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A recently published paper sheds more light on the Warbling Vireo taxonomy (Lowell et al. 2021).

The abstract:

Eastern (Vireo gilvus gilvus) and western (V.  g.  swainsoni) forms of the Warbling Vireo have essentially allopatric breeding ranges across north-central North America, but come into contact in central Alberta, Canada. In 1986, Jon Barlow presented preliminary morphological and song evidence suggesting that the Warbling Vireo complex might comprise more than one valid species. However, to date, Barlow’s suggestion is supported by only limited DNA evidence, demonstration of molt and migration differences between the taxa, and anecdotal accounts of differences in song, morphology, plumage, and ecology. We analyzed variation in both mitochondrial and nuclear DNA in birds from Alberta and surrounding areas to determine the levels of genetic differentiation and hybridization occurring in the contact zone, and whether the two taxa warrant recognition as separate biological species. Our analyses reveal that Warbling Vireos in Alberta and the surrounding areas are separated into two welldefined, genetically differentiated, and monophyletic clades corresponding to previously recognized taxonomic groups. The two taxa come into contact in a narrow (~85 km) zone in Barrhead County, northwest of Edmonton, Alberta. They show evidence of limited hybridization. The distinct genetic differences are maintained in the contact zone, where individuals of the two taxa may occupy neighboring territories. Differences in spring arrival dates, molt schedules, and migration routes indicate that a migratory divide may play an important role in reproductive isolation. We suggest that the two taxa are distinct cryptic species: an eastern form, Vireo gilvus, and a western form, Vireo swainsoni.

Of relevance is a post from earbirding.com (a great resource!):

Identifying Eastern and Western Warbling Vireos – Earbirding

 

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Further on this, some skilled Colorado eBirders have been working hard on discerning occurrence patterns of the two taxa and found them in contact in the South Platte drainage. Wyoming and Montana birders are still figuring out where the two forms occur, but, in general in the two states, Eastern breeds in low-elevation riparian gallery forest dominated by Plains Cottonwood, while Western breeds in montane aspen forest and cottonwood riparian (Narrowleaf Cottonwood).

Eastern WAVI (summer)

Western WAVI (summer)

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Yup. Alberta and British Columbia are a gold mine for those interested in hybrids. I'd love to spend a couple summers up there! A more-inclusive list:

Franklin's and Spruce grouse

all three YBSA-type sapsuckers

both wood-pewees

the three Solitary vireos

both WAVI

the three Canada jays

the three White-breasted nuthatches

the three Brown creepers

both Stub-tailed wrens

the three Hermit thrushes

both Swainson's thrushes

three of the Orange-crowned warblers

Mac and Mourning warblers

both Yellow-rumped warblers

Townsend's and Black-throated Green warblers

Timberline and Brewer's and Clay-colored and Chipping sparrows

three of the Fox sparrows

Oregon and Slate-colored juncos

White-crowned and Golden-crowned sparrows

all three Pine grosbeaks

two or three Evening grosbeaks

Red Crossbills

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3 hours ago, Tony Leukering said:

A recently published paper sheds more light on the Warbling Vireo taxonomy (Lowell et al. 2021).

The abstract:

Eastern (Vireo gilvus gilvus) and western (V.  g.  swainsoni) forms of the Warbling Vireo have essentially allopatric breeding ranges across north-central North America, but come into contact in central Alberta, Canada. In 1986, Jon Barlow presented preliminary morphological and song evidence suggesting that the Warbling Vireo complex might comprise more than one valid species. However, to date, Barlow’s suggestion is supported by only limited DNA evidence, demonstration of molt and migration differences between the taxa, and anecdotal accounts of differences in song, morphology, plumage, and ecology. We analyzed variation in both mitochondrial and nuclear DNA in birds from Alberta and surrounding areas to determine the levels of genetic differentiation and hybridization occurring in the contact zone, and whether the two taxa warrant recognition as separate biological species. Our analyses reveal that Warbling Vireos in Alberta and the surrounding areas are separated into two welldefined, genetically differentiated, and monophyletic clades corresponding to previously recognized taxonomic groups. The two taxa come into contact in a narrow (~85 km) zone in Barrhead County, northwest of Edmonton, Alberta. They show evidence of limited hybridization. The distinct genetic differences are maintained in the contact zone, where individuals of the two taxa may occupy neighboring territories. Differences in spring arrival dates, molt schedules, and migration routes indicate that a migratory divide may play an important role in reproductive isolation. We suggest that the two taxa are distinct cryptic species: an eastern form, Vireo gilvus, and a western form, Vireo swainsoni.

Of relevance is a post from earbirding.com (a great resource!):

Identifying Eastern and Western Warbling Vireos – Earbirding

 

Very interesting. Thanks!

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6 hours ago, Tony Leukering said:

Yup. Alberta and British Columbia are a gold mine for those interested in hybrids. I'd love to spend a couple summers up there! A more-inclusive list:

Franklin's and Spruce grouse

all three YBSA-type sapsuckers

both wood-pewees

the three Solitary vireos

both WAVI

the three Canada jays

the three White-breasted nuthatches

the three Brown creepers

both Stub-tailed wrens

the three Hermit thrushes

both Swainson's thrushes

three of the Orange-crowned warblers

Mac and Mourning warblers

both Yellow-rumped warblers

Townsend's and Black-throated Green warblers

Timberline and Brewer's and Clay-colored and Chipping sparrows

three of the Fox sparrows

Oregon and Slate-colored juncos

White-crowned and Golden-crowned sparrows

all three Pine grosbeaks

two or three Evening grosbeaks

Red Crossbills

And three or four of the Red-tailed Hawks, right?

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