Drive, No. 11

Petaluma, CA 94954

September 2012

PRBO Contribution No. 1892

Summary

In partnership with the Tejon Ranch Conservancy, PRBO Conservation Science conducted 20 Le Conte’s Thrasher (Toxostoma lecontei) transects across the southern Tehachapi Mountains in February and March 2012. We focused our surveys (16 transects) near Joshua tree (Yucca brevifolia) stands on Tejon Ranch lands on the southeastern toe of the Tehachapi foothills. We also surveyed the northwestern slope of the Tehachapi Mountains at Comanche Creek south of Arvin (4 transects, Figures 1 and 2).

We detected two and perhaps three mated Le Conte’s Thrasher pairs and one unmated (but territorial) male during surveys, totaling three to four Le Conte’s Thrasher territories at Tejon Ranch. We were unable to locate active nests on these territories, despite repeat visits during the Le Conte’s Thrasher breeding season. However, the 2011-2012 winter was notably dry, and during drought many Mojave Desert resident species will delay nest initiation for several weeks. The lack of nesting we observed at Tejon Ranch matched concurrent March and April observations at PRBO study sites in the Mojave and Colorado Desert. In addition, we discovered a number of old and inactive Le Conte’s Thrasher nests during surveys, and it is likely that Le Conte’s Thrashers breed in small numbers on the southeastern edge of Tejon Ranch.

Introduction

Le Conte’s Thrashers are large-bodied, resident songbirds endemic to arid, desert scrub habitats of southern California, southern Nevada, extreme southwestern Utah, Arizona, Sonora, and Baja California (Fitton 2008, Sheppard 1996). They are uncommon across their range, and the vast majority of their distribution lies within the Mojave and Sonoran Desert. A small, disjunct population persists in California’s San Joaquin Valley, Cuyama Valley, and Carrizo Plain, and this Central Valley metapopulation is recognized on the California Bird Species of Special Concern list due to its isolation and vulnerability to habitat loss and degradation (Fitton 2008). They are a focal species for both Arizona and California Partners In Flight (Latta 1999, CalPIF 2009) and they are listed as Yellow Status on the Audubon Society Watch List, (http://birds.audubon.org/species/le-con).

Demographic data on Le Conte’s Thrasher are sparse. Like other members of their genus, they have relatively large home ranges (40-100 ha over multiple seasons) and exist in relatively low densities (< 5 pairs per km2) for a songbird (Sheppard 1996). They hold low fecundity rates and may withhold nesting during drought (CM personal observation, Sheppard 1996). Authors list a lack of suitable habitat as a primary limiting factor to population size distribution (Sheppard 1996, Fitton 2008). While habitat availability is undoubtedly a critical constraint for the isolated Central Valley metapopulation, reports on carrying capacity in Mojave and Sonoran Desert habitats are few. Given the prevalence of what appears to be suitable habitat in desert locales where they can found to be absent, it is not always apparent why Le Conte’s Thrashers exist in the densities (or absence thereof) in which they are found across their range (CM personal observation). The Tejon Ranch’s southeastern, Antelope Valley portion holds Mojave desert scrub, and it includes one of the westernmost Joshua tree stands in the US (http://www.tejonconservancy.org/science/index.html), It is on the periphery of the expected Le Conte’s Thrasher range in the Mojave Desert (Sheppard 1996). Though to our knowledge there were no historic records of Le Conte’s Thrasher presence on Tejon Ranch prior to 2012, Tejon Ranch Conservancy surveyors had accumulated multiple potential sightings before our surveys. In addition, the online eBird database holds multiple records within 20 miles of the eastern boundary of Tejon Ranch (www.ebird.org).

Methods

Survey locations were selected by the Tejon Ranch Conservancy and PRBO Conservation Science based on known Le Conte’s Thrasher habitat preferences. Southeast of the Tehachapi Mountains, surveys were largely centered on and near previously-mapped stands of Joshua trees from 3200 to 4500 feet above Sea Level (Figure 1). Northwest of the Tehachapi Mountains, surveys were placed at low elevations with higher concentrations of low shrubs, in particular California broomsage (Lepidospartum squamatum) and bladder pod (Peritoma arborea) from roughly 600 – 700 feet above Sea Level (Figure 2).

Figure 1. Le Conte’s Thrasher survey transects, Tejon Ranch, Kern County, CA. Transects (red) were conducted from February 18-23 and March 14-16, 2012. Joshua tree woodland represented by pale green polygons, and Tejon Ranch boundary in yellow.

Southeast of Tehachapi Mountains: Mojave Desert transects

PRBO Conservation Science and Tejon Ranch Conservancy Staff systematically surveyed isolated Joshua tree stands southeast of the Tehachapi Mountains from February 18 through February 23, 2012. The survey period was timed to coincide with the beginning of the Le Conte’s Thrasher nesting season (Sheppard 1996) in order to maximize our detection rate. This survey period also increased our chances that we might encounter thrashers building nests and/or incubating eggs.

Joshua tree stands in the study area are generally located in northwest-to-southeast draining canyons bottoms, save for two large stands at 270th and 280th Streets (Figure 1). Tejon Ranch Conservancy staff had previously surveyed these large Joshua tree stands, and thus we focused initial survey effort on the Joshua canyon bottoms to the north of these large stands, moving from east to west.

We surveyed transects from roughly one hour after local sunrise until one hour before sunset; each survey’s duration depended on the amount of Joshua tree habitat in each canyon bottom. Surveys typically started at the Tejon Ranch’s Contour Road and proceeded down-canyon to the edge of the Ranch’s property. Surveyors moved slowly back and forth across each canyon, pausing frequently to scan surrounding Joshua trees and California junipers for perched thrashers. All thrasher detections and behaviors were noted, and explicit care was taken to ensure that each detected thrasher was identified correctly to species (4 Mimid species were present: Le Conte’s Thrasher (Toxostoma lecontei), California Thrasher (Toxostoma redivivum), Sage Thrasher (Oreoscoptes montanus) and Northern Mockingbird (Mimus polyglottos). Transects typically required the surveyor to double back to the car, thus most of the canyons were essentially surveyed twice. We completed surveys of the Joshua tree canyon bottoms on February 22 and surveyed the large Joshua stands at 270th and 280th Streets on February 23.

We returned from March 14-16 to survey unvisited portions of the large Joshua stands surveyed initially on February 21 and on February 23. In addition, we revisited Le Conte’s Thrasher territories discovered in February, in order to 1] ensure that the birds were still present and 2] to search for signs of nesting.

When we detected Le Conte’s Thrashers, we spent up to three hours following the individual(s), noting behavior, searching for potential nests, and taking GPS waypoints from which we mapped their territory’s spatial extent.

Figure 2. Le Conte’s Thrasher survey transects, Tejon Ranch, Kern County, CA. Transects (red) were conducted on March 15, 2012. Tejon Ranch boundary in yellow.

Northwest of Tehachapi Mountains: Great Central Valley Transects

At the suggestion of the Tejon Ranch Conservancy, PRBO Conservation Science biologists surveyed the floodplain of lower Comanche Creek and terraces north of Comanche Point on March 15, 2012 (Figure 2). These areas hold more shrub cover than most other Tejon Ranch holdings on the western slope, and we regarded these locations as the most likely to be occupied in the north half of Tejon Ranch. We conducted these transects from 0900 in the morning to just before noon.

Vegetation Surveys

We conducted four 50 m transects at each of three Le Conte’s Thrasher territories found at Tejon Ranch in March 2012. The four 50 m transects were then surveyed in each cardinal direction from the nest. For two territories, we centered transects at old Le Conte’s Thrasher nests found while following pairs of territory. For an unmated male, we surveyed four 50 m transects in cardinal directions from the first location at which he was found. We measured the amount of cover on each transect by species, and noted whether shrubs were over or under 50 cm in height. For each territory, we recorded the total cover provided by Joshua trees within 50 m of the transects’ center point, as well as the height of the tallest Joshua tree within the 50 m-radius plot.

Species List

We maintained running tallies of all species of birds seen on transects and noted any breeding behavior. We submitted checklists to the online database eBird (www.ebird.org), and a species list is presented in Appendix 1.

Results and Discussion

We detected two Le Conte’s Thrasher pairs (Figure 3, Territories 1 and 2) and a lone, unmated male (Territory 3) that remained present and singing on his territory through both our February and March survey periods. All detections were associated with large Joshua tree stands near 270th and 280th Streets. In addition, we repeatedly detected one to two individuals near the 270th Street gate (Figure 3), with birds moving off Tejon Ranch property in each instance. The gate placed them between Territories 1 and 2, and we were uncertain as to whether these birds represented a new pair or if they were simply re-sighted Territory 1 and/or Territory 2 individuals. If the 270th Street gate detections represent a fourth territory, a large proportion of this territory is southeast of Tejon Ranch property.

Figure 3. Le Conte’s Thrasher territories (white polygons), Tejon Ranch, Kern County, CA. Transects (red), Ranch boundary (yellow), and Joshua tree woodland represented by pale green.

We followed detected Le Conte’s Thrashers throughout their territories, logging approximately ten hours total without noting nesting behavior. In two instances, we found pairs behaving suspiciously near partially-constructed Le Conte’s Thrasher nests in Joshua trees, but in each case the nest was left unfinished. We ultimately judged these nests to be old, perhaps built in 2011.

The winter and spring of 2012 were exceptionally dry, and Tejon Ranch received less than 25% of its normal December through March precipitation (National Weather Service, http://water.weather.gov/precip/). Songbirds have been found to delay nesting during drought, and in extreme cases, they have been found to avoid nesting all together (CM in preparation, Patten and Bolger 2003). We suspect that the two Tejon Ranch pairs would have been nesting by mid-March in wetter circumstances, and that the pairs’ failure to exhibit nesting behavior during the usual nesting season was drought-related. Additional evidence to support this conclusion include our failure to note Le Conte’s Thrasher nesting behavior on a concurrent study in northeastern San Bernardino County through April 2012 (even though thrashers were present on several point count transects), and an absence of observed nesting behavior for any species through the end of our March 16 surveys in the Antelope Valley portion of Tejon Ranch.

Habitat

Though our sample size is small, there are habitat similarities between each of the three Le Conte’s Thrasher territories that merit mention. As stated, the territories were located in large Joshua tree stands near 270th and 280th Streets. Not only are these the largest contiguous stands of Joshua trees at Tejon Ranch, but they are downslope of the Tehachapi foothills and canyons, on relatively flat terrain. Previous reports describe Le Conte’s Thrashers territories as typically lacking physical relief (Sheppard 1996), suggesting an explanation for thrashers’ absence from Joshua tree stands in canyon bottoms to the northeast.

Though only Tejon’s largest stands of Joshua trees held Le Conte’s Thrashers, each territory existed on the stands’ edges, where Joshua trees were grew more sparsely. Less than one-half of one percent of each of our three 50 m radius plots was covered by Joshua trees, even though Joshua trees reached much higher densities nearby (Table 1). Conversely, we never detected Le Conte’s Thrashers in denser reaches of these Joshua tree stands.

Two located nests were each constructed at roughly 1 m in height, in dense lower branches of 2 meter- tall Joshua trees. Though denser congregations of Joshua trees were within sight, the nest sites were relatively isolated from these nearby, denser Joshua trees stands.

Anecdotally, the three Le Conte’s Thrasher territories as a whole tended to hold a more developed woody shrub understory than found over much of our other transects, and notably, the unmated male’s territory held significantly less shrub cover than the two mated pair’s territories (Table 1). The majority of woody cover was provided by rubber rabbitbrush (Ericameria nauseosa), though we also encountered Ericameria cooperi, Ericameria linearifolia, and Eriogonum plumatella on vegetation transects. Rubber rabbitbrush often reached over 1 m in height on the two mated territories, and we suspect that Le Conte’s Thrashers may use rabbitbrush as a nest substrate alternative to Joshua tree.

Though Le Conte’s Thrashers favor open habitats with less shrub cover than their congeners (Sheppard 1996), past reports indicate that they require some shrub cover that provides shade, nesting substrate, and leaf litter and microclimates for prey (Jongsomjit et al. 2012 and Sheppard 1996). We found the two Le Conte’s Thrasher pairs foraging amidst the rabbitbrush on their territories throughout each of our encounters, and we suspect that while the presence of Joshua trees appears to be a critical reason for thrasher presence or absence, rabbitbrush understory is an essential component of thrasher presence on Tejon Ranch.

Table 1. Vegetation characteristics of 3 Le Conte’s Thrasher territories at Tejon Ranch. Territory numbers correspond to territory numbers in Figure 3. % Total Cover (Shrub Layer) equals the percent of four summed 50 m transects covered by vegetation greater than 50 cm in height. % Total Cover (Herb Layer) equals the percent of four summed 50 m transects covered by vegetation under 50 cm in height (regardless of whether this vegetation was woody or non-woody). % Total Woody Cover equals the percent of four summed transects covered by woody vegetation (regardless of height class). Maximum height equals the highest Joshua tree within 50 m of the plot’s center point. % Joshua Tree and % Rabbitbrush equal the absolute cover provided by Joshua tree and rabbitbrush regardless of height class. % Total Cover % Total Cover % Total Woody Maximum % Joshua Tree Territory (Shrub Layer) (Herb Layer) Cover Height (m) Cover % Rabbitbrush Cover

1 5.20 8.70 8.30 2.2 0.40 5.30 2 12.50 2.00 14.50 2.2 0.30 13.05 3 1.35 4.40 3.85 3.2 0.02 1.65

Comanche Creek Surveys

We did not detect Le Conte’s Thrashers on the Comanche Creek floodplain, nor north of Comanche Point. Comanche Creek held occasionally shrub cover, but we only detected California Thrashers on the creek. The terraces north of Comanche Point did not appear to hold sufficient shrub cover at low elevations to support Le Conte’s Thrashers. There are no known Le Conte’s Thrasher sightings east of Interstate Highway 5 since the 1970’s (Fitton 2008).

Detection Rate, Estimated Population Size, and Additional Habitat at Tejon Ranch

Le Conte’s Thrashers are secretive in nature, hold large territories, and move through their territories by running more often than by flight. They sing and vocalize irregularly, and in short, present surveyors with challenges to achieving high detection rates. Though we moved slowly and often covered Joshua tree stands multiple times, it is to be expected that our detection rate was not 100%.

We had a low sample size and our transect locations were not chosen randomly, precluding us from calculating occupation rates and density empirically. However, if we use three habitat criteria ( 1] presence of Joshua tree woodland, 2] presence of shrub understory, and 3] minimal amount of topographic relief), we can map potential Le Conte’s Thrasher habitat at Tejon Ranch and perhaps estimate size of the Tejon Ranch Le Conte’s Thrasher population (Figure 4). Given that we detected at least three territories, given observed territory spacing and size, and given the amount of available habitat, we believe that it is not out of the question that an additional three to four Le Conte’s Thrasher territories exist on Tejon Ranch. As we detected five individuals (and seven if birds observed near the 270th Street gate represented a fourth territory), we estimate that Tejon Ranch may support 5 to 15 Le Conte’s Thrashers.

Figure 4. Potential Le Conte’s Thrasher habitat, Tejon Ranch (Kern County, CA). Occupied territories in white, Joshua tree woodland represented by light green, and Tejon Ranch boundary in yellow. Orange polygons represent habitat that has: 1] sparse Joshua tree woodland, 2] significant shrub understory, and 3] on relative flat terrain. The yellow polygon west of the 270th Street gate has flat terrain and Joshua tree woodland, but generally lacks the shrub understory present on occupied territories. The yellow polygon in the northeast corner of the map holds Joshua tree woodland and a shrub understory, but it is in a relatively narrow canyon with higher relief than occupied territories.

Recommendations

The following are survey and habitat recommendations meant to help the Tejon Ranch Conservancy sustain the Tejon Ranch’s Le Conte’s Thrasher population:

1] Additional Surveys. Though Le Conte’s Thrashers likely breed on Tejon Ranch, we have not confirmed breeding. We were only able to survey on eight days. We scheduled surveys during the typical Le Conte’s Thrasher nesting season, but drought conditions likely delayed the nesting season in 2012, circumventing our survey schedule. Future surveys should occur during the same February – March window, and additional April – May surveys should be added in the event of drought conditions that might push back nesting to later dates.

We were only able to gather transect data on a limited number of territories (n= 3). The Tejon Ranch Conservancy can increase replicates by surveying for Le Conte’s Thrashers on nearby habitat to the east of the Ranch and by assessing these off-Ranch territories habitat characteristics. Off-Ranch surveys would also give the Tejon Ranch Conservancy valuable information on habitat connectivity and on whether and to what degree the Ranch’s Le Conte’s Thrasher population is isolated from other population segments to the east. Off-Ranch surveys might thus guide partnerships between the Tejon Ranch and its neighbors.

Finally, we were unable to conduct randomly generated ‘non-use’ habitat surveys for comparison with our ‘use’ plots. Our habitat assessment thus remains more anecdotal and less quantitative than desired. Ideally, the Tejon Ranch Conservancy might conduct 20 – 30 ‘use’ and ‘non-use’ vegetation plots to better quantify the habitat attributes responsible for Le Conte’s Thrasher presence or absence within the Ranch.

2] Habitat. We have identified areas within Tejon Ranch which we believe are most likely to hold Le Conte’s Thrashers, and we have outlined some broad habitat characteristics (sparse Joshua tree stands amidst larger Joshua woodlands in relatively open and flat terrain, presence of rabbitbrush understory) which our detected Le Conte’s Thrasher territories held in common. The Tejon Ranch Conservancy should manage its grazing activities to sustain and promote Joshua tree and rabbitbrush habitat in the southeastern part of the Ranch. As Le Conte’s Thrashers rely on accumulated leaf litter beneath shrub vegetation to support prey, the Ranch can help Le Conte’s Thrashers by not overgrazing shrub understories and by preventing the sculpting of shrub vegetation that can occur during overgrazing. Appendix 1. Species list with numbers observed, southeast-slope days unshaded, northwest-slope survey shaded gray.

Species Scientific Name 18 Feb 20 Feb 21 Feb 22 Feb 23 Feb 14 Mar 15 Mar 16 Mar

California Quail Callipepla californica 11 23 70 2 27

Turkey Vulture Cathartes aura 23

Osprey Pandion haliaetus 1

Golden Eagle Aquila chrysaetos 1 1

Northern Harrier Circus cyaneus 1 1 1 2 2 1

Cooper's Hawk Accipiter cooperii 2

Swainson's Hawk Buteo swainsonii 1 1

Red-tailed Hawk (Western) Buteo jamaicensis calurus 1 2 1 1 1

Ferruginous Hawk Buteo regalis 1 1 1

Rough-legged Hawk Buteo lagopus 1

Killdeer Charadrius vociferus 1

Whimbrel Numenius phaeopus 9

Long-billed Curlew Numenius americanus 3

Mourning Dove Zenaida macroura 1 3 1

Greater Roadrunner Geococcyx californianus 1 1 2 2 1

Great Horned Owl Bubo virginianus 2

Burrowing Owl Athene cunicularia 1 1

Anna's Hummingbird Calypte anna 1 1

Rufous Hummingbird Selasphorus rufus 2

Ladder-backed Woodpecker Picoides scalaris 1 1 2 4

Northern Flicker (Red-shafted) Colaptes aurus [cafer Group] 1 2 1

American Kestrel Falco sparverius 1 1 1 1

Merlin (Taiga) Falco columbarius columbarius 1

Prairie Falcon Falco mexicanus 1

Black Phoebe Sayornis nigricanus 1

Say's Phoebe Sayornis saya 1

Loggerhead Shrike Lanius ludovicianus 1 7 6 7 3 6 1 5 Aphelocoma californica [californica Western Scrub-Jay (Coastal) Group] 1 1 7

American Crow Corvus brachyrhynchos 1 1

Common Raven Corvus corax 6 8 6 5 4 9 10 10

Horned Lark Eremophila alpestris 6 194 7 45 8 14 1 14

Northern Rough-winged Swallow Stelgidopteryx serripennis 3 1

Tree Swallow Tachycineta bicolor 3 42 2 1

Species Scientific Name 18 Feb 20 Feb 21 Feb 22 Feb 23 Feb 14 Mar 15 Mar 16 Mar

Violet-green Swallow Tachycineta thalassina 3 2 2

Barn Swallow Hirundo rustica 3

Cliff Swallow Petrochelidon pyrrhonota 3 4 1

Rock Wren Salpinctes obsoletus 1

House Wren Troglodytes aedon 2 2

Marsh Wren Cistothorus palustris 1

Bewick's Wren Thryomanes bewickii 1 4 1 6 2 1 5

Cactus Wren Campylorhynchus brunneicapillus 1 13 13 11 17 12 9

Western Bluebird Sialia mexicana 5

Mountain Bluebird Sialia currucoides 2 58 3 5 21 1

Hermit Thrush Catharus guttatus 1

American Robin Turdus migratorius 3

Northern Mockingbird Mimus polyglottos 5

Sage Thrasher Oreoscoptes montanus 28 75 32 74 72 41 2 32

California Thrasher Toxostoma redivivum 6 2

Le Conte's Thrasher Toxostoma lecontei 2 4 4

European Starling Sturnus vulgaris 58 5 6 1 2

American Pipit Anthus rubescens 1

Yellow-rumped Warbler (Audubon's) Setophaga coronata auduboni 1 2 2 6

Spotted Towhee Pipilo maculatus 2

Vesper Sparrow Pooecetes gramineus 1 4 1 2

Lark Sparrow Chondestes grammacus 1 3

Black-throated Sparrow Amphispiza bilineata 2 3 1 Amphispiza bellii Sage Sparrow (Interior) nevadensis/canescens 2

Savannah Sparrow Passerculus sandwichensis 27 2

Song Sparrow Melospiza melodia 2

Lincoln's Sparrow Melospiza linconii 1

White-crowned Sparrow (Gambel's) Zonotrichia leucophrys gambelii 156 202 82 98 168 424 66 486

Dark-eyed Junco (Oregon) Junco hyemalis [oreganus Group] 29 16 17 2

Western Meadowlark Sturnella neglecta 7 67 52 4 8 29 2

Brewer's Blackbird Euphagus cyanocephalus 70

House Finch Haemorhous mexicanus 47 235 19 123 67 19 2 24

Lesser Goldfinch Spinus psaltria 2

Lawrence's Goldfinch Spinus lawrencei 26

Literature Cited

CalPIF (California Partners In Flight). 2009. Version 1.0. The Desert Bird Conservation Plan: a Strategy for Protecting and Managing Desert Habitats and Associated Birds in California. California Partners in Flight. http://www.prbo.org/calpif/plans.html

Fitton, S.D. 2008. Le Conte’s Thrasher (Toxostoma lecontei). In California Bird Species of Special Concern: A ranked assessment of species, subspecies, and distinct populations of birds of immediate conservation concern in California. Shuford, W.D. and Gardali, T., editors. Studies of Western Birds 1. Western Field Ornithologists, Camarillo, California, and California Department of Fish and Game, Sacramento.

Jongsomjit, D. J. R. Tietz, S. Michaile, T. Fonseca, G.R. Geupel. 2012. Le Conte’s Thrasher Monitoring in the Carrizo Plain National Monument: Report to the Bureau of Land Management. PRBO Conservation Science, Petaluma, CA.

Latta, M.J, C.J. Beardmore, T.E. Corman. 1999. Arizona Partners In Flight Bird Conservation Plan. Version 1.0. http://www.azgfd.gov/pdfs/w_c/partners_flight/APIF%20Conservation%20Plan.1999.Final.pdf

Patten . M.A. and D.T. Bolger. 2003. Variation in top-down control of avian reproductive success across a fragmentation gradient.

Sheppard, J.M. 1996. Le Conte’s Thrasher (Toxostoma lecontei), The Birds of North America (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology.