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Distribution and Abundance of Least Bell’s Vireos (Vireo bellii pusillus) and Southwestern Willow Flycatchers (Empidonax traillii extimus) at the Mojave River Dam, San Bernardino County, California—2025 Data Summary

Data Report 1218
Ecosystems Mission Area–Species Management Research Program
By:  and 
https://doi.org/10.3133/dr1218

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Executive Summary 

We surveyed for Least Bell’s Vireos (Vireo bellii pusillus; vireo) and Southwestern Willow Flycatchers (Empidonax traillii extimus; flycatcher) at the Mojave River Dam study area near Hesperia, California, in 2025. Four vireo surveys were completed between April 23 and June 26, 2025, and three flycatcher surveys were completed between May 16 and June 26, 2025.

We detected two territorial male vireos, both of which were paired, and one transient vireo. No juvenile vireos were observed during surveys. Vireos were reported in two habitat types: riparian scrub dominated by narrowleaf willow (Salix exigua) or mule fat (Baccharis salicifolia) and willow-cottonwood dominated by red or arroyo willow (Salix laevigata or lasiolepis). One transient willow flycatcher of unknown subspecies was observed in willow-cottonwood habitat dominated by Fremont cottonwood (Populus fremontii).

Suggested Citation

Howell, S.L., and Kus, B.E., 2026, Distribution and abundance of Least Bell’s Vireos (Vireo bellii pusillus) and Southwestern Willow Flycatchers (Empidonax traillii extimus) at the Mojave River Dam, San Bernardino County, California—2025 data summary: U.S. Geological Survey Data Report 1218, 8 p., https://doi.org/10.3133/dr1218.

ISSN: 2771-9448 (online)

Study Area

Table of Contents

  • Acknowledgments
  • Executive Summary
  • Introduction
  • Methods
  • Results
  • Summary
  • References Cited
Publication type Report
Publication Subtype USGS Numbered Series
Title Distribution and abundance of Least Bell’s Vireos (Vireo bellii pusillus) and Southwestern Willow Flycatchers (Empidonax traillii extimus) at the Mojave River Dam, San Bernardino County, California—2025 Data Summary
Series title Data Report
Series number 1218
DOI 10.3133/dr1218
Publication Date March 25, 2026
Year Published 2026
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Western Ecological Research Center
Description vi, 8 p.
Country United States
State California
County San Bernardino County
Other Geospatial Mojave River Dam
Online Only (Y/N) Y
Additional publication details