A couple of days ago, I saw this gull at McDonalds hoping for handouts. Although the colors seemed basically right for a Thayer’s Gull, its bulk did not. It seemed very comparable in size/shape to other Glaucous-winged Gulls that were nearby, and qualitatively it struck me as significantly larger than I tend to associate with Thayer’s Gulls. I tried to get photos of it with other birds for comparison, but was unable to do so as there was a particularly aggressive bird that appeared to me to be a second-cycle Glaucous-winged Gull. It chased several birds away from the car, presumably to keep the potential food source itself (of course, while chasing the other birds away, it typically missed opportunities to grab a bite). On a number of occasions it chased this particular mystery gull.
For now I am thinking this bird is probably a Glaucous-winged x Herring Gull hybrid due to the large bulk, dark eyes, and dark wingtips. I am definitely not completely confident with this choice, and would be interested to hear alternative suggestions.