Fine question, Jerry (in Chicago).
As brief background, in 2017 the American Ornithological Society (AOS) voted down a measure that would have split Yellow-Rumped Warbler into different species* (see two notes, below, for more details related to the aforementioned sentence).
Many of you already know the Yellow-rumped Warbler currently occurs as at least two, three, or four subspecies (varying among taxonomic organization plans). Among the current taxonomic plans, the “Myrtle” group (coronata), and the “Audubon’s” group (auduboni) are accepted in every one, with “Black-fronted” (nigrifrons), and “Goldman’s” (goldmani) also noted in at least one other taxonomic plan.
NOTE #1: In the N.A. Birds Online account for this species, the following additional subspecies is described in the "Myrtle group (below photo)": D. c. hooveri McGregor, 1899: Breeds in central and s.-central Alaska, se. Alaska, Yukon Territory, Mackenzie, and nw. British Columbia; intergrades with auduboni known from Stikine River, AK (Gibson and Kessel 1997a). Like nominate coronata, slightly larger, with longer wing (minimum wing length 73.5 mm in females, 75.5 mm in first-year males, and 78.0 mm in adult males); more streaked below (Alternate-plumaged males) or paler brown (females). Characters broadly clinal where range meets that of nominate coronata; for this reason, hooveri not recognized by Hubbard (Hubbard 1970b). More recently, hooveri maintained as valid (Godfrey 1986, Gibson and Kessel 1997a, R. Dickerman and P. Unitt pers. comm.).
(Audubon's Yellow-rumped subspecies appears in photo, below.)
NOTE #2: The International Ornithological Congress (IOC) differs from the AOS assessment of this species. It ALREADY splits the Yellow-rumped Warbler, thereby recognizing Audubon's and Myrtle as two species).
(* = The recent AOS rejection vote against creating additional divisions of the Yellow-rumped complex was based, in part, by several committee members who suggested the need for further genetic analysis and determination of the extent of interbreeding in the subspecies’ contact zones where the “Myrtle” group (coronata), and the “Audubon’s” group (auduboni) mix in western Canada. The status of two other subspecies — “Black-fronted” (nigrifrons), and “Goldman’s” (goldmani) remain unchanged according to at least one taxonomic organization plan.
Black-fronted is a resident in Mexico, and Goldman’s occurs only in southernmost Mexico and Guatemala. Neither of these two subspecies has been observed in the American Birding Association geographical area.
As for why, the IOC considers the Myrtle and Audubon's to represent two distinct species, the following 10 naming rules appear to guide the IOC's reasons for adopting name choices, with one or more the reason why the IOC divides the Yellow-rumped Warbler into two species: Myrtle Yellow-rumped Warbler and Audubon's Yellow-rumped Warbler:
1. - Each species should have one name only.
2. - A species name must be unique.
3. - Anglicized names are acceptable.
4. - Established names should prevail.
5. - Local names should not have priority.
6. - Offensive names should be changed.
7. - Patronyms are acceptable without bias for or against.
8. - Simplicity and brevity are virtues.
9. - Use of the word “island” will be limited.
10. - Species in the same genus may have different group names.
For more information, see: