This is a supplement to my post about identification of Warbling Vireos, which covers the basics of distinguishing them by song
In 2025 AOS and eBird split Warbling Vireo into two species, named Eastern and Western Warbling Vireo. Differences in song provide the best way to identify the two species, but there is variation, including regional variation and apparently intermediate songs around the contact zone. Here I describe some of that variation.
Western-like song in the east
Eastern birds often include one or two higher phrases or trilled phrases among their songs, but the overall sound is still recognizable as Eastern. Here is a song from central New York that has significant western-like characteristics: a short song with longer phrases and longer pauses, several phrases above 4 kHz, abrupt changes in pitch, and several trilled phrases. To my ear it still has the more gentle sing-song quality with limited frequency range of Eastern, and the song shown here is the most western-like among all of the songs in the recording. Still, it raises questions about identification by song. We assume this is a pure Eastern because of the location, but how would we assess a recording like this from farther west?
Recorded by Chris Wood, Jun 2025, Cortland County, NY; recording here: https://macaulaylibrary.org/asset/637432089
Eastern-like song in the west
I did not find any examples of presumed Western Warbling Vireo singing Eastern-like songs, but my review was very limited.
Intermediate songs in the hybrid zone
I have long thought that I heard confusingly intermediate songs where the range of these two species overlap (e.g central Montana). And songs from Boulder Colorado, identified as Eastern in Floyd (2014), are more western-like than typical songs of Eastern from farther east.
In my recent review of audio recordings in the Macaulay Library and Xeno-Canto, multiple songs from across central and eastern Montana seem intermediate, combining and blending many of the distinguishing features described above. A couple of examples are shown here. I have included comparison examples of typical Eastern and Western, and for each spectrogram I have added an image with dark blocks covering the extent of each phrase to help visualize the tempo and frequency range of each song.
I also measured a few variables in these spectrograms and created a chart. I chose three variables that are relatively easy to measure, and that capture some of the key differences that I hear in the songs. As expected it shows the rate of phrases-per-second higher in the east, while high-pitched phrases and trilled phrases are more frequent in the west. The Montana recordings are intermediate in all three measurements. This is a subjective and informal estimate with a very small sample size, but it matches my impression of many recordings from the contact zone.
To my ear the Montana songs linked below sound closer to Eastern, and maybe that’s what they are (see New York recording above), but this apparent shift towards intermediate songs in the contact zone deserves more attention. A more comprehensive and objective study is needed to understand the variation.
Typical Eastern
Intermediate Montana
Both of the following examples are identified in eBird as Eastern, in a region where both species can occur. In both cases the songs appear intermediate to me. In comparison to the typical Eastern above, they have more varied phrases, multiple trilled phrases, wider pitch range, and longer phrases. Conversely, compared to typical Western songs they are less varied, with narrower pitch range, some successive phrases only slightly different in pitch, and some phrases short and rapid.
Intermediate song eastern Montana
Here are two successive songs from a single bird in northeastern Montana.
Recorded by Steve Flood, Jun 2021, Phillips County, MT; recording here: https://macaulaylibrary.org/asset/644325987
Intermediate song western Montana
Here is a song from about 100 miles east of the Rocky Mountains in Montana.
recorded by Steve Flood, Jun 2021, Choteau County, MT; recording here: https://macaulaylibrary.org/asset/644354970
Typical Western
Regional variation in song
One important question not addressed by recent research is the significant regional variation in song within Western Warbling Vireo. I have always been struck by the distinctive sound of Warbling Vireos in the canyons of southeastern Arizona, and a review of recordings confirms this, such as this example from the Chiricahua Mountains.
Recorded by Richard Webster, 8 Jun 2022, Cochise County, AZ; recording here: https://xeno-canto.org/780060
Songs in that area average much shorter overall, with simpler phrases and slightly longer phrases. They sound slower and less varied than Western Warbling Vireo from farther north, and are often described as finch-like. They can be much less varied, simply repeating a similar up-down phrase combination two or three times.
These birds have always been considered part of subspecies brewsteri, but the difference in song suggests a distinct population. Other subspecies with further song variation occur in the mountains of western Mexico. It is surprising that the multi-decade studies of Warbling Vireos in Alberta never expanded to include these intriguing populations farther south. Hopefully someone will take that on in the near future.
In addition, Carpenter et al 2022a found that northern populations of Western sang, on average, shorter and slower-paced songs with slightly lower pitch than southern populations. These differences are slight, but it is certainly plausible that more regional song variation exists within populations of the western US.
Sources
Carpenter et al 2022a – Amanda M Carpenter, Brendan A Graham, Garth M Spellman, John Klicka, Theresa M Burg. 2022. Genetic, bioacoustic and morphological analyses reveal cryptic speciation in the warbling vireo complex (Vireo gilvus: Vireonidae: Passeriformes). Zool J Linn Soc 195: 45-64. – https://doi.org/10.1093/zoolinnean/zlab036
Carpenter et al 2022b – A. M. Carpenter, B. A. Graham, G. M. Spellman, and T. M. Burg. 2022. Do habitat and elevation promote hybridization during secondary contact between three genetically distinct groups of warbling vireo (Vireo gilvus)?. Heredity 128: 352-363. – https://doi.org/10.1038/s41437-022-00529-x
Cicero 2025 – Carla Cicero. 2025. Treat Warbling Vireo Vireo gilvus as two species. AOS Checklist committee proposal. – https://americanornithology.org/wp-content/uploads/2025/05/2025-C.pdf
Floyd 2014 – Ted Floyd. 2014. Documentation by sound spectrogram of a cryptic taxon, _Vireo g. gilvus_, in Boulder County, Colorado. Western Birds 45; 57-70. – https://archive.westernfieldornithologists.org/archive/V45/WB-45\(1\)-Floyd.pdf
Howes-Jones 1985a – D. Howes-Jones. 1985. Relationships among song activity, context, and social behavior in the warbling vireo. The Wilson Bulletin – http://www.jstor.org/stable/4162033
Howes-Jones 1985b – D. Howes-Jones. 1985. The complex song of the Warbling Vireo. Canadian Journal of Zoology 63; 2756-2766.. – https://cdnsciencepub.com/doi/abs/10.1139/z85-411
Lovell 2010 – S. F. Lovell. 2010. Vocal, Morphological, and Molecular Interactions between Vireo Taxa in Alberta. Doctoral dissertation, University of Calgary, Department of Biological Sciences. https://ucalgary.scholaris.ca/items/468a37d9-f0fc-4773-85f3-6addca3ecbb8/full
Lovell et al 2021 – Scott F. Lovell, M. Ross Lein, Sean M. Rogers. 2021. Cryptic speciation in the Warbling Vireo (Vireo gilvus). Ornithology 138: 1-16. – https://doi.org/10.1093/ornithology/ukaa071
Mlodinow et al 2025a – Mlodinow, S. G., A. M. Carpenter, T. Gardali, G. Ballard, P. Pyle, G. M. Kirwan, S. M. Billerman, and A. J. Spencer (2025). Western Warbling Vireo (Vireo swainsoni), version 1.0. In Birds of the World (B. K. Keeney, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.wewvir2.01
Mlodinow et al 2025b – Mlodinow, S. G., A. M. Carpenter, T. Gardali, G. Ballard, P. Pyle, A. J. Spencer, G. M. Kirwan, and S. M. Billerman (2025). Eastern Warbling Vireo (Vireo gilvus), version 1.0. In Birds of the World (B. K. Keeney, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.eawvir1.01
Spencer 2012 – Andrew Spencer. 2012. Identifying Eastern And Western Warbling Vireos. Earbirding blog post. https://earbirding.com/blog/archives/3667