Yellow Rumped Warbler with finch-like upper mandible?

Nico Stuurman

While enjoying the Tropical Kingbird at South Lake this morning, Stephen (sp?) and I spotted a Yellow Rumped Warbler with a highly enlarged upper mandible that made it look like a finch bill.  I am not sure whether this was external material stuck on the bill (the bird was sooty as well), or that it is a genetic variation (in which case it would be quite interesting that this bird made it almost into the next season already).  Curious to learn mode.  One photo attached, more in my ebird list: