Yellow-rumped Warbler split opinion? Why? (i.e., Why does the American Ornithological Union (AOU) and the International Ornithological Union (IOC) split their opinion as to whether the YRWA warrants splitting into full species or should remain as subspecies)?

Andrea, your question to the Warbler Guy (above) shows me you are as passionate about warblers as me.

So put on your Focus Cap for a few seconds of intense reading pleasure that aims to educate:

As brief background, in 2011 the AOU voted down a measure that would have split Yellow-Rumped Warbler into two, three or four species*.

(Audubon's Yellow-rumped subspecies appears in photo.)

As many of you already know, the Yellow-rumped Warbler currently occurs as four subspecies, according to many researchers: the “Myrtle” group (coronata), and the “Audubon’s” group (auduboni), “Black-fronted” (nigrifrons), and “Goldman’s” (goldmani)

The taxonomy of these Yellow-rumped Warbler subspecies was under consideration in 2011 for change by an AOU committee and, currently, is not under consideration for a status change by this committee (according to my "sources").

(NOTE  #1: In the N.A. Birds Online account for this species, the following additional subspecies is described in the "Myrtle group (below photo)":  Yellow-rumped Warbler subspecies: D. c. hooveri (McGregor, 1899). This subspecies 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 1997). 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 (1970). More recently, hooverimaintained as valid (Godfrey 1986, Gibson and Kessel 1997, R. Dickerman and P. Unitt pers. comment.

NOTE #2: The IOC splits two subspecies in this group and recognizes Audubon's and Myrtle as two species).

(* = The AOU vote was 7–4 against any divisions of the Yellow-rumped complex. The committee members 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.

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: http://www.worldbirdnames.org/english-names/principles/