"Dissenting Answer"
Something is wrong with the analysis presented in the original answer. Hybrid TOWA x HEWA rarely shows the yellowish wash on the underparts, particularly in the vent area. One or more field marks may not be evident in the scan, perhaps due to overexposure in the original photograph or poor scanning. TOWA still can be safely eliminated due to the amount of yellow on the underparts as can GCWA. The bird is most likely a BLACK-THROATED GREEN WARBLER where the some facial details, particularly the outline of the auricular area, have been washed out in the image presented.
"Original Answer"
The picture of our mystery warbler is not the best; it's either overexposed or poorly scanned, but still has enough detail that we should be able to identify it. It has a short dark bill that's relatively stout at the base, a pale face that's entirely yellow with what appears to be an eye line fore and aft of the eye. The eye lacks and any noticeable eye ring; the crown is yellow suffused with green extending all the way down the nape. The back is yellowish green or greenish yellow, with noticeable black streaking on it. The wings are black with two noticeable white wing bars and the tertials are edged in white. The throat is blotchy black on a white background and there is streaking running down the (sides of the?) upper breast from the throat. The flanks are streaked with black and the belly is appears to be yellowish white, though it's hard to tell from the picture. The undertail coverts are white and the undetail is effectively not visible. The legs appear to be dark, but the feet may have some yellow in them, but again it's hard to tell from the photo.
If I saw this bird flitting around in the canopy around San Francisco in spring or fall, I'd probably call it a HERMIT WARBLER and go on to the next bird, depending on how much activity there was. However, there are some problems with calling this a HEWA. First, HEWA rarely show a well-defined eye line fore and aft of the eye. Admittedly the photo is overexposed, but the green on the face see in some plumages of HEWA tends to be more diffuse and ill-defined that one would imagine even in a photo of this quality. The real killer for HEWA though is the streaking on the breast and the flanks; no plumage of HEWA shows this type of streaking.
So we need to go further with our search. Our mystery warbler is probably in the genus dendroica, since it does resemble a HEWA. However, few warblers from this genus show dominantly yellow faces, so the choices are relatively small. BLACK-THROATED GREEN WARBLER has a bright green back with little or no streaking on it and the face is suffused with a greenish auricular outline, neither of which appear to be present in this photo. TOWNSEND'S WARBLER shows a dark cheek in all plumages which is clearly not present in this bird.
GOLDEN-CHEEKED WARBLER is similar to BTNW but has clear yellow cheeks with distinct eye line that angles posteriorly and no dark lower border to cheeks. Backs of males have heavy streaking. However, the eye line on a GCWA is well-defined and runs from the base of the bill through the eye to the rear of the check and then up to the crown in all plumages. This bird doesn't appear to show that. As well, there is no yellow on the underparts of GCWA. While this may be ambiguous in the photo, it does appear that there is some suffusion of yellow on some of the underparts.
So what gives? There is one other option, however. TOWA and HEWA do hybridize and the identifiable hybrids do show intermediate characteristics between the two species. Were this a hybrid TOWA x HEWA, this would explain why the bird appears superficially to be a pure HEWA, but having elements of a TOWA including streaking on the breast and flanks and a semi-distinct eye line. In fact, I do indeed agree with two of the three respondents that this is a hybrid TOWA x HEWA, likely a spring female.
Which answer is correct? Let me know!
Last updated 30 April, 1999