Technical Report

2024 Middle Rio Grande Southwestern Willow Flycatcher Study Results Middle Rio Grande Project, New Mexico, Upper Colorado Basin Region; Technical Report No. ENV-2025-059

URL: https://webapps.usgs.gov/mrgescp/documents/2024-Middle-Rio-Grande-Southwestern-Willow-Flycatcher-Study-Results.pdf

Date: 2025/04/01

Author(s): Bullard M.

Publication: Prepared by Bureau of Reclamation, Technical Service Center, Denver, Colorado with funding by Bureau of Reclamation Albuquerque Area Office, Middle Rio Grande Conservancy District, and New Mexico Interstate Stream Commission

Abstract:

During the summer of 2024, the Bureau of Reclamation (Reclamation) conducted surveys and nest monitoring of the federally endangered Southwestern Willow Flycatcher (SWFL). The surveys were completed in seven distinct reaches along approximately 99 river miles of the Rio Grande in New Mexico between the Isleta Pueblo and Elephant Butte Reservoir. Due to difficulties in hiring temporary employees, Reclamation only surveyed seven sites in the Belen reach. However, Tetra Tech Inc. was contracted to survey the remaining 29 sites in the Belen Reach. The survey efforts were selected to meet compliance needs for Reclamation, the Middle Rio Grande Conservancy District, and the New Mexico Interstate Stream Commission projects. Certain sites were also selected to contribute to current baseline population data, monitor population trends, and determine the current distribution of SWFLs along the Middle Rio Grande (MRG).

During 2024, 792 resident SWFLs were documented. These residents formed 368 pairs and established 424 territories and resulted in 56 unpaired males. As in previous years, the San Marcial Reach of the Rio Grande was the most productive supporting 340 territories and 293 pairs. However, comparisons to previous years’ data are difficult due to previous years’ reduced survey effort. The Bosque del Apache Reach supported 40 territories, an increase from 2022 and 2023 when 35 and 27 territories were recorded, respectively. Only 10 territories were recorded in the Belen Reach. However, the southern half of the reach, which contained more than 80 territories in 2022, was not surveyed by Reclamation in 2024. There were 18 territories documented in the Sevilleta/La Joya Reach, 17 of the 18 territories were confirmed as nesting pairs. In the Escondida Reach, 16 territories were detected and 12 were confirmed to be nesting pairs. The San Acacia and Tiffany Reaches were surveyed in their entirety, but no territories were documented in either reach. Overall, 60 more territories were documented in MRG sites during 2024 than during 2023, but this increase is likely due to the additional survey effort in 2024 as opposed to an actual increase in SWFL territories.

Additionally, nest searching and monitoring were conducted in occupied sites, and nest fates were often determined. Nests were monitored for success rates, productivity, depredation, abandonment, and Brown-headed Cowbird parasitism. The San Marcial Reach provided the most productive habitat, where 365 nests and 348 SWFL fledglings were documented. Overall nesting success for the MRG was 49 percent.