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
On Thu, Aug 14, 2025 at 9:04 AM Peter Ruprecht <pruprecht...> wrote:
> 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
>
> On Thu, Aug 14, 2025 at 8:22 AM Brandon <flammowl17...> wrote:
>
>> 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
>>
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