These small gray vireos can be found irregularly through Oasis Valley from May through September, most migrating in spring in fall. Some may be wanderers up from their breeding spots downriver in Ash Meadows (Nevada) and Tecopa-China Ranch (California) along the Amargosa. I wonder if they are looking to breed in Oasis Valley, as I hear their characteristic "talkative" squeaky song in spring in the willow and mesquite thickets. They require dense riparian trees and shrubs along streams to nest and find food for their chicks, and heavy cattle grazing can eliminate the lower vegetation in thickets that they favor. In addition, feral burros in Oasis Valley browse the riparian trees where they can reach them as well. Fortunately the preserves managed by The Nature Conservancy, such as Torrance Ranch, may in the future provide the vireos with excellent, protected habitat if they choose to colonize the area.
^In a shady Russian olive tree (Eleagnus angustifolius), an introduced plant.
^Taking off to fly to the next twig.
^Picking insects out of Honey mesquite blossoms (Prosopis glandulosa).
^In an elm planted in a ranch.
The endangered California Least Bells' vireo (Vireo bellii pusillus) was collected from Ash Meadows in the 1890s by the Death Valley Expedition, and continues to breed along the Amargosa River in Inyo County, California, and at Ash Meadows in southern Nye County, Nevada. The Least Bells' vireo has also been found migrating through parts of Death Valley (J. Grinnell and A. Miller, 1944, The Distribution of the Birds of California. Pacific Coast Avifauna 27).
The Arizona race (V. b. arizonae) breeds along the lower Colorado River and its tributaries in the southeastern part of Nevada, and south along that river in California-Arizona.
Least Bell's vireo populations began to decline in the 1930s, and crashed by the 1970s in most places, due to habitat destruction, water diversions, and Brown-headed cowbird nest parasitism. Arizona Bells' vireos have had similar problems.
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