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Distribution and Abundance of Least Bell’s Vireos (Vireo bellii pusillus) and Southwestern Willow Flycatchers (Empidonax traillii extimus) on the Middle San Luis Rey River, San Diego County, Southern California—2020 Data Summary

Data Series 1134
By:  and 
https://doi.org/10.3133/ds1134

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Abstract

We surveyed for Least Bell’s Vireos (Vireo bellii pusillus; vireo) and Southwestern Willow Flycatchers (Empidonax traillii extimus; flycatcher) along the San Luis Rey River, between College Boulevard in Oceanside and Interstate 15 in Fallbrook, California (middle San Luis Rey River), in 2020. Surveys were conducted from April 13 to July 13 (vireo) and from May 14 to July 13 (flycatcher). We found 192 vireo territories, at least 150 of which were occupied by pairs. Vireo territories increased by 40 percent from 2019 to 2020 in the portion of the middle San Luis Rey River that burned as a result of a wildfire in 2017. In contrast, vireo territories decreased by 5 percent from 2019 to 2020 in the unburned portion of the middle San Luis Rey River. 

Vireos used six different habitat types in the survey area: (1) willow-cottonwood, (2) mixed willow riparian, (3) riparian scrub, (4) upland scrub, (5) willow-sycamore, and (6) non-native. Forty-nine percent of the vireos were detected in habitat characterized as willow-cottonwood, and 93 percent of the vireos were detected in habitat with greater than 50-percent native plant cover. Of the 17 banded vireos detected in the survey area, 6 were resighted with a full color-band combination. Two other vireos with single (natal) federal bands were recaptured, identified, and color-banded in 2020. Eight vireos with a single dark blue federal band, indicating that they were banded as nestlings on the lower San Luis Rey River (LSLR), could not be recaptured for identification. One vireo with a single gold federal band, indicating that it was banded as a nestling at Marine Corps Base Camp Pendleton (MCBCP), could not be recaptured for identification. The two natal vireos that were recaptured on the middle San Luis Rey River dispersed from 2.6 to 6.2 kilometers (km) from their natal territories. Banded vireos with a known age ranged from 1 to 8 years old. 

One resident flycatcher was observed in the survey area in 2020. The resident flycatcher (male) was detected in a territory of mixed willow habitat with greater than 50-percent native plant cover. He was detected as a single male from May 27 to July 2, 2020, and no evidence of pairing or nesting was observed. The male flycatcher was resighted with a unique color-band combination and had occupied the same territory since 2018.

Suggested Citation

Allen, L.D., and Kus, B.E., 2021, Distribution and abundance of Least Bell's Vireos (Vireo bellii pusillus) and Southwestern Willow Flycatchers (Empidonax traillii extimus) on the middle San Luis Rey River, San Diego County, southern California—2020 data summary: U.S. Geological Survey Data Series 1134, 11 p., https://doi.org/10.3133/ds1134.

ISSN: 2327-638X (online)

Study Area

Table of Contents

  • Executive Summary
  • Introduction
  • Methods
  • Least Bell’s Vireo
  • Southwestern Willow Flycatcher
  • Summary
  • Acknowledgments
  • References Cited
Publication type Report
Publication Subtype USGS Numbered Series
Title Distribution and abundance of Least Bell's Vireos and Southwestern Willow Flycatchers on the middle San Luis Rey River, San Diego County, southern California—2020 data summary
Series title Data Series
Series number 1134
DOI 10.3133/ds1134
Year Published 2021
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Western Ecological Research Center
Description iv, 11 p.
Country United States
State California
County San Diego County
Other Geospatial Middle San Luis Rey River
Online Only (Y/N) Y
Google Analytic Metrics Metrics page
Additional publication details