Yellow-rumped Warblers in winter plumage are being reported by many birders who are offering suet at their feeders. It seemed that two or three were in my yard all day today. Some of them are fairly easy to identify as either the "Audubon's" or "Myrtle" form, but this one has me puzzled. According to the Nat'l Geographic guide, the lack of a whitish line over the eye makes it an Audubon. According to Sibley, the prominent streaks on the breast make it a Myrtle. Maybe it's half-and-half.
This one seems to have more of an eyeline, and the wite of the throat extends farther back on the side of the neck, so that indicates that it is likely a Myrtle.