Proposal (1065) to South American Classification Committee

Status of Vireo gilvus (Warbling Vireo) in SACC area: (A) Treat Vireo gilvus as two species, and (B) Remove Vireo gilvus from SACC list pending additional information

Background: NACC has recently voted to treat the two North American groups of Vireo gilvus as two species: Western Warbling-Vireo (V. swainsonii) and Eastern Warbling-Vireo (V. gilvus). The relevance to SACC is that although only one of these two has been recorded in the SACC area (a single photographic record from in Ecuador) as a vagrant, whether the photograph (Freile et al. 2009) can be identified to species is uncertain.

A. Treat Vireo gilvus and two species. Before we can address the record from Ecuador, first we have to address species limits. Carla Cicero wrote a long, detailed, comprehensive proposal to split them, and this was unanimously endorsed by NACC. The proposal is 50+ pages long and given that we have only one record of one individual bird in South America, presenting the proposal in full here seems over-the-top.

The full proposal (NACC 2025-C-3) is here and deals with virtually every aspect of these taxa relevant to species limits:

https://americanornithology.org/wp-content/uploads/2025/05/2025-C.pdf

The concluding statement of the proposal is:

“Recommendation:

“Multiple studies and lines of evidence suggest that Eastern and Western Warbling Vireos have separate evolutionary histories and should be elevated to full species. These include differences in genetics, morphology, song, molt, migration, ecology, and response to brood parasitism. Furthermore, separate studies of contact zones have shown relatively low levels of hybridization and cytonuclear discordance across a large area, even where males of the two taxa occupy adjacent territories. On the basis of concordance in different suites of traits, I recommend that we recognize Eastern and Western Warbling Vireos as separate species.”

Comments and votes from NACC members are here:

https://americanornithology.org/wp-content/uploads/2025/09/2025-C-comments-forweb.pdf

This situation is far beyond our geographic purview, and I would be extremely reluctant to not endorse the NACC decision on this. Nevertheless, if you see any fatal flaws, speak out. I have read the proposal and was already familiar with most of the evidence, and I strongly recommend a YES on this one.

B. Remove Vireo gilvus from SACC list pending additional information. We previously voted to accept broadly defined Vireo gilvus based on a published photo from Ecuador and an even poorer recording of its call: SACC 849. However, the photo is of sufficiently poor quality that two of you voted against the record.

In my opinion, the photos are definitely Vireo gilvus sensu lato, but are not identifiable to species: https://ebird.org/checklist/S35871298

Although obviously identifiable in the hand (and from song), I am not certain that photos can be reliably identified yet, and until that is established one way or another, we have a problem. Obviously, Eastern Warbling-Vireo is by far more likely in South America; it winters farther south and east than does Western Warbling-Vireo, although the winter ranges of these two now have to be re-examined with care. AOU (1998) gave the southern extreme of the winter range of swainsonii as southern Mexico, but I have not checked for newer information. Our SACC rules for inclusion are based on verifiable, tangible evidence, which no longer applies in my opinion, so I recommend a YES on a proposal to remove it from the SACC list pending further information.

If we accept the split, then we could move the record to the Hypothetical List as Eastern Warbling-Vireo (V. gilvus).

References:

Chesser, R. T., S. M. Billerman, K. J. Burns, C. Cicero, J. L. Dunn, B. E. Hernández-Baños, R. A. Jiménez, O. Johnson, N. A. Mason, P. C. Rasmussen. 2025. Sixty-sixth supplement to the American Ornithological Society’s Check-list of North American Birds. Ornithology 142: 1–18. https://doi.org/10.1093/ornithology/ukaf015

Van Remsen, October 2025

Vote tracking chart:

https://www.museum.lsu.edu/~Remsen/SACCPropChart1044+.htm