Warbling Vireo, Vireo gilvus
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.
In fresh plumage and as colorful as this drab species ever appears, with olive above and pale yellow below, instead of the usual gray-brown and white.
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.
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.
The same relatively bright bird in fresh fall plumage as is shown second from the top, feeding on blackberries in post-breeding dispersal, preparing to migrate to the wintering grounds.