Technical Report
2008 Southwestern Willow Flycatcher Study Results
Date: 2009/04/01
Author(s): Moore D., Ahlers D.
Publication: Bureau of Reclamation Report, 71 p.
Abstract:
During the summer of 2008, the Bureau of Reclamation (Reclamation) conducted surveys and nest monitoring of the federally endangered Southwestern Willow Flycatcher (Empidonax traillii extimus) (SWFL) in eight distinct reaches along approximately 200 kilometers of the Rio Grande in New Mexico between Velarde and Elephant Butte Reservoir. Surveys were performed to contribute to current baseline population data of the SWFL along the Middle Rio Grande and also to meet Reclamation’s Endangered Species Act (ESA) compliance commitments. There were 480 resident SWFLs documented in 287 territories and forming 193 breeding pairs. As in previous years, the San Marcial reach of the river was by far the most productive containing 235 territories and 168 pairs.
Nest monitoring was conducted at all sites where nesting pairs were detected. Nests were monitored for success rates, productivity, and Brown-headed Cowbird (Molothrus ater; BHCO) parasitism. The San Marcial reach proved most productive, producing 186 nests and fledging 209 SWFL young. The Sevilleta reach produced 13 nests and fledged 12 SWFL young. Unknown nest fates accounted for the reduction in fledglings. Overall, parasitism decreased, predation increased and abandonment and success were similar to the past several years.
Other studies were initiated or continued in 2008. These include: (1) BHCO point counts, (2) livestock grazing study, (3) SWFL nesting hydrology study and (4) vegetation/habitat mapping. These studies are designed to provide further insight into potential threats to and habitat requirements of SWFL populations.