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Since the Solitary Vireo was split into three species (Blue-headed, Cassin's and Plumbeous Vireos), birders have struggled to separate these taxa when any one of them is reported outside of its normal range. In the Fall, separating putative Blue-headed Vireos on the West Coast is complicated by the fact that freshly-molted Cassin's Vireos are nearly as bright, colorful, and contrasting in plumage (Heindel 1996). Similarly, a suspected Cassin's Vireo found east of the Rocky Mountains must first be proven not to be a Blue-headed.
Vireos sport their brightest and most contrasting appearance in the Fall after going through their only molt cycle of the year. Like other vireos, Cassin's Vireos seen during Spring and Summer can be quite dull and virtually colorless. The contrast and color one sees in the Fall are lost to feather wear and fading. During this season, it's hard to imagine how a Cassin's might be confused with a Blue-headed, but Plumbeous...that's another story.
One of the by-products of any species split is a predictable surge in reports of the new species from locales where the former subspecies was either never, or rarely recorded. Trying to get a handle on the actual status and distribution of the new taxon can be a challenge because the first wave of such sightings comes before field guides catch up and authoritative ID articles are published. As we rush to fill the blank spots on the new scorecard, we do so without the help of well-vetted resources. Most birders don't concern themselves with trying to recognize subspecies, but once one is elevated to full species status and becomes available for us to tick off, the sense of urgency to do so ratchets up.
The breeding range of Plumbeous Vireo spans most of the Great Basin–Nevada, s. Idaho, Utah, w. Wyoming, n. and e. Arizona, and much of New Mexico (Goguen and Curson 2012). The northernmost breeding records are from Montana and to the south the breeding range extends into Mexico along the Sierra Madre (Goguen and Curson 2012). Although some presume that this species breeds in Oregon, there is no clear evidence for this being the case.
Prior to 1997, when Plumbeous Vireo attained full species status, there were but a handful of colloquial and published "Plumbeous" Solitary Vireo records from Oregon. Most were not fully documented. Additionally, no specimen taken in Oregon has ever been assigned to this form. However, with the anticipation and eventual assignment of full species status the occasional claims of a Plumbeous Vireo, which formerly came every few years or so, have mushroomed into multiple reports per year.
Most of Oregon's reports of Plumbeous Vireos have fallen late May to early June. The majority of such sightings coming from desert oases/migrant traps in the southeastern corner of the state, where visiting birders arrive primed and ready to find vagrants. Note that this section of Oregon is sparsely populated and only a handful of active birders live in this subregion. I'm not quite sure how, or why it happened, but in the wake of the species split the Oregon birding community quickly came to accept that Plumbeous Vireo should be expected in southeastern Oregon at this time of year and that it probably breeds semi-regularly in this corner of the state. The evidence supporting either of these notions is at best, scant. There are but seven reports of Plumbeous Vireos that have been accepted by the Oregon Bird Records Committee (OBRC). Four of these are single-observer sight records and only two of the seven accepted records are supported by photos.
The species account in Birds of Oregon: A General Reference (BOGR) may have contributed to the misunderstanding of Plumbeous Vireo's status in Oregon (Nehls 2003). It starts by stating that the status of this species is "poorly understood." Then, there is a cautionary note about identification, which references Matt Heindel's excellent ID article published in Birding (Heindel 1996). It further points out that there are no Oregon specimens. Following these caveats, there is a roster of 11 cited records, including a putative nest found along Kelly Creek in southern Lake County in 1996. Apparently, the nest site was logged shortly after this discovery, thus no further documentation or photos were obtained. The account also offers that the species "has been reported in Harney County every spring since 1992." It's hard to read this account and not come away believing that the regular occurrence of Plumbeous Vireo in Oregon is well established. Many of the reports listed in BOGR (Nehls 2003) were never submitted to the OBRC and of those that were, two were not accepted.
The issue with over-reporting of Plumbeous Vireos is not confined to Oregon. In California, ballooning spring reports of this new species from that state's coast and desert oases have also raised questions (Paul Lehman pers. comm.). According to Lehman and others, Plumbeous Vireos are still "scarce-to-rare" during Spring even in Southern California, despite an apparent increase in the numbers found there during fall and winter. Away from Southern California (Los Angeles south) Plumbeous Vireos are casual visitors at any season. Long-time North American Birds regional editor Guy McCaskie is of the opinion that there are only a few "ironclad" spring records from Southern California and that all good records are from the desert, with coastal reports being suspect (Lehman pers. comm.). As they have been in Oregon, reports from California's desert outposts are often poorly documented, as many birders believe that a spectacled vireo seen in desert/Great Basin habitats is by default a Plumbeous. From Oregon, Alan Contreras points out that northbound Cassin's Vireos continue to trickle through Harney County oases into early June, even though this species is not thought to breed in Harney County, or neighboring Malheur County to the east.
When seemingly credible sources repeat the same thing with enough frequency, claims that lack hard supporting evidence gain traction and start to become accepted 'fact.' Political parties are experts at this sort of manipulation of reality, often swaying public opinion with the repetition of half-truths, or murky data.
The birding world is not immune to being influenced by apparent critical mass. Unlike the manipulation that occurs in the political arena, birders tend to unknowingly perpetuate fallacies. When visiting sites where vagrant birds are found regularly, or birding a locale where a desired species is "expected," it can be a challenge to avoid the pitfalls that accompany expectation. Objectivity and healthy skepticism are too often replaced by an excited mind that is ripe for being duped. It's not hard for us to shoehorn the first plausible candidate into a species that we are hoping to see. It becomes easy to overlook what one subconsciously doesn’t ‘want’ to notice and see only those aspects of the bird that seem to best fit the species that one hopes to add to their list. If you’ve been birding for any length of time, you’ve probably stepped into this pothole a time or two.
When birding desert migrant traps in southeastern Oregon–where Cassin's Vireos are low density spring migrants–the sight of an apparently dull and colorless spectacled vireo gets the blood pumping. If the bird stays inside the canopy and out of direct sunlight (typical for this species group), it's not going to be difficult to convince yourself that you aren't seeing yellows and greens in the plumage, because you probably aren't. Even knowing how dull Cassin's can be in the spring, I have twice been fooled into thinking that I might be looking at a Plumbeous in Oregon, only to have my camera or exposure to direct sunlight reveal color that I wasn't seeing when the bird was in the shade. Even in good light, the subtle colors of a spring Cassin's come and go with changes of angle.
At this point, the actual status of Plumbeous Vireo in Oregon remains a mystery, in part because our birding community has applied little rigor to the examination of reports of this species. Acceptance of a presumed status is based almost entirely on a set of unreviewed or undocumented reports, hence there is a prevailing notion that reports of this species don’t require thorough documentation. I contend that we should start from ground zero and start treating Plumbeous Vireo as though this species is extremely rare in Oregon, which it may well be. Only then, will we be able to get a handle on their actual status. Getting that genie back into the bottle is easier said than done.
Goguen, Christopher B. and David R. Curson 2012. Plumbeous Vireo (Vireo plumbeus), The Birds of North America Online (A. Poole, Ed.) Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell,edu/bna/species366 doi:10.2173/bna366
Heindel, M.T. 1996. Field identification of the Solitary Vireo complex. Birding 28:458-471.
Nehls, H. B. 2003. Plumbeous Vireo. p. 401 in Birds of Oregon: A General Reference. D.B. Marshall, M.G. Hunter, and A.L. Contreras, Eds. Oregon State University Press, Corvallis, OR.