East meets West, spreadwing style

With Lake Michigan iced over to the horizon I didn't know if I would find any gulls at the landfill today.  Fortunately there were still large numbers of herring gulls (the ring-billed's seemed to be gone).   I found a first cycle glaucous, a 2nd cycle glaucous, and a 1st cycle lesser black-backed, but all of these were flyby's or far away.  The dumping site currently is much closer than usual and without an 8 foot chain link fence, so I decided to work on some spread-wing herring gull images just to see what kind of variability I could find.  I just shot at random, focusing on the densest areas of foraging gulls and clicking when I saw wings go up.

This is a 4th winter bird, aged by the blackish on the primary coverts.  It also has a tiny bit of black on P4 which adults don't generally show.  It also has slightly smaller white tips to the the feathers than a full adult would have.  The overall pattern though, of only P10 having a small-medium sized mirror was the most common one I documented.

Here's a full adult with the same pattern (and another more blurry one in the foreground).  This, per Olsen and Larsson, is the most common pattern of American herring gulls on the west coast.

This bird, however, has a large mirror on P10, as well as medium sized mirror on P9 as well.  This pattern is the typical one found on the East coast.
Based on my (small number of photos), the ratio between the two patterns was about 5:1.