Warbling and Philadelphia Vireos can be quite similar in appearance. This identification challenge is not necessarily straightforward, most especially when dealing with these two species flitting about in green, leafy vegetation on a sunny day. Birds in these environments can reflect unusually bright greenish tones and can potentially mislead an observer to the true distribution and depth of hue throughout.
Some points of confusion: The approximations given in the Sibley and Nat Geo guides, such as the dark wedge to the loral area (Philadelphia) or the lack of this feature (Warbling) are simply averages; Warbling Vireo can show a dark wedge-like mark to the loral area and some Philadelphia’s (esp. first year examples) can show a greatly reduced dark wedge. Other features such as the shape and extent of the supercilium (eyebrow) and the presence/absence of a contrast between a greenish back and a grayish crown (typically, a Philadelphia Vireo trait) have great variability across the board and these secondary and tertiary identification points of interest should be used with caution and in conjunction with other, primary indicators, such as the distribution of yellow to the underparts and the GISS of each species.
Identification notes: with Philadelphia Vireos, the yellowish tones are most intense to the throat and upper breast; the intensity of yellow here is as great or greater than adjacent areas throughout the underparts. With Warbling Vireos, the opposite is true; the intensity of yellowish tone is weakest to the throat and upper breast area, greatest along the flanks and undertail. A general rule of thumb regarding underpart coloration: individuals in spring/early summer are often at their drabbest overall (due to feather wear and color fading) and at their most colorful in fall (in fresh plumage). Differences in GISS (General Impression of Size and Shape) between the two species are also notable. Comparing Philadelphia Vireo to Warbling Vireo, the collective impression of its slightly stouter bill, slightly rounder head shape, pot belly, and slightly shorter tail provides the expression of a plump n portly vireo; a “look” I find impossible to find in any examples of Warbling.
Photo: An example of an autumn Warbling Vireo showing “typical” coloration throughout its underparts (left; photo taken at Britannia C.A., mid August, courtesy of Norbert Haché) and a relatively drab fall Philadelphia (right; photo taken at Britannia C.A. mid September 2015, taken by the author). This particular Warbling has a high-contrast face pattern and dark loral wedge but lacks the contrasting bluish-gray blush to the crown, as is typically seen on even the drabbest examples of Philadelphia Vireo. Note the distribution, depth and concentration of yellow to the underparts of both these birds and also the chunky, pot-bellied build of the Philadelphia.
Kaufman, K. 1990. Advanced Birding. Houghton Mifflin Co., Boston, MA.
Sibley, D. A. 2014. The Sibley Guide to Birds. 2nd ed. New York: Alfred A. Knopf.