Date: 11/27/25 6:59 pm From: '<greatgrayowl...>' via Colorado Birds <cobirds...> Subject: [cobirds] Warbling vireos and Colorado eBird
All:
With the split of Warbling Vireo into two species (EasternWarbling Vireo [EWVI] and Western Warbling Vireo [WWVI]), eBirding has becomemuch more difficult and problematic, and we Colorado birders get to be theguinea pigs for learning about the distributions of the two species on theColorado plains. While there are pre-existing data, there are nowhere nearenough, as few birders cared about the subspecies Warbling Vireo (Eastern) andWarbling Vireo (Western).
The first and most intractable problem is that there are NOconsistent plumage color or pattern differences between the two species.Western TENDS toward the grayer end with a darker crown, and Eastern TENDStoward the brighter end with a paler crown, but the overlap in plumage tone isvirtually complete. Western has a shorter, thinner bill than does Eastern, butthe usable in-hand differences are in the half-millimeter range, something thatwill be nearly useless in field conditions.
The primary take-home message from this post:Recordings of SINGING birds provide the only truly definitive documentation. Notcalling birds. Not whining birds. Singing birds, and singing birdssinging full songs. That means that all of our phones’ audio recorderswill be getting a workout come May. That also means that non-singing birds are essentiallyunidentifiable, and should be recorded as “Eastern/Western Warbling Vireo” (orsome such entry).
Because there are relatively few definitively identifiedrecords of either species on the Colorado plains, where the two species meetand overlap, we don’t know the true extent of either species’ breedingdistributions there. Yes, the farther east one goes, the more likely it is thatEWVI is the breeding species, and WWVI is more likely as one approaches thefoothill edge. West of the foothill edge, all breeders are WWVI… probably. Bothspecies are suspected to breed at Barr Lake S.P., and if they hybridize thereor elsewhere where the two species meet, then virtually all bets are off whenit comes to definitively identifying even singing warbling vireos.Additionally, the extremities of the two species’ breeding ranges probablydiffer greatly between the South Platte drainage and the Arkansas drainage. Thesame is true for many “eastern” species of birds, such as Red-belliedWoodpecker, Bell’s Vireo, Baltimore Oriole, and Indigo Bunting, all of whichbreed much farther west in the South Platte drainage than in the Arkansasdrainage. More on this, below.
As I noted in the first sentence, the reason I am posting inthis venue is to give everyone birding on the Colorado plains next spring andsummer a heads-up as to how the Colorado eBird filters will be dealing withthis worst-ever bird-ID conundrum to visit the state’s birders.
eBird filters provide a framework for the abundance (or lackthereof) for all species occurring in a given filter region (e.g., AdamsCounty, 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 for more on that eBird process). Individual species have upper limits on thenumber of individuals that can be submitted to eBird from a particular location,beyond which the entry will flag for relatively atypical abundance for limitsof 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. For other aspects of eBird relative to Colorado, check out the blog housing the above-linked essay: Colorado & Wyoming eBird
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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...
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Colorado & Wyoming eBird
Everything eBird for Colorado and Wyoming, from the keyboards of the region's eBird reviewers
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I return you to our regularly scheduled programming.
Front Range, Wet Mountains, and Sangre de Cristos andwest – This is the only portion of the state that is/has been simple tocreate the eBird filter limits that will govern which species will be availableon filters: All filters from these areas will allow Western Warbling Vireoat various limits of >0 during the seasons of typical occurrence (on a grossscale, May through September). Somewhat unfortunately, all foothill-edgecounties in Colorado straddle the foothill edge, so those counties (Larimer,Boulder, Jefferson, Douglas, El Paso, Pueblo, Huerfano, and Las Animas) will alsohave Eastern Warbling Vireo on those filters, but with the limit set to zero oneach. That means that any reports of Eastern Warbling Vireo in thosecounties 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 endof the breeding season (probably May through mid-August), but will have 0limits for Western Warbling Vireo (the species may be a casual or raremigrant through these counties, as the species breeds in the Black Hills ofSouth Dakota and may traverse eastern Colorado to and fro).
Western portions of the Arkansas River drainage on theplains (Elbert, El Paso, Lincoln, Crowley, and Otero counties): EasternWarbling Vireo will have limits of 0 in all seasons and all counties. Becausewe CO birders are not sure of the distributions of the two species in Crowleyand Otero, BOTH species will have a limit of 0 all year; documentation will berequired, even in migration, when Western Warbling Vireo is probably a fairlycommon spring (and fall) migrant.
Eastern portions of the South Platte River drainage onthe plains (Logan, Morgan, and Washington counties): The expected breedingspecies here is Eastern Warbling Vireo, but in Washington, possibly onlyalong the South Platte and at Prewitt Reservoir. Western Warbling Vireois probably of reasonably regular occurrence as a spring migrant. EWVI willhave non-zero filter limits from May through early August, but WWVI will havefilter limits of 0.
The problem children (Weld, Adams, and Arapahoecounties): As I noted earlier, both warbling vireo species have been notedsinging at Barr Lake (Adams Co.) during the breeding season. Thus, in AdamsCounty, both species will have filter limits >0 from May to earlyAugust, but both filters will have 0 limits in the fall (essentially after 7August). I strongly suggest providing recordings for reports of eitherspecies in the county so we can begin to fully understand the breedingdistributions and the relative abundances of the two species. In both Weld andArapahoe counties, the filter limits of both species will be 0, sodocumentation for both will be required.
Hopefully, eBirders will provide a lot of recordings thiscoming spring and summer, so that the various Colorado eBird reviewers can betterunderstand the two species’ spring and summer occurrence patterns. That wouldprovide those reviewers the opportunity to refine filters for subsequent breedingseasons.
Tony LeukeringDenver, COeBird blog
eBird commentaryPhotos
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