Nelson's Gull: Lake County Illinois

My prize for enduring the wet and cold rain yesterday, under brutally overcast conditions, was this putative Nelson's Gull:

Presumed Glaucous x Herring hybrid (adult). Lake County, Illinois. 22 Dec 2014.

These hybrids are rather straightforward in definitive adult plumage. They appear paler than a typical Herring, have "slatyish" outer primaries, or paler, and large apicals. Most are big-bodied and the head often looks blocky with a thicker neck and heavier chest.

One thing I've noted on the last few Nelson's that I've seen is that there's more white on the back and sides of the neck where the white begins to meet the gray. I've circled this region in black on the edited photo below:

This increased white (or reduced gray) could simply be due to posture, but I believe it has more to do with the larger surface area of the body. The tertials and adjacent secondaries also seem longer and tend to form a large cloak over the base of the primaries (compare with the surrounding HERGs) - a couple of features that I'll be keeping an eye out for in the future.