Changing the mantle

After entirely too many posts (don't think it'll stop me in the future) on Gull-nasty, the dark-mantled Chandeleur type gull (Kelp x Herring), I have a different dark-mantled hybrid for your viewing pleasure.

This bird was off New Buffalo.  When I first picked it up, seeing a nearly entirely white headed gull with a noticeably darker mantle than the average Herring Gull, but lighter than a typical graelsii Lesser Black-backed I had hopes for a Vega or Yellow-legged Gull.  As we got closer though the mantle seemed a bit dark for either and the bird was globe-headed with a heavy bill even for a Herring Gull.  You can get a sense of the heft of the bill and the jaw compared to the HERG in the foreground:

A flight shot ...

The one pic of GBBG x HERG in Howell and Dunn has a slightly larger mirror on P9, but I'm sure this is still within the range of variation.  It shares the subterminal white spot "pearl" on P7.

It made a couple close passes at our bread.  I kept shooting to try to get the orbital color...

It's orange or red-orange, both black-backs have red orbitals per Sibley, HERG has yellow-orange.

So, to summarize the intermediate characters on this bird, body size and bill larger and heavier than HERG arguing strongly for GBBG, mantle intermediate between HERG and a black-back, orbital intermediate, the feet are pink pretty close to a typical HERG and not like the much paler pink feet of a GBBG.  The intensely pink feet, large size, and minimal head markings are pretty strong arguments against LBBG as the black-backed parent.

While it's always hard to compare shades of gray in a photo, I think that these pics are pretty representative of the relative shades, here's a Lesser Black-back from the same flock at virtually the same time:

Lesser seems to moult later than do the other gulls, this one is a bit asymmetric with an old P10 on one side and a fresh P10 still growing in on the other.