AOU has split Warbling Vireo

[Brandon]

We now have two species of Warbling Vireos.  Eastern Warbling-Vireo and Western Warbling-Vireo both breed here in Colorado.  The songs are different.  I know the birds I have breeding up here in Custer County in Aspen trees are western, what about the breeding birds at Lake Pueblo State Park in Cottonwood trees?  Just trying to figure out how far West the eastern ones are.  I am pretty sure they are all easterns that breed in Otero, Bent, Prowers, along the Arkansas River in SE Colorado.

Thanks for any info on their range in Colorado.

Brandon Percival
Pueblo West, CO

[Peter Ruprecht]

Just anecdotally, I have heard both songs within 100m of each other at Walden Ponds in E. Boulder County. So it seems like there's some range overlap.

I guess this situation is a reason why I should do a better job of documenting subspecies when they are clearly differentiable.

Peter Ruprecht

Superior

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[Nathan Pieplow]

Hi all,

Western Warbling-Vireo is the default breeding species in the foothills and mountains in Colorado. Their habitat is aspen and cottonwood stands surrounded by coniferous forest. In some places, they breed along cottonwood corridors a few miles out onto the eastern plains, but usually not very far. 

Eastern Warbling-Vireo is the default breeding species in cottonwood corridors along the eastern plains, in many places coming right up to the foothills.

The best way to identify the two is song. Ben Guo is working on ways to identify them by calls, and it looks like he's having some success with that, but ID by call is always going to be difficult. The best visual cue is bill size, but I think there's overlap in that character; other people can say more about the visuals.

Here's a quick county-by-county breakdown of range for Front Range birders:

In Larimer County, it looks like Fossil Creek Reservoir and Arapahoe Bend have mostly Easterns, while everything west of Fort Collins is a Western. Here's a nice Eastern from Arapahoe Bend: https://macaulaylibrary.org/asset/619886593

In Boulder County, Walden Ponds and Pella Crossing have mostly Eastern Warbling-Vireos, but are near the contact zone and both species are possible. All 18 recordings from Pella Crossing are Easterns, except possibly for this bird (https://macaulaylibrary.org/asset/101593911). But this bird from Crane Hollow Road, basically at the western edge of the park, appears to be a Western (https://macaulaylibrary.org/asset/629682703). At Walden Ponds, both species have been recorded, and some of the birds there (and elsewhere) have songs that are kind of hard to classify. Eastern is the expected breeder at White Rocks Trail and Teller Lakes and along 95th Street where the Anhinga has been. Western is the expected breeder along the Mesa Trail and at Gregory Canyon. 

Jefferson County has 43 audio recordings of Warbling Vireos on eBird. I don't see any Easterns from JeffCo at this time. About 95% are identifiable as Western Warbling-Vireos and the other 5% are not identifiable because the recordings don't contain song phrases or the song phrases are too faint to assess. 

It's a little hard to say what's going on in Denver County since there are only a few recordings and I'm having some trouble classifying the audio from Bluff Lake.

Douglas County doesn't have many recordings but they are all Westerns from what I can see.

El Paso County: There might be some Easterns hiding in the mix, but nothing jumped out at me in a quick scan. 

Pueblo County has a few Eastern recordings, maybe all migrants. 

I'm only seeing Westerns from Huerfano and Las Animas. I'm only seeing Easterns from Otero and Bent. 

We can all look forward to sorting the situation out a little better down the road!

Nathan Pieplow

Boulder

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[Eric DeFonso]

In the Fall 2023 issue of Colorado Birds, I discussed this very topic in my column on Birding By Ear. Unfortunately we'll have to wait until next spring before we can again practice separating the two species in the field, but indeed we in the Front Range live in a transition zone between them. I'm thrilled that through this split, more attention will be brought to these lovely songsters who in some locations are remarkably abundant!

Eric

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Eric DeFonso
Boulder County, CO

On Thu, Aug 14, 2025 at 9:04 AM Peter Ruprecht <prup...@gmail.com> wrote:

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