The least Bells vireo (Vireo bellii extimus) and southwestern willow flycatcher (Empidonax traillii extimus) are migratory species dependent upon riparian habitat for breeding. Extensive loss and degradation of riparian woodlands throughout the southwestern U.S., coupled with increased parasitism by the brown-headed cowbird (Molothrus ater) has resulted in steep declines in these species during the past half-century, with the result that both are now endangered. Development of effective and realistic Recovery Plans for these species requires an understanding of their population structure and dynamics, as well as habitat and other requirements influencing management decisions. This project extends research conducted by the P.I. since 1986 on the distribution, habitat requirements, nesting ecology, wintering distribution, foraging behavior, singing behavior, and demography of the least Bells vireo, and expands previous investigations of the distribution of willow flycatchers to include breeding ecology and dispersal.
We will continue our demographic studies of Least Bell’s Vireos and Southwestern Willow Flycatchers at our long-term study sites on the San Luis Rey River and Marine Corps Base Camp Pendleton, California, as well as our more recent study site, the San Diego River. Our work continues to address issues of management concern in southern California; namely, the response of endangered birds to habitat management within the contexts of exotic vegetation removal and channel maintenance for flood control purposes. Our proposed investigation of the response of Least Bell’s Vireos to re-implementation of cowbird control was delayed yet another year by the funding agency, and will be pursued in 2010. Genetic analyses of willow flycatcher populations are underway and will be completed in 2010, facilitating a shift in focus to vireo genetic analyses. Our long-term data on Bell’s Vireos will be analyzed for relationships between migration patterns (i.e. arrival date on breeding grounds) and the factors influencing them (i.e. precipitation, temperature) and demographic variables including breeding productivity and survival, with the goal of identifying responses of this species to climate change scenarios
USGS Contact For This Project