This bird looks like a juvenile Thayer's Gull with the slightly darker wing tips (to be expected), but the tertials on this bird don't look as solid and uniform to the outer edge, as the tertials appear on many juvenile Thayer's Gulls I've seen. The outer edge of the tertials on this bird almost have an Iceland Gull-like appearance. Note the bill size too... doesn't seem exceeding small for a Thayer's Gull bill, but would likely be towards the female-sized bill if this bird is a Thayer's Gull.