Townsend's Warbler x Black-throated Green Warbler hybrid, Crow Valley, Weld County (Colorado, USA), 2nd September 2015 - copyright Steve Mlodinow
(photo ID: 2283)
Best I let Steve explain this one...
"This bird's initial appearance is in the eye of the beholder. Many will see Townsend's Warbler, as its cheek and crown are black and there is extensive yellow below the black throat. However, the black throat expands laterally to join the side streaking, not as much as in Black-throated Green Warbler but more so than in Townsend's Warbler. The auricular patch is narrower, particularly towards rear, than in Townsend's Warblerand the yellow crescent below the eye is larger. The olive back was utterly unstressed, as in Black-throated Green Warbler but wrong for adult male Townsend's Warbler. Interestingly, both adult male Townsend's Warbler and Black-throated Green Warbler have gray uppertail coverts and an olive rump that are streaked. Finally, the undertail coverts are unstressed, a rare finding in Townsend's Warblerat any age, and particularly so in adult males."Steve refers to a paper in the Journal of Avian Biology, "Hybridization between Townsend's Dendroica townsendi and black-throated green warblers D. virens in an avian suture zone" (Toews, Brelsford and Irwin, 2011) (Journal of Avian Biology 42: 434-446). Steve tells us that the authors found that only 4 of the 137 birds scored to phenotype by were “intermediate” with most largely resembling one parent or the other. And there's apparently more information on this hybrid in "Two recent records of apparent Black-throated Green Warbler x Townsend’s Warbler in Colorado" (Mlodinow, Walbek and Leatherman, 2014) published in Colorado Birds 48: 102-106.
Townsend's Warbler x Black-throated Green Warbler hybrid (same bird as in photo ID 2283 above), Crow Valley, Weld County (Colorado, USA), 2nd September 2015 - copyright Steve Mlodinow
(photo IDs: 2284-2286)
Townsend's Warbler Setophaga townsendi (formerly Dendroica townsendi)
Black-throated Green Warbler Setophaga virens (formerly Dendroica virens)