Please forgive my priorities: I didn't take a break from fishing to try and get some pictures. As this is a new ID for me, I regret the decision.
I was shore-fishing off three mile harbor in East Hampton when I saw these. The birds were white and each had some kind of black band. I don't remember where the bands were. The birds were noticeably smaller than Gulls, but weren't tiny. There were many over the water, but the birds were separated into individual flocks. I didn't count - something like 4-8 to a flock. Their behavior was different than that of the Gulls. They weren't perching anywhere or looking for spots to perch. I only saw the birds gliding. Their wings were always fully extended and not flapping. Not sure if it helps with the ID at all, but I've been fishing that same spot this time of year since 2015, and haven't seen them before. Maybe some migratory patterns have changed.
Any ideas? I have the images clearly in my head, so if I look up a likely prospect, I'll be able to quickly verify it it was what I saw.