This would be a reasonable photoquiz if I hadn't given away the answer in the title
This bird had the classic half-hood and slender bill of a young Franklin's Gull.
The moult of the primaries was somewhat less classic.
Another (blurry) view of the primaries. I didn't trust the auto-focus to choose the right gull with the birds constantly crossing in front and behind each other (or it could have bailed on the gulls altogether and chosen the ground) and was manually focusing here. The secondaries also appear to be entirely dark based first-fall feathers.
It has completely moulted its tail.