The gulls use this parking lot to rest after feeding at the nearby landfill to the west. Once there's no water for them to bath in or drink, they readily abandon this site and move out. My feeling is that they go to the Zion landfill which is closer to Lake Michigan - just a guess.
Here's a somewhat pale-eyed Thayer's. Howell and Ellitott found that only about 14% of Thayer's Gulls (in California) could be called "truly dark-eyed", while 20% were found to be "pale-eyed". They went on to say that about 66% of birds are somewhre in between.
|Thayer's Gull (adult). Lake County, IL. 24 November 2013.|
|Lesser Black-backed Gull (sub-adult). Lake County, IL. 24 November 2013.|
|Lesser Black-backed Gull (1st cycle). Lake County, IL. 24 November 2013.|
Notice the post-juvenile scapulars on the upper back. I've marked the photos below with N (for new prealternate feathers) and J (for juvenile, 1st basic, scapulars):
Here's the unmarked image: