Status and Breeding Biology of an Isolated Population of Bendire's Thrashers in California: a Study Proposal
Bendire's Thrasher (Photo by Richard Ditch)
Project Update - 2003
Kristie N. Nelson: email "storm_petrel at hotmail dot
The Bendire's Thrasher (Toxostoma bendirei) remains poorly understood in several important aspects including population size, breeding biology, and habitat requirements. California populations are small and isolated (Grinnell and Miller 1944, England and Laudenslayer 1989a) and therefore vulnerable to disturbance due to off-road vehicle use, grazing, and habitat degradation (Remsen 1978). Remsen (1978) estimated the state's total breeding population to be less than 200 pairs. Coupled with the sparse and isolated nature of their populations within the state and the apparent declining trend of the species throughout its range (Sauer et al. 2001), the Bendire's Thrasher has been listed as a California Species of Special Concern and a Bird of Conservation Concern (CDFG and PRBO 2001, USFWS 2002) and is on the Audubon Society Watch List (Audubon 2002).
In 1986 and 1987 England and Laudenslayer (1989a) conducted the only extensive statewide survey of Bendire's Thrasher to date in California. This study represents the most thorough analysis of the species' range, yet basic breeding biology and population parameters of Bendire's Thrashers still remain unstudied. England and Laudenslayer (1989a) found that Bendire's Thrasher breeding range extended further than previously documented and located several new isolated populations. They also found Bendire's Thrashers to be "curiously lacking" from the western Mojave and other areas where apparently suitable habitat was present. They concluded that "habitat variables limiting the distribution of Bendire's Thrashers have yet to be quantified with the detail necessary to understand its complex distribution."
England and Laudenslayer (1989a, 1989b) were intrigued by the nature of these small, isolated breeding populations in California. They were curious as to whether these populations were permanent or ephemeral. Local population extirpation and recolonization over time presents a unique parameter for land managers to consider when managing for susceptible species. Existing populations must be carefully monitored and provided for, yet other suitable habitat should be available for Bendire's Thrashers to colonize over larger temporal and spatial scales. England and Laudenslayer recommend monitoring isolated Mojave Desert populations over a relatively long time scale to investigate factors such as population size, breeding incidence, breeding limitation variables, and breeding biology (England and Laudenslayer 1989b). These topics are essential to determine population status, understand habitat requirements, and provide insight into future management actions.
Studies are lacking in California and elsewhere on basic population parameters, nesting requirements, and breeding biology of the Bendire's Thrasher. Only rudimentary vegetation attributes have been measured to determine habitat selection of Bendire's Thrashers (England and Laudenslayer 1989b). As of 1989, only 5 nests had been discovered within California, 4 of which were only visited once (England and Laudenslayer 1989a), and we know of no subsequent nest discoveries.
This project will be conducted at Lee Flat in Death Valley National Park, which harbors the only known Bendire's Thrasher breeding population in the northern Mojave. At 5780' it is also the highest known breeding elevation for this species, and its unique understory vegetation differs from the habitats described for other locations (England and Laudenslayer 1989b). England and Laudenslayer found relatively high numbers of thrashers at Lee Flat yet none in nearby areas with similar vegetation. Bendire's Thrashers have been seen at Lee Flat during subsequent breeding seasons, and breeding was confirmed in 1990 (J Heindel, pers. comm.).
The population status of the Bendire's Thrashers in California remains unknown (England and Laudenslayer 1989b). Breeding Bird Survey Data suggests a decline throughout its range, yet the sample size is too small to lend much power to this trend (Sauer et al. 2001). Determining the status of the isolated Lee Flat population will provide the first quantitative, long-term, population-based trend data for this species.
Our efforts will examine the breeding biology and habitat requirements of the Bendire's Thrasher. No published data is available on many aspects of its life history including nest building, nest-site selection, development of pair bond, role of each sex in brooding, singing, and incubation date (England and Laudenslayer 1993). Population and demographic information such as recruitment, reproductive success, age at first breeding, breeding-site fidelity for migratory populations, and natal philopatry have never been recorded for Bendire's Thrashers (England and Laudenslayer 1993). We will investigate these life history traits and demographics as well as population change over time, all of which are of particular interest for the management of small isolated populations.
Monitoring the status of this population is especially important to assess the possible meta-population behavior of Bendire's Thrashers in California and to properly manage for their habitat. Data collected in this project will be provided to Death Valley National Park land mangers, the forthcoming California Partners in Flight Desert Bird Conservation Plan, and other interested parties. We will attempt to publish our findings in peer-reviewed journals.
Kristie N. Nelson
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England, A.S. and W.F. Laudenslayer, Jr. 1989b. Review of the status of Bendire's Thrasher in California. Sacramento, Calif.: California Dept. of Fish and Game Adm. Rep. 89-2.
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