Date: 11/29/25 6:03 am From: tom none <jtcurt325...> Subject: [cobirds] Re: Warbling vireos and Colorado eBird
If the two "species" hybridize where they overlap, might we dealing with a
cline. Maybe the western birds can't breed with eastern counterparts, but
are interfertile at the relative edges of their respective ranges.
Have fun,
Tom Curtis
On Thursday, November 27, 2025 at 7:59:47 PM UTC-7 <greatg......> wrote:
> All:
>
> With the split of Warbling Vireo into two species (Eastern Warbling Vireo
> [EWVI] and Western Warbling Vireo [WWVI]), eBirding has become much more
> difficult and problematic, and we Colorado birders get to be the guinea
> pigs for learning about the distributions of the two species on the
> Colorado plains. While there are pre-existing data, there are nowhere near
> enough, as few birders cared about the subspecies Warbling Vireo (Eastern)
> and Warbling Vireo (Western).
>
> The first and most intractable problem is that there are NO consistent
> plumage color or pattern differences between the two species. Western TENDS
> toward the grayer end with a darker crown, and Eastern TENDS toward the
> brighter end with a paler crown, but the overlap in plumage tone is
> virtually complete. Western has a shorter, thinner bill than does Eastern,
> but the usable in-hand differences are in the half-millimeter range,
> something that will be nearly useless in field conditions.
>
> *The primary take-home message from this post*: Recordings of SINGING
> birds provide the only truly definitive documentation. Not calling birds.
> Not whining birds. Singing birds, *and* singing birds singing *full*
> songs. That means that all of our phones’ audio recorders will be getting a
> workout come May. That also means that non-singing birds are essentially
> unidentifiable, and should be recorded as “Eastern/Western Warbling Vireo”
> (or some such entry).
>
> Because there are relatively few definitively identified records of either
> species on the Colorado plains, *where the two species meet and overlap*,
> we don’t know the true extent of either species’ breeding distributions
> there. Yes, the farther east one goes, the more likely it is that EWVI is
> the breeding species, and WWVI is more likely as one approaches the
> foothill edge. West of the foothill edge, all breeders are WWVI… probably.
> Both species are suspected to breed at Barr Lake S.P., and if they
> hybridize there or elsewhere where the two species meet, then virtually all
> bets are off when it comes to definitively identifying even singing
> warbling vireos. Additionally, the extremities of the two species’ breeding
> ranges probably differ greatly between the South Platte drainage and the
> Arkansas drainage. The same is true for many “eastern” species of birds,
> such as Red-bellied Woodpecker, Bell’s Vireo, Baltimore Oriole, and Indigo
> Bunting, all of which breed much farther west in the South Platte drainage
> than in the Arkansas drainage. More on this, below.
>
> As I noted in the first sentence, the reason I am posting in this venue is
> to give everyone birding on the Colorado plains next spring and summer a
> heads-up as to how the Colorado eBird filters will be dealing with this
> worst-ever bird-ID conundrum to visit the state’s birders.
> eBird filters provide a framework for the abundance (or lack thereof) for
> all species occurring in a given filter region (e.g., Adams County, Crowley
> and Otero counties, and the San Luis Valley’s five counties). Those filters
> are what cause entries to flag or not (see eBird Data Quality : Help
> Center
> <https://ebird.freshdesk.com/en/support/solutions/articles/48000795278-ebird-data-quality> for
> more on that eBird process). Individual species have upper limits on the
> number of individuals that can be submitted to eBird from a particular
> location, beyond which the entry will flag for relatively atypical
> abundance for limits of 1 or greater, or will flag for rarity for a limit
> of zero.
>
> [Those interested in a deeper dive into the hows and wherefors of Colorado
> eBird filters, check out Colorado & Wyoming eBird: Stone Age to
> Industrial Age: The evolution of eBird's filter system
> <https://cowyebird.blogspot.com/2015/02/stone-age-to-industrial-age-evolution.html>.
> For other aspects of eBird relative to Colorado, check out the blog housing
> the above-linked essay: Colorado & Wyoming eBird
> <https://cowyebird.blogspot.com/> >
> Stone Age to Industrial Age: The evolution of eBird's filter system
>
> Did you ever wonder why eBird asks for details about a report of American
> Dipper from Adams County, Colorado, bu...
>
> <https://cowyebird.blogspot.com/2015/02/stone-age-to-industrial-age-evolution.html> >
> Colorado & Wyoming eBird
>
> Everything eBird for Colorado and Wyoming, from the keyboards of the
> region's eBird reviewers
> <https://cowyebird.blogspot.com/> > .]
>
>
> I return you to our regularly scheduled programming.
>
> *Front Range, Wet Mountains, and Sangre de Cristos and west* – This is
> the only portion of the state that is/has been simple to create the eBird
> filter limits that will govern which species will be available on filters:
> All filters from these areas will allow *Western Warbling Vireo* at
> various limits of >0 during the seasons of typical occurrence (on a gross
> scale, May through September). Somewhat unfortunately, all foothill-edge
> counties in Colorado straddle the foothill edge, so those counties
> (Larimer, Boulder, Jefferson, Douglas, El Paso, Pueblo, Huerfano, and Las
> Animas) will also have Eastern Warbling Vireo on those filters, but with
> the limit set to zero on each. That means that any reports of *Eastern
> Warbling Vireo* in those counties will require documentation of the
> occurrence.
> *Colorado’s eastern edge* (Sedgwick, Phillips, Yuma, Cheyenne, Kit
> Carson, Kiowa, Prowers, and Baca counties): These filters will have limits
> of >0 for *Eastern Warbling Vireo* from arrival to the end of the
> breeding season (probably May through mid-August), but will have 0 limits
> for *Western Warbling Vireo* (the species may be a casual or rare migrant
> through these counties, as the species breeds in the Black Hills of South
> Dakota and may traverse eastern Colorado to and fro).
>
> *Western portions of the Arkansas River drainage on the plains* (Elbert,
> El Paso, Lincoln, Crowley, and Otero counties): Eastern Warbling Vireo will
> have limits of 0 in all seasons and all counties. Because we CO birders are
> not sure of the distributions of the two species in Crowley and Otero, BOTH
> species will have a limit of 0 all year; documentation will be required,
> even in migration, when Western Warbling Vireo is probably a fairly common
> spring (and fall) migrant.
>
> *Eastern portions of the South Platte River drainage on the plains*
> (Logan, Morgan, and Washington counties): The expected breeding species
> here is *Eastern Warbling Vireo*, but in Washington, possibly only along
> the South Platte and at Prewitt Reservoir. *Western Warbling Vireo* is
> probably of reasonably regular occurrence as a spring migrant. EWVI will
> have non-zero filter limits from May through early August, but WWVI will
> have filter limits of 0.
>
> *The problem children* (Weld, Adams, and Arapahoe counties): As I noted
> earlier, both warbling vireo species have been noted singing at Barr Lake
> (Adams Co.) during the breeding season. Thus, in *Adams County*, both
> species will have filter limits >0 from May to early August, but both
> filters will have 0 limits in the fall (essentially after 7 August). I
> *strongly* suggest providing recordings for reports of either species in
> the county so we can begin to fully understand the breeding distributions
> and the relative abundances of the two species. In both Weld and Arapahoe
> counties, the filter limits of both species will be 0, so documentation for
> both will be required.
> Hopefully, eBirders will provide a lot of recordings this coming spring
> and summer, so that the various Colorado eBird reviewers can better
> understand the two species’ spring and summer occurrence patterns. That
> would provide those reviewers the opportunity to refine filters for
> subsequent breeding seasons.
>
> Tony Leukering
> Denver, CO
> eBird blog <http://cowyebird.blogspot.com/> > eBird commentary <https://ebirdcommentary.blogspot.com/> > Photos <http://www.flickr.com/photos/tony_leukering/> >
>
>
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