I haven't seen a hybrid quite like this before and I'm curious about its parentage, for each potential combination there is something that doesn't quite sit comfortably. The only thing I would put money on, is that Glaucous-winged is involved. Money, but not the house. And if my arm were twisted, I'd plump for Vega as the other progenitor.
|So here's the bird. The smallish size, to an extent structure and aspects of the plumage could be highly suggestive of second-winter Thayer's. The head looks fairly rounded in this image and notice the white far-wing.|
|Another angle and the head looks much heavier, as is the bill towards the tip, more of a Glaucous-winged feel than Slaty-backed. The body was only about the size of a smallish Vega.|
|In this view the stronger light means the upperside of the primaries looks much greyer compared to the previous images, while shadow creates the darker underside.|
|The tail is still quite solidly dark and there's a ghost of a p10 mirror.|
|The upperside of the primaries has a Glaucous-winged feel with a clear difference between inner and outer webs.|