On the surface, the average observer might think "adult" RBGU and move on. The first thing I noticed was the lack of any white primary tips. After observing this bird for a few more minutes I found an all white tail:
2nd cycle birds do occasionally, if not regularly, have all white tails. The dark iris and lack of any white primary markings makes this identification a simple one. Adults tend to have clear yellow irides and bigger apical spots. This is a 2nd cycle Ring-billed Gull.
We take Ring-billed Gulls for granted here in Chicago, but I know a birder who drove 2300 miles (~3700 km) to see an adult Ring-billed Gull in Germany.