Endangered Species Recovery Program

8. Le Conte's Thrasher (San Joaquin Valley Population) (Toxostoma lecontei lecontei)

• Taxonomy
• Description
• Identification
• Historical Distribution
• Current Distribution
• Food and Foraging
• Reproduction
• Demography
• Behavior and Species Interactions
• Activity Cycle
• Habitat and Community Associations
• Reasons for Decline
• Threats to Survival
• Conservation Efforts
• Conservation Strategy

Taxonomy.-- The genus Toxostoma is comprised of 10 species of thrashers, all of which are found in North America, including Mexico. Most thrasher species breed in the arid southwestern United States and northwestern Mexico. California species within the genus include Le Conte's thrasher (T. lecontei), California thrasher (T. redivivum), crissal thrasher (T. crissale), and Bendires thrasher (T. bendirei) (Peterson 1990). The type specimen of Le Conte's thrasher was described by Lawrence (1852) from a single specimen collected in Yuma County, Arizona by John L. Le Conte. The American Ornithologists Union, in 1957, recognized two subspecies of T. lecontei: the desert thrasher (T. l. arenicola) of the west coast of Baja California, and Le Conte's thrasher (T. l. lecontei) of the San Joaquin Valley and Mojave and Colorado Deserts of California and Nevada southward into northeastern Baja California, Mexico, and points farther south; and the Sonoran Desert of Utah, Arizona, and Mexico. In 1965, based on plumage coloration, Phillips described the population of Le Conte's thrasher found in the San Joaquin Valley as T. l. macmillanorum from four birds collected near Buttonwillow, Kern County, California. Phillips (1965, according to Sheppard 1973) described the San Joaquin population as having a slightly darker crown than back, with slightly lighter sides, flanks, and breast when compared with the T. l. arenicola. A comparison of measurements between the T. l. arenicola and T. l. lecontei and the San Joaquin Valley population indicated no significant difference (Sheppard 1973), and Sheppard concluded that T. l. macmillanorum is a synonym of T. l. lecontei.

The San Joaquin Valley population apparently is isolated from other populations of Le Conte's thrasher and is resident; individuals do not migrate (Grinnell 1933b, Sheppard 1996). Sheppard (1973) suggested that the exchange of genetic material between the San Joaquin population and others probably does not occur. Recent DNA analysis (Zink and Blackwell as reported in Sheppard 1996) found no mtDNA sequence differences between the San Joaquin Valley population (T. l. macmillanorum) and other samples from the southwestern United States. The T. l. macmillanorum subspecies recognition, Sheppard suggests, should be withheld until some set of characters shows clear and abrupt divergence from west Mojave and Colorado desert populations.

Description.-- The Le Conte's thrasher is a medium-sized songbird, about the same size as the northern mockingbird (Mimus polyglottos). The total length and weights are nearly identical for both sexes: 240 to 280 millimeters (9.4 to 11 inches; Ridgway 1907) and 54.5 to 75.5 grams (1.9 to 2.6 ounces; Sheppard 1973). The Le Contes thrasher has a plain grayish--or sandy--colored body without wing bars or spots.

Identification.-- The Le Conte's thrasher (Figure 68) is distinguishable from songbirds other than thrashers by its long, nearly black, tail (about 12 centimeters, about 4.7 inches), and its distinctly-decurved black bill (about 2.7 centimeters, about 1 inch). The adult Le Conte's thrasher is distinguished from other thrashers by its unspotted breast, pale buffy crissum (undertail feathers), dark eye, lack of distinct superciliary stripe (above the eye), and dark tail contrasting sharply with the much paler body. The California and crissal thrashers are lrger and darker. The California thrasher has a cinnamon crissum. The crissum of the crissal thrasher is a deep chestnut color (Sheppard 1996). The San Joaquin Valley population of Le Conte's thrasher has a slightly darker crown than back, and slightly lighter sides, flanks, and breast than the desert thrasher (Phillips 1965).

Figure 68. Illustration of a Le ConteÆs thrasher. Drawing by Wendy Stevens (© CSU Stanislaus Foundation).

Historical Distribution.-- Le Conte's thrasher occurs in two separate geographic areas: the Colorado and Mojave deserts down into Baja California, Mexico, where the species is widespread (Laudenslayer et al. 1992), and the southern San Joaquin Valley. Most Le Conte's thrashers are found between sea level and 1,150 meters (3,800 feet) (Sheppard 1973). The northernmost location for Le Conte's thrasher was Mono County, California; the southernmost was on the west coast of Baja California. The historical range for the San Joaquin Valley population of Le Contes thrasher included the west side of the San Joaquin Valley, from the Panoche Mountains, Fresno County, in the north, to Maricopa, Kern County, in the south (Figure 69) (Dobkin and Granholm 1990). Grinnell (1933b) used a reverse "J" shape to describe the range: the northern extent stopped at Huron, the valley floor of the San Joaquin Valley was excluded, and neither the Carrizo Plain nor Cuyama Valley were included. Sheppard (1970 and 1973) added the Carrizo and Cuyama based on his personal observations, added the Valley floor based on specimens near Wasco collected after Grinnell, and added the Panoche Mountains based on an observation by a birder.

Current Distribution.-- The current distribution of the San Joaquin Valley population of Le Conte's thrashers is determined largely by the presence, structure, and vigor of saltbush, proximity to other saltbush areas, size of habitat fragment, and presence of California thrashers. The picture is of a complex of islands with relatively insurmountable distances of unsuitable habitat separating them. Irrigation and land development have eliminated a considerable amount of former habitat in the San Joaquin Valley, restricting the San Joaquin Valley population of Le Conte's thrashers to a small portion of its former range (Laudenslayer et al. 1992). There are five known and one potentially extant population areas. Each area is a mosaic of habitats ranging from unsuitable to fair habitat (only two of the five areas have good to excellent habitat). A brief discussion of each area follows:

1. McKittrick - Maricopa. This area extends from Belridge just north of McKittrick, south to Devils Gulch south of Maricopa, east to the California Aqueduct between Lokern Pumping Station and Pentland, and west to the lower third of the Temblor Mountains. This is by far the largest and best habitat area. The highest concentrations of Le Conte's thrasher are near McKittrick and Maricopa. The southwest corner of the Belridge oil field has several hundred acres of good habitat. Several pairs of thrashers persisted here through the drought. However, areas of unsuitable nesting habitat exist. In early May 1997, a wildfire burned 40,000 acres in the area known as the Lokern, including burning half of a grazing experiment study area. On 22 July 1997, USBLM burned another 1,000 acres on the Lokern Study Area to keep the 4 square mile experimental area similar. Bird data gathered just prior to the fire in 1997 documented Le Conte's thrashers adjacent to seven of eight plots while none were detected in April and May of 1998. Observations of Le Conte's thrashers several miles from the study plots indicate that the lack of observations in 1998 in the study area is likely a result of the nearly complete mortality of saltbush (charred skeletons remain) and not a decline of the species in the local area (S. Fitton pers. comm.).
2. Lost Hills. This area extends north from Highway 46 for less than 9.6 kilometers (6 miles) with the Califrnia Aqueduct as the eastern boundary. Habitat patches are small and highly fragmented with probably fewer than 20 pairs of thrashers. Significant distances of plowed ground separate this subpopulation from the Maricopa and Kettleman Hills subpopulations.
3. Kettleman Hills. This area is from Highway 41 north to almost Jayne Road. The eastern boundary is Interstate 5, and western boundary is the near Highway 33. There is little good habitat in the Kettleman Hills, probably supporting fewer than 20 pairs. In the 1960s, J. M. Sheppard (pers. comm.) estimated this subpopulation to be 200 pairs. This area is now entirely surrounded by plowed ground, however, there is good potential for habitat improvements on all the domes of the Kettleman Hills and the adjacent alluvial fans. Without grazing, the Kettleman Hills accumulate a thick and tall mulch that is generally avoided by Le Conte's thrashers. (Note: A 8,100 hectare (20,000 acre) wildfire, started from Interstate 5 in 1995, typifies the threat of fire to this species habitat. The fire destroyed most of the occupied habitat on the Middle Dome of the Kettleman Hills leaving habitat on only about half of the North Dome from about Skyline Boulevard, State Route 269, north to end of the hills (S. Fitton pers. comm.)
4. Carrizo - Elkhorn Plain. This area is composed of two subunits. One is the Elkhorn Plain, extending from Wallace Creek in Panorama Hills on the north, south to Beam Flat. The other subunit is within the southern end of the Carrizo Plain. The birds of these two subunits probably come into contact with each other as well as with Le Conte's thrashers from the McKittrick - Maricopa area. They may also come in contact with the Cuyama area birds. The Carrizo - Elkhorn Le Conte's thrashers overlap with California thrashers.
5. Cuyama Valley. Since the time Sheppard (1970) first found Le Conte's thrashers in Cuyama Valley, much of the habitat has been overgrazed or converted to agriculture (J. Sheppard pers. comm., S. Fitton pers. comm.) Now, after extensive surveying, the species is only found in a small area dominated by ephedra, from the mouth of Ballinger Canyon north to CA Highway 166. Many areas now seem to support California thrashers. There are probably fewer than 10 pairs of Le Conte's thrashers in the Cuyama Valley (S. Fitton and L. Saslaw unpubl. observ.). If the alluvial fans east of CA Highway 33 reverted to native shrublands, Le Conte's thrashers would no doubt respond by expanding into the habitat. Le Conte's thrashers in the Cuyama Valley are surrounded by excellent, occupied California thrasher habitat as well as a nearly continuous, narrow belt of California thrasher habitat along the Cuyama River.
6. Panoche Mountains. Recent surveys, from 1989 to 1998, have not located Le Contes thrashers north of Kettleman Hills (S. Fitton unpubl. observ.). While some of the habitat looks suitable, only California thrashers have been seen recently. It is possible that Le Conte's thrashers occur in the Panoche Mountains at very low numbers and isolated from other subpopulations.

Other areas that historically have had Le Conte's thrashers or appear to be suitable and have been surveyed over several years, 1989 to 1998, without success are: Panoche Hills, Panoche/Silver Creeks, Tumey Hills, Antelope Hills, Sunflower Valley, alluvial fans on the south side of Caliente Mountain, portions of the Carrizo Plain, Warthan Creek, Los Gatos Creek, Guijarral Hills, Skunk Hollow, Poso Creek north of Bakersfield, and isolated patches of saltbush along Interstate 5 from Stockdale Avenue north to Twisselman Road (S. Fitton and L. Saslaw unpubl. data).

Figure 69. Distributional records for the San Joaquin Valley population of Le Conte's thrasher (Toxostoma lecontei macmillanorum)

Food and Foraging.-- The Le Conte's thrasher occupies a highly specialized niche within the ecosystem (Sheppard 1973). The Le Conte's thrasher forages in the leaf litter nder saltbush plants, on the ground surface, or 5 to 7.6 centimeters (2 to 3 inches) into the substrate for arthropods, including scorpions, spiders, beetles, grasshoppers, and butterfly and moth larvae. Occasionally, this species will feed on seeds, small lizards, or other small vertebrates (Bent 1964, Sheppard 1970). Le Contes thrashers are not known to drink water; their diet is their only source of water (Sheppard 1970).

Reproduction.-- Singing starts in mid-autumn and peaks in late December and January, as nest building begins. The species is not migratory and pairs remain together throughout the year. Mated pairs appear to have site fidelity until one bird dies. Thick, dense, and thorny desert shrubs (such as saltbush) are preferred for nesting sites (Sheppard 1996). Such plants are often along well established drainages, or are older, well formed plants on upland sites. Le Conte's thrashers do not use habitats without this structure (S. Fitton unpubl. data).

The breeding season for Le Conte's thrasher begins in late January and extends through early June, with the peak ranging from mid-March to mid-April. This species may have up to three broods during the reproductive season. Clutch size is usually 3 or 4 eggs (range 2 to 5). Eggs are incubated for 14 to 20 days by both parents. Young fledge 12 to 20 days after hatching, with the male continuing to feed the young if the female is incubating the next clutch. At approximately 30 days old, fledglings disperse approximately 400 meters (1,300 feet). Dispersal movements may continue until the young are clear of occupied territory (Sheppard 1970, 1996). Based on dispersal of young, it is estimated that if isolated habitat fragments are greater than 10 to 15 kilometers (6 to 9 miles) apart, colonization or recolonization may be precluded (S. Fitton as reported in Sheppard 1996).

Demography.-- Grinnell (1933b) estimated 2.3 pairs per square kilometer (less than 1 pair per square mile) near McKittrick, Kern County, California during late February and March, when adults are less obvious. Average January density at Maricopa was 4.6 pairs per square kilometer (12 pairs per square mile) (Sheppard 1996). San Joaquin Valley Le Conte's thrashers banded near Maricopa used from 20 to 50 hectares (50 to 125 acres) per pair over 1 year (Sheppard 1973). Home range may vary in size and shape depending on time of year and interactions with neighbors. It is estimated that about 7 hectares (18 acres) are needed per pair for nesting territory (Sheppard 1970). Since the late 1960s, densities of the San Joaquin Valley population of Le Conte's thrasher have declined except in a few core areas (Laudenslayer et al. 1992).

Behavior and Species Interactions.-- The Le Conte's thrasher is a resident species, remaining year round in suitable habitat. In general, the Le Conte's thrasher is a terrestrial bird, running among shrubs rather than flying. Flying occurs irregularly, such as during nest building and when bringing food to the young. The Le Conte's thrasher is highly territorial through much of the year. Males become less territorial during the summer months when they are molting and young are dispersing. The territory is most actively defended between early December and early February (Sheppard 1970, S. Fitton unpubl. data).

Potential competitors for food and nesting sites include California thrasher, sage thrasher (Oreoscoptes montanus) northern mockingbird, loggerhead shrike (Lanius ludovicianus), and greater roadrunner (Geococcyx californianus) (Sheppard 1973). At Maricopa, California, San Joaquin Valley Le Conte's thrashers and loggerhead shrikes often nest within 20 meters (65 feet) of each other (Sheppard 1973). Species known to prey upon the eggs, young, and adults of Le Conte's thrashers include hawks, owls, greater roadrunners, antelope ground squirrels, cats, dogs, coyotes, and various species of snakes (Sheppard 1973).

Activity Cycle.-- The San Joaquin Valley Le Conte's thrasher is active during daylight, throughout the year. Little or no activity takes place during eriods of higher temperatures (above 35 to 38 degrees Celsius [95 to 100 degrees Fahrenheit]) (Sheppard 1970).

Habitat and Community Associations.-- Le Conte's thrashers are generally found in open desert scrub, alkali desert scrub, and desert succulent scrub. In the San Joaquin Valley, the species is found primarily in habitats dominated by saltbush, and often frequents desert washes and flats with scattered saltbush (Laudenslayer et al. 1992). Nesting habitat mainly is in taller, bushier shrubs. Sheppard (1970, 1973) found San Joaquin Le Conte's thrashers most commonly associated with sandy and alkaline soils, but believed that, except for texture, soils had little direct effect on the distribution of the species.

Within the Maricopa region, Le Conte's thrashers are in contact with California thrashers wherever patches of willow and/or big saltbush are found, and along the foothills of the Temblor Mountains wherever the slope increases and eastwoodia and narrowleaf goldenbush begin to dominate on north-facing slopes. California thrashers occupy moister and shadier locations (even as a microclimate).

Reasons for Decline.-- Habitat degradation and loss to agriculture, urbanization, oil and gas development, fire, and over-grazing by livestock are the primary reasons for decline of the San Joaquin Valley population of Le Conte's thrasher (Laudenslayer et al. 1992). Pesticides may have historically been responsible for nesting failure. In Maricopa several clutches from 1968 to 1971 failed to hatch and DDT and DDE poisoning were suspected (P. Owens as reported in Sheppard 1996). Prior to the 1972 ban, DDT spraying was conducted in this area each winter.

Threats to Survival.-- Because of the San Joaquin Valley Le Conte's thrashers limited mobility and susceptibility to habitat fragmentation and degradation, it is vulnerable to becoming isolated and eventually disappearing from a nesting area. The loss of expansive areas of suitable nesting and foraging areas is a considerable threat to the population of Le Conte's thrasher within the San Joaquin Valley. Though a significant amount of saltbush-dominated communities has been converted to agricultural land use, there remains substantial acreage of annual rangelands on the west side of the San Joaquin Valley that may be suitable for this species. Whether these habitats are occupied depends on the structure of the saltbush overstory, the size of the habitat patch, and the connectivity among habitat patches.

Much of the remaining habitat is predominately used for livestock grazing and petroleum production. Suitable saltbush structure can be eliminated by heavy livestock grazing which mechanically damages plants and reduces leaf litter. Many acres of suitable habitat have been eliminated through grazing practices that remove saltbush structure or restrict seedling establishment. However, suitable habitat can be reestablished with modification of livestock grazing practices that allows for seedling establishment and the development of plants greater than 1.5 meters (4.9 feet) in height and scattered across the landscape. As was evidenced in the Carrizo Plain following reestablishment of saltbush, Le Conte's thrasher will recolonize new saltbush stands.

Wildfires that burn large acreages of saltbushes eliminate suitable Le Conte's thrasher habitat. The duration of such habitat loss may depend on fire frequency, climatic conditions that favor saltbush reestablishment, and livestock grazing practices. While fire kills saltbushes (D. Germano and L. Saslaw unpubl. data), the site can be repopulated with saltbushes under favorable climatic conditions and compatible grazing practices (S. Fitton unpubl. data).

Dense cover of herbaceous vegetation, especially introduced annual grasses and filaree that result in thick mats of dead vegetation, reduce foraging habitat for this species and increases the risk of wildfire.

Oil and gas development continue to be a threat. Intensive petroleum development that eliminates all vegetative cover over large acreage eliminates Le Conte's thrasher habitat. However, light and modrate petroleum activities that maintain the saltbush community between wells and facilities, and tall saltbushes along drainages, do provide substantial habitat for this species. Most of the oil fields in the western foothills of the southern San Joaquin Valley provide suitable thrasher habitat. Oil sumps not properly maintained have proven fatal to young and adults who become entrapped (Sheppard 1996).

Brood parasitism by cowbird species (Molothrus spp.) has not been widely noted, however, S. Rothstein (as presented in Sheppard 1996) tested active Le Conte's thrasher nests at Maricopa, to artificial introduction of brown-headed cowbird (Molothrus ater) eggs. All of the eleven thrasher pairs in the experiment accepted these eggs as their own.

Conservation Efforts.-- The San Joaquin Valley population of Le Conte's thrasher is not a candidate for Federal listing, but is considered a species of concern (USFWS 1996). It is also a California Species of Special Concern (Remsen 1978). No areas of habitat have been set aside specifically for this thrasher. However, conservation areas such as the Carrizo Plain set aside for other species in jeopardy (e.g., San Joaquin kit fox, giant kangaroo rat, blunt-nosed leopard lizard, etc.) also have benefited this species.

The maintenance of saltbush communities has been identified as a management objective in the Carrizo Plain Natural Area, Elk Hills Naval Petroleum Reserves in California, on USBLM lands in western Fresno, Kings, Kern and San Luis Obispo Counties, and at the Lokern Area. The maintenance of saltbushes in drainage channels and conservation of natural lands in the oil fields are also being addressed in the Habitat Conservation Plans. However, the lands in conservation programs are a small percentage of the available habitat.

Conservation Strategy.-- A systematic review, distributional survey, and population monitoring of the San Joaquin Le Conte's thrasher are needed to clarify the birds distributional and population statuses, potential threats of endangerment (Laudenslayer et al. 1992), and listing status.

Maintenance of the saltbush communities in the oil fields will be a key component for conservation. Management practices that avoid saltbush drainages, minimize habitat disturbance, and promote reclamation of degraded saltbush communities will aid in conservation. Reintroduction of Le Conte's thrashers into patches of suitable saltbush larger that 405 hectares (1,000 acres) should be investigated.

Maintenance of remaining saltbush communities and connecting fragmented stands of suitable habitat in southwestern Kern County would significantly reduce the threats to this species. Annual rangelands found on deeper alluvial soils that are capable of supporting tall stands of common saltbushes should be promoted on public and private rangelands. Grazing management practices that aid in the establishment and maintenance of common saltbush on suitable sites should be introduced to livestock producers for management and economic evaluation. Appropriate grazing management practices on Federal, CDFG and other conservation lands should be implemented to maintain suitable saltbush and herbaceous structure. Key conservation areas include the Naval Petroleum Reserve in California #2, Occidental of Elk Hills, Lokern Area, USBLM lands around Taft and Maricopa, and the Elkhorn Plain. If such provisions are included and implemented in upcoming Habitat Conservation Plans, long-term conservation probably can be achieved.

The status of the San Joaquin Valley population of the Le Conte's thrasher should be reevaluated within 5 years of recovery plan approval or when new information is available, whichever is less.