The Warbling Vireo is known to many by its complex warbling song ushering in the warm days of spring. “Fresh as apples and as sweet as apple blossoms comes that dear, homely song from the willows” ( Dawson 1923 ). Indeed, this vireo is highly noted for its persistent singing—even from the nest.
A small gray-olive bird with white underparts sometimes slightly washed yellow, the Warbling Vireo is far less conspicuous to the eye than to the ear. Males and females are alike in plumage. Two subspecies predominate in North America: in the east (Vireo gilvus gilvus) and in the west (V. g. swainsoni). These differ in overall size, gilvus averaging larger than swainsoni in bill size (depth and length), wing chord, and mass.
Corresponding with its broad breeding distribution (Figure 1), this species occupies a variety of deciduous forest habitats, predominantly riparian. It builds its nests in the forked limbs of trees from 1 to 40 meters above the ground at elevations ranging from sea level to over 3,000 meters. The species appears well adapted to human landscapes, as nests have been found in neighborhoods, urban parks, orchards, and farm fencerows. Its reproductive success in these areas has never been quantified, however.
The winter range of this vireo—western Mexico and northern Central America—is much smaller than its breeding distribution. Habitats on the wintering grounds are diverse, from shade coffee plantations to thorn forest to pine-oak woodland (Pinus-Quercus). During winter in western Mexico, this vireo is rarely encountered outside of a mixed species feeding flock.
The vocalizations of this species have been especially well studied: vocal behavior and development of young ( Howes-Jones 1984 ), the complex song ( Howes-Jones 1985c ), relationships between song activity, context, and social behavior ( Howes-Jones 1985b ), and the call note system ( Howes-Jones and Barlow 1988 ).
Population trends for Warbling Vireos vary with geography. In California, for example, they have been declining steadily for the past 20 years, while long-term trends in Ontario indicate that some populations are increasing there.