An interesting chickadee found whilst looking for Boreal Chickadees at Moose Bog, Wenlock WMA in the North East Kingdom, Vermont. This bird's unusual appearance prompted the question of hybridization between Boreal and Black-capped? A request for information on ID Frontiers revealed that there are apparently no fully documented cases of hybrids between this particular species pairing.
This bird was foraging close to two Black-capped Chickadees but responded immediately to sound recordings of Boreal. Although the digiscoped images (taken in dull overcast conditions) are rather poor they do reflect much of what I experienced in the field - a bird somewhat resembling Boreal Chickadee but lacking the brown-cap and gray neck sides of that species, and showing a rather white-cheek similar to Black-capped Chickadee. In the field, the wings appeared near uniform lacking the white edging to the secondaries and greater coverts of Black-capped and more closely resembling Boreal. At all times the bird appeared black-capped, though the images show a subtle brown tone to the top of the crown which I didn't see in the field. Further odd features include a clear contrast between the black cap and the mantle, and rather weakly colored buffy flanks (not rich buff as in typical Boreal).
I left the bird feeling quite puzzled and thought about the hybrid question even before I'd had chance to look over the images. Although I don't expect the images to be in anyway conclusive, they do appear to show a reasonably good candidate for a Boreal x Black-capped Chickadee hybrid.
I'm very grateful to Peter Pyle, David Sibley, Cameron Eckert and Steve Mlodinow for commenting on hybridization in chickadees and for offering to comment on these images.