Warbling Vireo, Vireo gilvus
Warbling Vireo, singing male, 6/3/10, Arastradero Preserve
Making the warbling song "Rosita, Rosita, Ro-zeet." Warbling Vireos often sing all day long during spring, and like other vireos are much more easily heard than seen.
Warbling Vireo, eastern male, 5/20/10, Magee Marsh, OH
Above is an eastern Warbling Vireo; the species has distinct eastern and western populations that are candidates for separate species status. The eastern bird is said to be distinguished by larger size, a bill thicker at the base, paler crown, and larger pale area on upper mandible of the bill (see Birds of North America species account.) All the other birds shown on this page are western.
Warbling Vireo, 7/30/06, Russian River, Healdsburg, Sonoma Co

Warbling Vireo, 7/30/06, Russian River, Healdsburg, Sonoma Co
Warbling Vireo, 6/8/08, Yuba Pass Summit, Sierra Co
Warbling Vireo, male singing on nest, 6/23/09, Yuba Pass Summit, Sierra Co
This male Warbling Vireo was singing from the nest, a well-known behavior I've also observed in Cassin's Vireo. In both cases the nest was at eye level in a conspicuous spot, here in a campground, with the Cassin's at a trailhead.