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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) and to present results of management actions implemented to attract flycatchers and enhance flycatcher habitat at Marine Corps Base Camp Pendleton (MCBCP, or Base). Surveys for the flycatcher were done on Base between May 6 and July 23, 2025. All MCBCP’s historically occupied riparian habitat (core survey area) was surveyed for flycatchers in 2025. None of the non-core survey areas were surveyed in 2025.
No resident flycatchers were detected on Base in 2025. The one resident (female) present in 2024 did not return to the territory she occupied in 2024, and she was not detected within the historically occupied habitat surveyed in 2025.
Eight transient Willow Flycatchers of unknown subspecies were observed on two of the five drainages surveyed in 2025: Las Flores Creek and the Santa Margarita River. No Willow Flycatchers were detected at Fallbrook, Pilgrim, or San Mateo Creeks. Transients in 2025 occurred in mixed willow and riparian scrub habitats, dominated by multiple willow species (Salix spp.). Exotic vegetation was recorded in most flycatcher locations and was dominant (cover of exotics greater than 50 percent) in more than half of all transient locations. The most common exotic plant in habitat used by flycatchers was poison hemlock (Conium maculatum). All six of the flycatchers that were observed closely enough to determine banding status were unbanded.
Two measures were initiated in recent years to attract and retain resident breeding flycatchers on MCBCP: conspecific attraction using flycatcher song broadcasts and installation of artificial seeps to enhance flycatcher habitat. We surveyed plots with and without speakers that broadcast flycatcher vocalizations throughout the breeding season and detected two transient Willow Flycatchers within 20 meters of one speaker in 2025. We set up permanent vegetation sampling points surrounding artificial seeps and nearby sites without artificial seeps (Reference sites) to determine the effects of surface-water enhancement by seep pumps. Vegetation cover was highest near the ground and decreased with increasing height. Woody vegetation made up most of the cover at all height categories. Soil saturation in 2025 was higher at the sites near seeps than at the Reference sites and was associated with higher native herbaceous cover and lower non-native cover.
Suggested Citation
Lynn, S., Howell, S.L., and Kus, B.E., 2026, Distribution, abundance, breeding activities, and restoration efforts for the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2025 Annual Report: U.S. Geological Survey Open-File Report 2026–1016, 37 p., https://doi.org/10.3133/ofr20261016.
ISSN: 2331-1258 (online)
Study Area
Table of Contents
- Acknowledgments
- Executive Summary
- Introduction
- Study Areas and Methods
- Results
- Discussion
- Conclusions
- References Cited
- Appendix 1. Southwestern Willow Flycatcher (Empidonax traillii extimus) Survey Areas at Marine Corps Base Camp Pendleton, 2025
- Appendix 2. Vegetation Sampling Locations and Vegetation Sampling Data Sheet, Marine Corps Base Camp Pendleton, 2025
- Appendix 3. Locations of Willow Flycatchers (Empidonax traillii) at Marine Corps Base Camp Pendleton, 2025
| Publication type | Report |
|---|---|
| Publication Subtype | USGS Numbered Series |
| Title | Distribution, abundance, breeding activities, and restoration efforts for the Southwestern Willow Flycatcher at Marine Corps Base Camp Pendleton, California—2025 Annual Report |
| Series title | Open-File Report |
| Series number | 2026-1016 |
| DOI | 10.3133/ofr20261016 |
| Publication Date | May 15, 2026 |
| Year Published | 2026 |
| Language | English |
| Publisher | U.S. Geological Survey |
| Publisher location | Reston, VA |
| Contributing office(s) | Western Ecological Research Center |
| Description | vii, 37 p. |
| Country | United States |
| State | California |
| Other Geospatial | Marine Corps Baase Camp Pendleton |
| Online Only (Y/N) | Y |
| Additional Online Files (Y/N) | N |