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Distribution and Abundance of Least Bell’s Vireo (Vireo bellii pusillus) and Southwestern Willow Flycatcher (Empidonax traillii extimus) at the Sepulveda Dam Basin, Los Angeles County, California—2025 Data Summary

Data Report 1219
Ecosystems Mission Area—Species Management Research Program
By:  and 
https://doi.org/10.3133/dr1219

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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) along Bull Creek, Haskell Creek, and the Los Angeles River (Sepulveda Dam project area) in Los Angeles County, California, in 2025. Four vireo surveys were completed between April 16 and July 2, 2025, and three flycatcher surveys were completed between May 21 and July 2, 2025. We found 11 territorial male vireos, 4 of which were confirmed as paired, and 3 transient vireos. Of the 11 territorial vireos, 6 were detected along the Los Angeles River, 3 along Haskell Creek, and 2 along Bull Creek. Forty-three percent of vireos were detected in habitat characterized as mixed willow riparian, and most vireos were detected in habitat with greater than 50-percent native plant cover. No flycatchers were observed in the Sepulveda Dam project area in 2025.

Suggested Citation

Allen, L., and Kus, B.E., 2026, Distribution and abundance of Least Bell’s Vireo (Vireo bellii pusillus) and Southwestern Willow Flycatcher (Empidonax traillii extimus) at the Sepulveda Dam Basin, Los Angeles County, California—2025 data summary: U.S. Geological Survey Data Report 1219, 7 p., https://doi.org/10.3133/dr1219.

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 Vireo (Vireo bellii pusillus) and Southwestern Willow Flycatcher (Empidonax traillii extimus) at the Sepulveda Dam Basin, Los Angeles County, California—2025 Data Summary
Series title Data Report
Series number 1219
DOI 10.3133/dr1219
Publication Date March 20, 2026
Year Published 2026
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Western Ecological Research Center
Description vi, 7 p.
Country United States
State California
County Los Angeles County
Other Geospatial Sepulveda Dam Basin
Online Only (Y/N) Y
Additional publication details