The least Bell's vireo is a Spring and Summer breeding resident, migrating south for Fall and Winter. It primarily inhabits riparian woodlands, scrub, and thickets for breeding. Population declines due to urban and agricultural development, habitat alteration, and brood parasitism by the brown-headed cowbird.
The vireo was listed as Federally Endangered in 1986, State Endangered in 1980. Federal "Critical Habitat" has been designated for upper Sweetwater Reservoir and immediately upstream habitat. "Critical habitat" is defined as the specific area occupied by a species that contain physical or biological features essential to the conservation of the species and that may require special management considerations or protection. On the Sweetwater River, the densest species concentration is between Rancho San Diego Golf Course and Sweetwater Reservoir (i.e. critical habitat area). A few scattered locales are known upstream of this area, including a few occurrences at upper Loveland Reservoir. Despite available habitat, the species is virtually non-existant in the lower Sweetwater River basin (only 2 locales known). Rangewide, brown-headed cowbird control (trapping and nest monitoring) have resulted in a nearly 10-fold population expansion over the last decade. The Federal Draft Recovery Plan (in circulation) describes the need for a long-term management plan for the Sweetwater River population.
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Photo by Anthony Mercieca©