Warbling Vireo is a mid-sized, slim, small-billed vireo that is reasonably long-winged, long-tailed, and has a "blank" expression caused by a white supercilium and extensive white below the eye (above). It is otherwise usually rather plain, with dull olive upperparts, a dull grayish cap, and whitish underparts with (often) yellow on the flanks or undertail coverts. Each year, a very few Philadelphia Vireos are reported. Philadelphia Vireo is a rare eastern vagrant to California, breeding mostly in alder-spruce forests from Alberta to Maine, and migrating to winter in Central and South America. Some of these reports will be correct, but a significant percentage will be erroneous. The experience of the California Bird Records Committee (CBRC) has been that about one out of every four reports of 'Philadelphia Vireo' is unconvincing; almost all of these are  misidentified Warbling Vireos. Therefore a review is warranted.

One problem is that too many observers rely on their field guides to identify vagrants. Field guides are not designed for this purpose, and cannot show the range of variation that may create problems. Field guides — even excellent new ones like Sibley or National Geographic — show 'standard' individuals. There is excellent literature about the Warbling v. Philadelphia Vireo problem, and more observers should review this literature before claiming vagrants. Excellent discussions are in Terrill & Terrill (1981), Zimmer (1985), and Kaufman (1990), and some highlights are in Roberson (1980, 1986, 1993). The Kaufman book — Field Guide to Advanced Birding — should be in everyone's library as an immediate reference. The points discussed below are all covered in the literature.

Photos right: top © D. Roberson, 3 Aug 1981 Pajaro R. mouth MTY; bottom © Bill Hill, 13 Sep 2004 Carmel R. mouth
 

© D. Roberson

The "wedge-shaped" character of the loral line of Philadelphia Vireo is consistent in all of these birds. The line is very broad directly in front of the eye — it is often broader here than behind the eye. This seems to never happen in Warbling. Obviously, determining the shape of the loral mark takes some good, close views. This is one reason I am skeptical of claims of Philly Vireo seen only at the top of a tall cottonwood. On the middle Philadelphia shown above, the front part of the loral line is obscured somewhat, looking to be an "incomplete" line in the field. This can happen in Philly Vireos but the part of the line next to the eye is still very broad and prominent.

There is something else to note in these facial patterns. On Warbling Vireo, the white supercilium usually expands above or behind the eye, and then continues on as a broad band for some distance before narrowing and fading toward the nape. There is variation in this pattern but the general shape just described is there on most birds. On Philadelphia Vireo, the broadest part of the white supercilium is often in front of the eye, and in many cases the supercilium stops rather abruptly behind the eye. This feature, combined with a shorter neck and more rounded head, gives Philly Vireo a 'cuter' expression than the 'blank-faced' look of Warbling Vireo.

Also note the variation in the white areas below the eye. In the top row of Warbling Vireos, birds #2 and #3 are the same bird but note how different the facial expression looks in different angles and lights. Birds #1, #2, and #5 show a lot of white below the eye; bird #4 shows very little. On Philadelphia Vireo, the amount of white below the eye is also variable, being sometimes as intense as the supercilium (e.g., #1, #2, and #4) but sometimes decidedly less noticeable (#3). It is very likely that the sketch (#5) overlooked the white below the eye as the focus of the observer was in capturing the shape of the dark loral area; note also how the sketch grossly overstates the thickness (but not the length) of the bill.

Photos above: top (L to R) Warbling: 22 Jun 1997 Big Sur R. mouth MTY © D. Roberson ; next two (same bird, different angles) 13 Sep 2004 Carmel R. mouth MTY © Bill Hill ; 15 Nov 1994 Big Sur R. mouth © BSOL (J. Booker); 3 Sep 1982 Pt. Reyes MRN © Albert Ghiorso.
bottom (L to R) Philadelphia: May 1983 Pt. Pelee, Ontario ©W. Ed Harper; 21 Sep 1993 Big Sur R. mouth MTY © BSOL (J. Booker) ; 27 Oct 1998 Big Sur R. mouth © BSOL (J.N. Davis); 6 Oct 2001 Big Sur R. mouth (BSOL banded, ph © D. Roberson); sketch 13 Sep 2003 Carmel R. mouth © D. Roberson.

In contrast, the most intense yellow on the underparts of bright Warbling Vireos are the flanks and undertail coverts, and especially the latter. Most Warbling Vireos lack yellow on the throat but, when it is there, it is not as bright as the sides, flanks, or undertail coverts. The Warbling (below) has a wash of yellow to the sides of the throat and to sides of breast, but it is most intense on the sides and the center of the breast is less yellow.

Philadelphia also looks shorter-tailed and more round-headed in the field than the typical Warbling, but this is so subjective as to be useful only to the observer who already has lots of experience. 
 

In fall migration in California, some claims of Philadelphia Vireos are unpersuasive at any date, but in peak periods for this vagrant (late September through October) this error rate is only about 15%. Mistakes are much more common in early September (when Warbling Vireos are very common and at their brightest) and in November (when either species is quite late). There are no acceptable claims of Philadelphia Vireo in August at all.

In fact. the four earliest dates of Philadelphia in California are:

• 4 Sep 1990 Smith R. estuary, DN    accepted 16th CBRC report
• 12-15 Sep 1986 SE Farallon I., SF    accepted 12th CBRC report; photographed
• 13-14 Sep 1986 Fairhaven, HUM    accepted 12th CBRC report
• 13-26 Sep 2003 Carmel R. mouth, MTY    occurred after CBRC stopped reviewing records, but seen by many over 2 week stay; sketch above