Posted by Phil Pickering (208.46.229.20) on May 30, 2002 at 19:48:58:
Wow, good detective work, Mark. I wasn't really considering Verdin because of the bill shape, but in 3rd-look retrospect it's (painfully) obvious that the malar area is not entirely feathered yet, making the bill seem much thicker-based and more sparrow-shaped than it would otherwise. Cover up the malar area and the bill shape looks fine for Verdin.
In addition to the points already mentioned, both the overall rounded tail shape and the very rounded individual rect shape seem to be better for Verdin than sparrow/junco. Also, the outer webs of Verdin rectrices are edged in very pale gray, noticeably paler than the rest of the feather (and presumably pale enough to be translucent). I think that's the case with this bird - if the edges were really white as in a junco, they would probably contrast more with the underparts in the left (not backlit) photo. The rect edges look the same color as the underparts in that photo, which is apparently perfect for Verdin. Juvenile Verdins can also have a white lower eye arc (yellow-headed adults have yellow eye arcs). And, according to Rising, juvenile Black-chinned Sparrows do have faint breast streaking, which, if not obvious, would likely at least be discernable in these close up photos.
A just-out-of-nest Verdin is the only thing that has made any real sense to this point.
Oh well. Good one, Joe!
Phil