On the closer bird, notice the primaries... not really black, but more of a pale slate/medium gray color. The amount of white in the windows (wing-tip region of the bird) can vary throughout Herring Gull and Thayer's Gull. I was unable to get any photos of this bird's underwing (especially the underside of the primaries).. be sure to look at the underside of the folded primaries and check the color... this bird had a very light gray underside to the primaries, whereas Herring Gull tend to be very dark 'underside-primaried'. This bird, when it flew, had very light gray under-primaries with the white windows. On a more zoomed-in scale, this bird has a slightly rounder-shaped head than a typical Herring Gull, with dark eye and more vibrant bill. The dark eye and more vibrant bill can be also be found on Herring Gulls. But overall, notice lack of pure-black on this bird's primaries, in addition to round head and dark eye.