LeConte’s Thrasher Status and Nest Site Requirements Study, Colorado Desert

In support of the Coachella Valley Multiple Species Habitat Conservation Plan’s (CVMSHCP’s) monitoring program, BioServices staff in 2019 investigated the status of the LeConte’s Thrasher in the Coachella Valley, where the species’ numbers had severely declined.

Led by Research Ecologist Dr. Lori Hargrove, museum biologists sought to (1) determine the current distribution of the thrasher in the Coachella Valley by locating and mapping any territories, (2) gain a better understanding of territory and nest site requirements in this region, and (3) identify likely causes of decline.

Anticipating low sample sizes, the team surveyed 40 sites in the Coachella Valley as well as sites in neighboring Joshua Tree National Park (higher elevations) and Anza-Borrego Desert (lower elevations), where larger populations are known to persist. The biologists used standardized survey protocols developed by the CVMSHCP’s Biological Working Group, and protocols developed by the Desert Thrasher Working Group that are part of a range-wide monitoring effort.

Unfortunately, the team confirmed that the thrasher has continued to decline since baseline surveys were conducted in 2004-2005, and that the thrasher is currently extirpated or nearly extirpated from the Coachella Valley. However, the biologists detected LeConte’s Thrashers at 11 of 12 sites in Joshua Tree National Park and nine of 20 sites in Anza-Borrego.

By comparing habitat features around 26 LeConte’s Thrasher nests to habitat features in unoccupied plots, staff identified the nest site requirements that are lacking in the Coachella Valley and the factors that have contributed to the thrasher’s regional decline.

Museum researchers also used multiple criteria to rank 20 sites in the Coachella Valley for their potential to sustain LeConte’s Thrasher populations in the future and identified management measures that could facilitate recolonization of the species.

The study’s results will serve as a basis for comparison with future thrasher studies and inform habitat management decisions for the region.