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Southwestern Willow Flycatcher (Empidonax traillii extimus) surveys and nest monitoring San Diego County, California, 2025

Published by U.S. Geological Survey | Department of the Interior | Catalog Last Checked: August 01, 2026 at 04:47 AM | Dataset Last Updated: December 12, 2025 at 12:00 AM
Data presented are results of surveys and monitoring in 2025 for Southwestern Willow Flycatcher (Empidonax traillii extimus) in San Diego County, California. Surveys were conducted at one location along the San Dieguito River (San Dieguito) and four locations along the upper San Luis Rey River (Cleveland National Forest, Rey River Ranch, Vista Irrigation District, and VID Lake Henshaw), and nest monitoring was conducted at Lake Henshaw. Locations were surveyed using a standardized protocol three to four times between 15 May and 31 July. The number, age (adult or juvenile), sex, banding status (color banded or not, and if so, color combination), and breeding status (paired, undetermined, or transient) of all Willow Flycatchers was recorded. The goal of nest monitoring was to collect baseline data on annual productivity, nest success, and parasitism rate. Monitored territories were visited weekly from mid-May to August.

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