Technical Report

2019 Middle Rio Grande Southwestern Willow Flycatcher Study Results

URL: https://webapps.usgs.gov/mrgescp/documents/Moore_2020_2019-Middle-Rio-Grande-Southwestern-Willow-Flycatcher-Study-Results.pdf

Date: 2020/03/01

Author(s): Moore D.

Publication: Bureau of Reclamation Report ENV-2020-024, 55 p.

Abstract:

During the summer of 2019, the Bureau of Reclamation (Reclamation) conducted surveys and nest monitoring of the Federally-listed endangered Southwestern Willow Flycatcher (SWFL). The surveys were completed in five distinct reaches along approximately 80 river miles of the Rio Grande in New Mexico between San Acacia Diversion Dam and Elephant Butte Reservoir. Surveys were performed to contribute to current baseline population data, monitor population trends, determine the current distribution of SWFLs along the Middle Rio Grande, and meet Reclamation’s and the Corps of Engineers’ Endangered Species Act compliance commitments. During 2019 surveys, 590 resident SWFLs were documented. These residents formed 264 pairs and established 326 territories. As in previous years, the San Marcial Reach of the Rio Grande was by far the most productive supporting 293 territories and 243 pairs. The Bosque del Apache Reach supported 24 territories. Overall, territory numbers in the Middle Rio Grande declined slightly in 2019 when compared to 2018. However, two reaches (Belen and Sevilleta/La Joya) that typically are occupied by resident SWFLs were not surveyed in 2019. Time permitting, nest monitoring was conducted at all sites where nesting pairs were detected. Nests were monitored for success rates, productivity, depredation, abandonment and Brown-headed Cowbird parasitism. The San Marcial Reach again proved most productive, producing 280 nests and fledging 272 SWFL young. The Bosque del Apache Reach produced 7 nests for which fates were all unknown due to limited site access caused by extensive flooding. Overall nesting success was 42 percent.