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

Data Report 1222
Ecosystems Mission Area—Species Management Research Program
By:  and 
https://doi.org/10.3133/dr1222

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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 Big Tujunga Creek in the Hansen Dam Basin in Los Angeles County, California, in 2025. Four vireo surveys were completed between April 17 and July 2, 2025, and three flycatcher surveys were completed between May 20 and July 2, 2025. We detected 62 territorial male vireos, 51 of which were confirmed as paired, and 2 transient vireos. Additionally, we detected 32 juvenile vireos during surveys. Seventy-seven percent of vireos were detected in habitat characterized as mixed willow, and 95 percent of vireos were detected in habitat with greater than 50-percent native plant cover. Most vireo territories were dominated by Goodding’s black willow (Salix gooddingii).

On May 20, 2025, we detected 18 transient Willow Flycatchers of unknown subspecies, none of which were confirmed to be paired, and no juveniles were detected. Mixed willow habitat was used by 78 percent of Willow Flycatchers, and all Willow Flycatchers were detected in habitat with greater than 50-percent native plant cover. Most Willow Flycatchers were detected in locations dominated by Goodding’s black willow.

Suggested Citation

Lynn, S., 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 Hansen Dam Basin, Los Angeles County, California—2025 data summary: U.S. Geological Survey Data Report 1222, 12 p., https://doi.org/10.3133/dr1222.

ISSN: 2771-9448 (online)

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 Hansen Dam Basin, Los Angeles County, California—2025 data summary
Series title Data Report
Series number 1222
DOI 10.3133/dr1222
Publication Date April 03, 2026
Year Published 2026
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Western Ecological Research Center
Description Report: vi, 12 p.
Online Only (Y/N) Y
Additional publication details