California Partners in Flight Desert Bird Conservation Plan
Le Conte's Thrasher - Cuitlacoche Pálido
Prepared by: James Weigand (firstname.lastname@example.org) and Sam Fitton, Ecologist
USDI Bureau of Land Management
California State Office
2800 Cottage Way
Sacramento, CA 95825
Weigand, J. and S. Fitton. 2008. Le Conte’s Thrasher (Toxostoma lecontei). In The Draft Desert Bird Conservation Plan: a strategy for reversing the decline of desert-associated birds in California. California Partners in Flight. http://www.prbo.org/calpif/htmldocs/desert.html
action plan summary
SPECIES: Le Conte's Thrasher (Toxostoma lecontei)
The taxonomy for subspecies of Le Conte’s thrasher populations does not have universal agreement. Three major and non-adjacent subpopulations may comprise three subspecies: the nominate T. l. lecontei, occurring in the Sonoran and Mojave Deserts of the states of California, Nevada, Arizona, Utah, Sonora, and Baja California; the San Joaquin Valley population in central California, T. l. macmillianorum; and the Peninsular Desert population on the Pacific Coast of Baja California Sur south of 28°56'N, T. l. arenicola. Banks et al. (2007) have not recognized a taxonomic split of T. l arenicola to T. arenicola in the A.O.U. Checklist, seventh edition, as suggested by Zink et al. (1997) based on mitochondrial divergence believed to have occurred since creation of the Baja California mid-peninsular seaway one million years ago. Zink et al. (op. cit.) also question the validity of a genetic discontinuity between the purported subspecies “lecontei” and “macmillianorum”.
Until 2006, the California Department of Fish and Game and the USDI Bureau of Land Management, California State Office, recognized Le Conte’s Thrasher as a Bird Special of Special Concern (Third Priority) and Sensitive Species respectively. Since 2006, both the Department and the Bureau recognize only the San Joaquin Valley population (“macmillianorum”) for special management designation by each agency.
This species is protected under the Migratory Bird Treaty Act.
The US Fish and Wildlife Service (2002) has designated Le Conte’s Thrasher as a bird species of conservation concern in the USA portions of Bird Conservation Regions 32 (coastal California) and 33 (Sonoran and Mojave Deserts) that lie within US Fish and Wildlife Service Regions 1 and 2. Le Conte’s Thrasher is also part of the USA National Bird Conservation Concern List.
The United States Watch List of Birds of Conservation Concern (2007) produced by the non-governmental organizations American Bird Conservancy and the National Audubon Society designated Le Conte’s Thrasher as a Rare Yellow List species, i.e., a species that has a stable population and is not facing extreme threats but remains nonetheless threatened because of the small range and small population size of the species.
In 2004, the International Union of Conservation of Nature and Natural Resources (IUCN) evaluated Le Conte’s Thrasher in the Least Concern category among IUCN Red List species.
I. HISTORICAL BREEDING DISTRIBUTION:
Le Conte's Thrasher [sensu stricto Zink (1997)] is non-migratory and occurs in two disjunct geographic areas: (1) the Sonoran and Mojave deserts from Mono County, CA, Esmeralda County, NV, and southwest Washington County, UT, south through western Arizona in the Colorado, Bill Williams, and Lower Gila Rivers, to Bahia San Felipe, northern Baja California and central coastal Sonora; and (2) the west side of the San Joaquin Valley from Huron, Fresno County, south to Maricopa, Kern County (Fitton and Saslaw 1998). The species is widespread but patchily distributed (Laudenslayer et al. 1992). Sheppard (1970, 1973) located populations of Le Conte’s Thrashers in the Carrizo Plain National Monument (San Luis Obispo County, the eastern Cuyama Valley (Santa Barbara County), and Central Valley Floor (Wasco, Kern County). Le Conte’s thrashers may have occurred or still occur sporadically in the Panoche Hills in western Fresno County, but recent records are few (S. Fitton pers. comm.).
II. CURRENT BREEDING DISTRIBUTION:
The IUCN (2004) has estimated global extent of the range of Le Conte’s Thrasher to be approximately 250 000 km². In California, Le Conte’s Thrashers nest west to near Julian, Palm Springs, Banning, the Antelope Valley, Walker Pass in Kern County, and, disjunctly, in the San Joaquin Valley. The northern range limit extends to the Owens Valley, Panamint Valley, and Death Valley and then east to Nevada and on to the Beaver Dam Mountains in the southwestern corner of Utah and southeastward to central and southern Arizona (Prescott 1999).
Habitat losses within the range from land use conversions to agriculture, urbanization, motorized recreation, and military infrastructure development may be transforming desert habitat substantially and creating patchier habitat for Le Conte’s Thrasher. Losses of original habitat are noteworthy in the Antelope Valley (Los Angeles County), the Victor and Lucerne Valleys (San Bernardino County), the Coachella Valley (Riverside County), and the Imperial Valley (Imperial County) in California, but quantitative data on land use changes and resulting impacts to Le Conte’s Thrasher populations globally have not been analyzed. It is unclear whether the species continues to nest in Mono County (Sheppard 1996).
Although Le Conte’s Thrashers avoid scrub agricultural areas (Small 1988), they can readily adapt to restored vegetation consisting of very large Atriplex canescens specimens along the second barrel of the Los Angeles Aqueduct (J Weigand pers. obs.).
Le Conte’s Thrashers in the San Joaquin Valley appear to be at greater conservation risk. Habitat losses in the western San Joaquin Valley foothills now restrict the San Joaquin Valley population of Le Conte's Thrashers to a small portion of their former range (Laudenslayer et al. 1992). Five known populations and one potentially extant population remain (Fitton and Saslaw 1998). Each area is a mosaic of habitats ranging from unsuitable to fair habitat. Only two of the five areas have parcels of good to excellent habitat.
Figure 1. Le Conte's Thrasher distribtion and density: http://www.mbr-pwrc.usgs.gov/bbs/htm03/ra2003_blue/ra07110.htm
I. AVERAGE TERRITORY SIZE
Estimates for breeding territory size range from 0.8 to 2.7 ha (Hensley 1954, Laudenslayer 1981). Foraging territory expands over a larger area (4.8 ha) in winter than during the nesting season (0.8 ha) (Smith 1967). Territorial boundaries are also subject to change in response to local population density fluctuations, changes in vegetation composition, and habitat disturbances.
II. TIME OF OCCURRENCE AND SEASONAL MOVEMENTS
Le Conte’s Thrasher is a non-migratory species, resident year round in it range. In the Central Valley of California, the species is not known to ever have occurred north of Fresno County although there are no topographic barriers to dispersal.
III. MIGRATION STOP-OVER NEEDS/CHARACTERISTICS
IV. NEST TYPE
Nests of Le Conte’s Thrashers are complex, variable, camouflaged structures as large as 28 cm in diameter and up to 23 cm high. Major construction materials are twigs for the outer nest shell; small twigs and herbaceous fibers in the mid-layer; and exclusively fine plant fibers in the innermost nest layer (Sheppard 1996). The layering is thought to provide insulation and cushioning to protect nestlings.
V. FORAGING STRATEGY
Le Conte’s Thrashers forage as generalists on bare ground and among vegetation litter beneath shrubs. Le Conte’s Thrashers readily scratch in the soil as they search and can overturn objects greater than their body weight with their bills (Sheppard 1996). Foraging may occur at any time during daylight hours except at the hottest times of the day during the hottest months of the year. Bands of birds may hunt communally in the immediately post-breeding period (J Weigand, pers. obs.).
Normally a secretive bird, Le Conte’s Thrashers become quite vocal and visible, even at mid-day, flying up to 15 m high when defending nesting territory and chasing intruding birds in mid- and late winter in the Mojave Desert (J. Weigand, pers. obs.).
VII. SOCIAL ORGANIZATION
A. Typical Breeding Densities
Sheppard (1996) estimates less than 0.2 pairs of Le Conte’s Thrashers per square kilometer throughout the species range in the United States. Because territories are commonly greater than 40 ha, densities of breeding pairs are low (Bury et al. 1977).
B. Mating System
Le Conte’s Thrashers are principally monogamous; observations come from the San Joaquin Valley populations only. Sheppard (1996) found that the sex ratio of Le Conte’s Thrashers over four years be 1:1.
C. Delayed Breeding
No known data.
D. Post-fledging Biology of Offspring
Fledged birds remain briefly in the nesting pair’s territory until no more than 18 days after fledging. Average dispersal distance in the San Joaquin Valley was 395 m from the nest after 30 days. Young of the year wander up to 4 km away from the nest (average distance 1.8 km) through early November (Sheppard 1996). Data on fledgling Le Conte’s Thrasher are not available from the Mojave and Sonoran Deserts.
D. Post-breeding Social Behavior
Juveniles often form loose bands of up to eight birds away from their birth nests through the late summer over a 20-ha area. Individual birds do not show any particular fidelity to a particular band. Pairs of birds may form from juvenile birds as early as mid-summer (Sheppard 1996).
VIII. CLUTCH SIZE
The usual range in two to five eggs per clutch, often depending on availability of food for the breeding pair. In the Mojave Desert, the average number of eggs 3.38, somewhat higher than for the Sonoran Desert (3.32). Nests at mid-season (March and early April) have larger clutches that earlier or later in the breeding season, across the entire range (Sheppard 1996).
IX. INCUBATING SEX
Both sexes alternate incubation nearly continuously during the day, but predominately by the female bird (Sheppard 1996).
Data from the San Joaquin Valley show that incubation ranges between fourteen and nineteen days, with incubation lasting longer during cold, rainy weather (Sheppard 1996).
XI. NESTLING PERIOD
Nestlings remain in the nest up to 17 days (on average 15.3 days).
XII. DEVELOPMENT AT HATCHING
Birds are born naked and with closed eyes (altricial) and remain inside the nest.
XIII. NUMBER OF BROODS
Depending on weather conditions and food availability, Le Conte’s Thrashers may have zero to three broods per year. When drought is prevailing, nesting may not occur.
XIV. CARE OF YOUNG
Female birds predominantly brood nestlings. Males do most food gathering, although parents exchange tasks of care and tending. Birds make more than 90 feeding trips a day while young are in the nest (Sheppard 1996).
A. Major Food Items
Le Conte’s Thrashers principally (92%) consume arthropods: Orthoptera (grasshoppers), Formicidae (ants), Lepidoptera (moths), Coleoptera (beetles), Scorpionida (scorpions), and spiders (Arachnida). Prey may weigh up to 4 g. Bird eggs, small lizards, and young snakes are minor vertebrate components of diet. Some seeds are also part of the diet (Sheppard 1996). Seasonal variation in diet is not described.
Drinking from water sources is very rare. Prey provide water needed (Sheppard 1996).
XVI. WINTERING GROUND NEEDS AND DISTRIBUTION
Wintering grounds are in the vicinity of breeding territories. No seasonal migration or habitat change occurs.
BREEDING HABITAT AND NEST SITE CHARACTERISTICS:
I. OVERVIEW OF BREEDING HABITAT
Le Conte’s Thrashers nest preferably in thorny shrubs or small desert trees. Where shade was available for a shrub, for example next to an arroyo wall, Le Conte’s Thrashers in the San Joaquin Valley nested preferentially (Sheppard 1996).
II. NEST SITE
A. Substrate Species
Cacti (Opuntia spp.), saltbushes (genus Atriplex) and other chenopod shrubs, yuccas (including small Joshua trees), and mesquites (Prosopis spp.) are favored plants for nest sites. In southeastern California, blue palo verde (Parkinsonia florida)and lotebush (Ziziphus obtusifolia) (C. McCreedy, unpubl. data).The common characteristic of most species with Le Conte’s Thrasher nests is thorniness.
B. Height of Nest and Nest Plants
Most nests are less than one meter from the ground in plants that are less than three meters tall (Sheppard 1996).
C. Nest Concealment
Nests are rarely visible from more than a three-meter distance (Sheppard 1996).
III. VEGETATION SURROUNDING THE NEST
A. Dominant Plant Communities for Nests and Canopy Cover
Le Conte’s Thrashers favor settings with higher fertility and above-ground biomass so that vegetation is thick and able to support and hide a substantial nest. Shrubs in the Chenopodiaceae plant family, especially in alkaline or saline soils, are common settings for nests. Although Le Conte’s Thrashers do not build nests in creosote (Larrea tridentata), they occur frequently in the widespread creosote – burrobush (Ambrosia dumosa) plant association, where desert-thorns (Lycium spp.) have stout stems to support thrasher nests (Hill 1980). Joshua tree (Yucca brevifolia) woodlands with abundant shrubs are also widely used in the Mojave Desert. Dense mesquite thickets close to extensive shrub lands are also good sites, but more massive Sonoran Desert woodlands do not support Le Conte’s Thrasher except at woodland-shrubland edges.
Le Conte’s Thrashers in the Mojave Desert prefer habitats with deep soils that encourage comparatively robust and dense shrub growth. In general, these sites are flat often at the base of alluvial fans. Rocky, steep slopes generally cannot support suitable sizes of shrubs for nesting.
Aspect does not appear to play an important role in flat sites. However, Le Conte’s Thrashers frequently nest where nest sites are shaded, as reported by Sheppard (1996).
D. Tree DBH
When Le Conte’s Thrashers are nesting in Joshua tree woodlands (not necessarily in Joshua trees), the proximity of a nest to one or more perch sites atop a three meter stem is common. Such short stems, the product of repeated fires, average about eight to ten inches dbh.
A Joshua tree snag may also serve as a perch near a nest.
F. Distance to Water
IV. LANDSCAPE FACTORS
Le Conte’s Thrasher habitat extends from -81 m in Inyo County, CA, to 1,500 m or higher in the Mojave Desert (Sheppard 1996).
Urbanization of the Sonoran, Mojave, and San Joaquin desert regions in recent decades has fragmented Le Conte’s Thrasher habitat. Habitat loss has been extensive habitat voids in the Antelope Valley, Los Angeles County; the Coachella Valley, Riverside County; the I-40 corridor in the West Mojave Desert, San Bernardino County; and the developing metropolitan areas around Las Vegas, NV, and Tucson, and Phoenix in Arizona. At smaller scales of residential development in the western Mojave desert, Le Conte’s Thrasher seldom occur where the composition and continuity of vegetation differs from the original landscape matrix.
C. Patch Size
Individual birds require tracts of about 20 ha or more of native vegetation during the non-breeding season. Because they are a sedentary species, habitat connectivity is important for dispersal during droughts. Breeding territory data from the San Joaquin Valley ranges up to 11 ha (Sheppard 1996). Data are not available from other areas.
The smallest fragment of isolated undisturbed habitat is about 130 ha for all habitat patches >2 km from nearest other habitat segment (Fitton unpubl. data, Sheppard 1996). Large gaps (> 10 km) created by urban settlement appear to prevent continuing occupation of small, fragmented natural patches of vegetation (Sheppard 1996).
D. Disturbance (natural or managed)
Habitat fragmentation from urbanization and vehicular traffic are the most frequently noted human disturbances (Bury et al. 1977). Extended droughts leads to population losses; flash floods can remove vegetation and soil from once productive habitat. The spatial scale of floods is likely to be shorter in duration than decadal drought in altering habitat quality for Le Conte’s Thrasher.
Reasons for decline: Leveling native vegetation over wide swaths creates a gap of at least 30 years of insufficient vegetation structure for Le Conte’s Thrasher in the West Mojave Desert (JW, pers. obs.). Buffers around urban areas create dead zones from increased predation from domesticated or feral animals. Repeated, random vehicle traffic churns up soil, reduces soil fertility, and batters slow-growing desert shrubs (Matchett et al. 2003). Once vehicle traffic leaves an area seasonally, provided vegetation structure is still present, Thrashers will reoccupy habitat (JW pers. obs.). The cumulative impact of vehicle traffic during operations at the many military installations is unknown (Prescott 1999).
Recommendations: (1) Maintain large intact patches of high-quality Le Conte Thrasher habitat free of vehicle travel, mining and gravel operations, and urban/industrial development; (2) prescribe focused revegetation at disturbed sites with careful attention to fostering species that benefit Le Conte’s Thrashers, particularly with species such as desert thorn (Lycium) and saltbush (Atriplex) species; and (3) design disturbances for multiple uses in desert landscape to create minimum impact and disturbance to habitat by careful engineering and project management practices.
E. Adjacent land use
Land uses for communities and industries create a gradient of habitat degradation in the buffer around land developments. Large blocks of open land within the center of Ridgecrest, CA, remain void of Le Conte’s Thrashers even though the vegetation species mix is adequate. Lack of connectivity and disturbances from the adjacent urban environment prevent long-term colony establishment and nesting. Irrigated agricultural landscapes do not provide suitable habitat for thrashers but desert residents report that Le Conte’s Thrashers will enter unirrigated gardens with pomegranates and pistachio trees on the outskirts of Ridgecrest, CA.
V. SPECIAL FACTORS
I. Brood parasitism
Sheppard reports that Bronzed Cowbirds (Molothrus aeneus) occasionally parasitizes Le Conte’s Thrashers nests. No confirmed natural parasitizing by Brown-headed Cowbirds (M. ater) has been recorded.
Lack of native invertebrate prey may be the ultimate factor limiting Le Conte’s Thrashers to undisturbed desert habitats.
III. Sensitivity to human-induced disturbance
Le Conte’s Thrashers are sensitive to human-induced disturbance, depending on its duration and magnitude. They do not adapt to environments modified by human land uses.
IV. Pesticide use
Evidence of toxicity from agricultural pesticides is limited thus far to San Joaquin Valley populations collected between 1968 and 1971 when DDE and DDT use was still occurring (Sheppard 1996).
Le Conte’s Thrashers are known to predate nestlings and eggs from nests of other thrashers. The following other nest predators reported are: snakes (various species), kit foxes (Vulpes macrotis), and domestic cats (Felis catus) in the San Joaquin Valley (Sheppard 1996) and antelope ground squirrel (Ammospermophilus harrisi) (Parker 1886). The only report of a raptor predating a Le Conte’s Thrasher is from Boyce (1985) of Prairie Falcon. Predation by Common Ravens has not been reported.
VI. Exotic species invasion/encroachment
It is not clear how severely Le Conte’s Thrashers are affected by fires from fine fuels created by non-native annual grasses in the genera Schismus and Bromus (Brooks and Berry 2006). Loss of native shrubs from increasing fire frequency may degrade habitat quality and the ability of thrashers to nest in fire-impacted areas. The increasing frequency of wild dogs and domestic cats may also increase nest predation. Burros (Equus asinus) may degrade shrub cover, and thus nesting and hiding habitat for Le Conte’s Thrasher.
All populations in the San Joaquin Valley and in regions around urban centers in the Mojave and Sonoran deserts have declined because of habitat loss. Sheppard (1996) reports that 26 percent of historical localities no longer had suitable Le Conte’s Thrasher habitat within three kilometers.
Figure 2. Le Conte's Thrasher population trend from Breeding Bird Surveys , 1966-2003. http://www.mbr-pwrc.usgs.gov/bbs/htm03/trn2003/tr07110.htm
I. Age and sex ratios
No data are available
II. Productivity measure(s)
Of 51 female Le Conte’s Thrashers studied in the San Joaquin Valley between 1968 and 1971, 84 of 124 nest attempts were successful in laying one or more eggs (67.7%) (Sheppard 1996).
The only data come from the San Joaquin Valley (Sheppard 1996): survivorship estimated at 0.185 for birds during the first ten to twelve months and 0.67 for adults.
Sheppard (1973) determined that that eight km was the greatest dispersion of banded birds. Data specifically from Mojave and Sonoran Desert birds is lacking, but birds appear to remain as permanent residents on their territories. Young disperse after the break-up of family bands in the early summer.
The principal management issues are:
(1) maintaining the best mix of suitable shrub species for nesting;
(2) reducing flammability of annual forb and grass biomass.
(3) limiting vehicle travel to designated off-highway vehicle routes
(4) better urban planning to reduce the urban footprint on the Mojave and Sonoran deserts
The following list of species would benefit from management of Le Conte’s Thrasher:
Scott’s Oriole (in Joshua tree habitat only)
(Mohave Ground Squirrel)
MONITORING METHODS AND RESEARCH NEEDS
Because Le Conte’s Thrashers have large territories, they have low density and thus low likelihood to be recorded with conventional point counts. Detecting them is also made difficult because their ventriloqual vocalizations carry over long distances, vocalizations are crepuscular, and birds are secretive.
Long-term tracking of resident birds would provide the best demographic and range data for local populations. The cost and labor for remote sensing to track individual birds and the remoteness of sites for monitoring are deterrents to study.
Le Conte’s Thrasher is protected under the Migratory Bird Treaty Act. Breeding bird survey data shows Le Conte’s Thrasher populations are declining throughout much of its range, especially where human populations are growing or incompatible land uses such as mining and irrigated crop agriculture are underway.
Le Conte’s Thrasher is a year-round resident in Mojave and Sonoran desert shrublands, mesquite bosque, and small-stature Joshua tree woodlands. Habitat continuity and connectivity are essential to maintain viable populations as territories are large (>10 ha).
Large-scale conversion of preferred natural vegetation types to new land uses punches large holes in the range of suitable habitat for all phases of life history. Large-scale housing and commercial developments dominated by exotic vegetation will not support Le Conte’s Thrashers, and habitat fragmentation from mining and gravel operations, pipelines, energy projects, and recreational vehicle travel threatens the species. Although Le Conte’s Thrasher occur at the urban-desert interface and are known to visit feeders (e.g., at Ridgecrest, California), no information is available about whether these interface habitats are marginal sink habitats for or whether they can function as viable nesting habitats.
Both non-native animals and plants may be increasing mortality through increased predation or increased fire frequency respectively. Because of its secretive habits and remote habitats, detailed information on virtually all aspects of Le Conte’s Thrasher biology remain poorly known. Very little information on the species comes from the Mojave and Sonoran deserts.
In 2007, the first instance of West Nile virus in Le Conte’s Thrasher (Anon. 2008).
Sustain and expand viable populations of Le Conte’s Thrashers in Mexico and the USA.
Focus first on large-scale landscape blocks where Le Conte’s Thrasher populations are robust to study which environmental factors contribute most to thrasher fecundity and survival.
Experiment with restoration treatments to accelerate or improve establishment of shrub species that provide nest sites, protective cover, and large invertebrate populations.
Secure large tracts of formerly or potentially high-quality habitat from ongoing human disturbance or unsustainable desert land uses.
Apply information from tested restoration treatments to remediate and recover lost habitats.
Invest in long-term remote sensing monitoring to track the viability of populations, particularly at the margins of the species range and on the urban-desert interface.
Control populations of non-native species in Le Conte's Thrasher habitat.
Coordinate bi-national efforts to conduct research and implement management to protect Le Conte's Thrasher populations.
Coordinate and maintain data sharing among appropriate agencies and organizations in the United States of America and the United States of Mexico, including the Bureau of Land Management, National Park Service, US Fish and Wildlife Service, US Department of Defense, US Department of the Navy, California Department of Fish and Game, Arizona Department of Game and Fish, and PRBO Conservation Science, Pro Natura, and SEMERAP.
Leave native vegetation, and in particular mesquite bosques and dense cactus and Atriplex shrublands intact.
Limit off-road vehicle use to designated travel routes
Control feral burro populations, particularly in desert wash ecosystems, to allow for natural regeneration and long-term survival of regenerating native shrubs for at least five continuous years out of every 25 years.In areas where urban or agricultural development is imminent, working with developers to retain native Sonoran shrubland patches of greater than 10-ha, especially along washes and arroyos, is critical. These patches must be interspersed throughout the urban or agricultural matrix at a distance of less than 0.5 kilometers.
Much remains to be learned about this species. Priorities for future research include the following: studies of behavior at nest, daily time budgets over seasons, food and water requirements, structural analysis of occupied/unoccupied habitat, thermal properties of nest lining and effects on eggs and nestling growth patterns, territory/home-range sizes in all parts of range, song repertoire of individual males over time and species-specific characteristics of song, barriers to dispersal, habitat restoration, interactions with congeneric species, physiological and behavioral responses to high temperatures, and effects of drought on prey base and population. Further study is needed to determine extent of geographic variation in nearly all aspects of Le Conte’s Thrasher biology, particularly reproductive efforts, vocalizations, and population densities.
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