Distribution, Abundance, and Breeding Activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2023 Annual Report

Open-File Report 2025-1001
Ecosystems Mission Area—Species Management Research Program
Prepared in cooperation with Assistant Chief of Staff, Environmental Security, U.S. Marine Corps Base Camp Pendleton
By:  and 

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Executive Summary

The purpose of this report is to provide the Marine Corps with an annual summary of the distribution, abundance, and breeding activity of the endangered Southwestern Willow Flycatcher (Empidonax traillii extimus; flycatcher) at Marine Corps Base Camp Pendleton (MCBCP or “Base”). Surveys for the flycatcher were completed on Base between May 8 and July 26, 2023. All of MCBCP’s historically occupied riparian habitat (core survey area) was surveyed for flycatchers in 2023. None of the non-core survey areas were surveyed in 2023.

In 2023, 14 transient Willow Flycatchers of unknown subspecies were observed on two of the five drainages surveyed, the Santa Margarita River and San Mateo Creek. No Willow Flycatchers were detected at Fallbrook, Las Flores, or Pilgrim Creeks. Transients occurred in a range of habitat types, including mixed willow (Salix spp.) riparian, and riparian scrub. Exotic vegetation, primarily poison hemlock (Conium maculatum), was present in most of the flycatcher locations.

In 2023, the resident Southwestern Willow Flycatcher population on Base consisted of one unpaired female occupying one territory. No territorial males were observed in 2023. The resident flycatcher population was restricted to the Santa Margarita River, and distribution was limited to the Air Station breeding area. The resident flycatcher territory was in mixed willow riparian habitat.

Nesting was initiated in late June and continued into late July. One nesting attempt was documented, which was ultimately unsuccessful because of infertile eggs. No instances of Brown-headed Cowbird (Molothrus ater) parasitism were observed. The flycatcher nest was placed in native sandbar willow (Salix exigua).

For the first time since 2012, a flycatcher that was originally banded as a nestling on MCBCP returned and established a breeding territory in 2023. The nestling (female) was originally banded in 2020, making her 3 years old. No other uniquely banded adult flycatchers present in previous years returned to MCBCP in 2023. No new adults or nestlings were banded in 2023. None of the transients observed during surveys were seen to carry bands. From 2000 to 2023, the adult annual survival of Southwestern Willow Flycatchers on MCBCP was 60±3 percent, while first-year survival was 20±3 percent.

Two measures were initiated in recent years to attract and retain breeding flycatchers on MCBCP: a conspecific attraction playback study (initiated in 2018) and an artificial seep study (initiated in 2019); both were repeated annually through 2023. The female resident flycatcher detected in 2023 was observed within 110 meters (m) of an automated playback unit, and within 90 m of an artificial seep.

Suggested Citation

Howell, S.L., and Kus, B.E., 2025, Distribution, abundance, and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2023 Annual report: U.S. Geological Survey Open-File Report 2025–1001, 33 p., https://doi.org/10.3133/ofr20251001.

ISSN: 2331-1258 (online)

Study Area

Table of Contents

  • Acknowledgments
  • Executive Summary
  • Introduction
  • Study Area and Methods
  • Results
  • Discussion
  • Conclusions
  • References Cited
  • Appendix 1. Southwestern Willow Flycatcher Survey Areas at Marine Corps Base Camp Pendleton, 2023
  • Appendix 2. Locations of Willow Flycatchers at Marine Corps Base Camp Pendleton, 2023
  • Appendix 3. Willow Flycatcher Detections at Marine Corps Base Camp Pendleton, by Drainage, 2000–23
  • Appendix 4. Southwestern Willow Flycatcher Territory Locations at Marine Corps Base Camp Pendleton, 2023
Publication type Report
Publication Subtype USGS Numbered Series
Title Distribution, abundance, and breeding activities of the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2023 Annual report
Series title Open-File Report
Series number 2025-1001
DOI 10.3133/ofr20251001
Year Published 2025
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Western Ecological Research Center
Description viii, 33 p.
Country United States
State California
Other Geospatial Marine Corps Base Camp Pendleton
Online Only (Y/N) Y
Google Analytic Metrics Metrics page
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