A hybrid gull... but what parentage?

The same day I saw the recent adult Thayer's I also came across this second-winter gull. I hadn't seen much of interest up to that point and this gull was the judges overwhelming choice for bird-of-the-day if it hadn't been for the Thayer's appearing at the death and pipping it to the accolade.


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.

Glaucous-winged x Slaty-backed is a logical choice in Japan but they both are relatively short-winged, typically with a deep-based and blunt-tipped primary projection but this bird is distinctly long-winged and that combination is highly unlikely to produce it. There isn't much overlap in breeding ranges of Glaucous-winged and Vega which may explain why I've never (knowingly) seen that hybrid. Vega x Slaty-backed, which I have seen, is more likely because of a greater area of sympatry according to range maps but I don't think there's much to suggest Slaty-backed about this gull.

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.

If it is a Glaucous-winged hybrid it might even be worth considering a New World combination, there seem to be plenty of hybrids across the water and Glaucous-winged x American Herring Gull ranges have significant overlap. This could account for the longer-winged appearance but surely such a hybrid would have a paler saddle and so seems a non-starter. The larger mirror of the underside of p10 is just visible on the upperside.