Technical Report

2023 Middle Rio Grande Southwestern Willow Flycatcher Study Results - Middle Rio Grande Project, New Mexico Upper Colorado Basin Region; Technical Report No. ENV-2024-018

URL: https://webapps.usgs.gov/mrgescp/documents/2023-Middle-Rio-Grande-Southwestern-Willow-Flycatcher-Study-Results.pdf

Date: 2024/08/01

Author(s): Moore S.D.

Publication: Prepared by Bureau of Reclamation, Technical Service Center, Denver, Colorado.

Abstract:

During the summer of 2023, the Bureau of Reclamation (Reclamation) conducted surveys and nest monitoring of the federally endangered Southwestern Willow Flycatcher (SWFL). The surveys were completed in five distinct reaches along approximately 61 river miles of the Rio Grande in New Mexico between the Isleta Pueblo and Elephant Butte Reservoir. Due to difficulties in hiring temporary employees, certain reaches were not surveyed in their entirety, but priority sites 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 2023, 665 resident SWFLs were documented. These residents formed 302 pairs and established 363 territories. As in previous years, the San Marcial Reach of the Rio Grande was the most productive supporting 321 territories and 268 pairs. However, comparisons to previous years’ data are difficult due to the reduced survey effort. The Bosque del Apache Reach supported 27 territories, a decrease from 2021 and 2022 when 39 and 35 territories were recorded, respectively. Only 12 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 in 2023. Only the southern-most portion of the Escondida Reach was surveyed in 2023 and two SWFL territories were documented. The Tiffany Reach was surveyed in its entirety for the first time since 2019 and one nesting pair was located. Overall, 141 fewer territories were documented in MRG sites during 2023 than during 2022, but this decrease is likely due to the reduced survey effort in 2023 as opposed to an actual reduction 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 (BHCO) parasitism. The San Marcial Reach provided the most productive habitat, where 317 nests and 125 SWFL fledglings were documented. Overall nesting success for the MRG was 39 percent.