From this angle, the bill pattern, darker color of the mantle (in comparison to the other birds facing at the same angle) and big white tertial crescent are all visible. The eye is dark, and if you notice the eye of the adult Ring-billed Gull in the lower left corner can be seen to be pale.
The legs were a greenish-yellow, and the larger size of the bird in comparison to the Ring-billed is also evident.