Paul, I think the "mark" Darius is referring to is not the "wing flag" but rather a whitish (or perhaps buff colored; hard to tell in the low-light understory they frequent) line which seems to be on the edge of the wing (the alula, as Darius said). I have seen it on at least two birds this fall, which I think were FEMALES, not males. As you said, Y-O-Y males are already moulted into a male-like plumage when they come through here, and only females would be brownish backed at this time. I think that perhaps they are young-of-the-year females, but I'm not certain. Also, both the birds I saw DID have (as far as I can recall) the wing-flag, albeit a rather small one. Sorry to cause any confusion. I have checked all my references and still cannot find any bird with this mark either, but it does exist, at least in a small population. Any bird-banders out there familiar with in-hand field marks on female Black-throated Blue Warblers? Rick Sussman Ashton, MD Warblerick@aol.com