Mari (in Phoenix), it's an interesting question that continues to be debated as researchers and American Ornithological Society (AOS) committee members continue to debate whether the Yellow-rumped Warbler species should potentially be split into two new species (Audubon's and Myrtle)....or, perhaps, even three or four (based on how two other subspecies — Black-fronted and Goldman's — within this species occur outside the USA's 48 lower states.Currently, the defining organization for this question — the AOS — does not have a new proposal to entertain a split that would result in species status for Myrtle and Audubon's. In fact, in recent years, an AOS committee turned down a proposal to create species status for more than Myrtle and Audubon's, but also, perhaps, Black-fronted and Goldman's subspecies within the Yellow-rumped complex.
For more current information, the following link is worth reading:
I'll provide more updates on this question as I learn of new information.
Regards to all, Daniel
Birding Guide (since the 1980s)