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 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.
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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.
