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Distribution and Abundance of Least Bell’s Vireos (Vireo bellii pusillus), Southwestern Willow Flycatchers (Empidonax traillii extimus), and Coastal California Gnatcatchers (Polioptila californica californica) at the Carbon Canyon Dam, Orange County, California—2025 Data Summary

Data Report 1221
Ecosystems Mission Area—Species Management Research Program
By:  and 
https://doi.org/10.3133/dr1221

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Executive Summary 

We surveyed for Least Bell’s Vireos (Vireo bellii pusillus; vireo), Southwestern Willow Flycatchers (Empidonax traillii extimus; flycatcher), and Coastal California Gnatcatchers (Polioptila californica californica; gnatcatcher) at the Carbon Canyon Dam study area near Brea, California, in 2025. Four gnatcatcher and vireo surveys were completed between April 22 and June 25, 2025, and three flycatcher surveys were completed between May 15 and June 25, 2025.

We detected 14 territorial male vireos, 12 of which were paired. We also detected a transient vireo. Juvenile vireos were observed in two territories during surveys. Of the five vireo nests incidentally located during surveys, three were parasitized by Brown-headed Cowbirds (Molothrus ater). Vireos were reported in four habitat types: (1) mixed willow riparian, (2) riparian scrub, (3) upland, and (4) non-native vegetation. The dominant tree species in vireo territories was Goodding’s black willow (Salix gooddingii). Most vireo territories (12) were in habitat with greater than 50-percent native vegetation. The most common exotic species in vireo territories was poison hemlock (Conium maculatum). No flycatchers or gnatcatchers were observed during surveys.

Suggested Citation

Howell, S.L., and Kus, B.E., 2026, Distribution and abundance of Least Bell’s Vireos (Vireo bellii pusillus), Southwestern Willow Flycatchers (Empidonax traillii extimus), and Coastal California Gnatcatchers (Polioptila californica californica) at the Carbon Canyon Dam, Orange County, California—2025 data summary: U.S. Geological Survey Data Report 1221, 11 p., https://doi.org/10.3133/dr1221.

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), Southwestern Willow Flycatchers (Empidonax traillii extimus), and Coastal California Gnatcatchers (Polioptila californica californica) at the Carbon Canyon Dam, Orange County, California—2025 data summary
Series title Data Report
Series number 1221
DOI 10.3133/dr1221
Publication Date March 19, 2026
Year Published 2026
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Western Ecological Research Center
Description vi, 11 p.
Country Unted States
State California
County Orange County
Other Geospatial Carbon Canyon Dam
Online Only (Y/N) Y
Additional publication details