While birding at Upper Beaver Meadows in RMNP, we encountered a small flock of migrating birds including several Mountain Chickadees and at least two Townsend's Warblers. Several days later, I posted some photos on COBIRDS of the one warbler that was cooperative for photos. I received an email from Steve Mlodinow who said he felt my photos depicted a Townsend's X Black-throated hybrid which he said was very unusual in Colorado.
According to Steve (in reference to my photos), "the main thing is how far the black throat of this adult male bulges onto the upper chest and then the black somewhat flows into the side streaking, reminiscent of a Black-throated Green male. Also, the yellow crescent under the eye seems particularly large and there is a lot of green mixed into the black of the auriculars."
Steve also referred me to an article in Colorado Birds and combined with his comments, I felt it was worthwhile to submit this report. Here is the article he mentioned:
Mlodinow, S.G., G. Walbeck, and D.A. Leatherman. 2014. Two recent records of apparent Black-throated Green Warbler × Townsend’s Warbler in Colorado. Colorado Birds 48: 102-106