Identificaciones de caspiantern01

A bit of a puzzling bird, 1st cycle I think, somewhat herring like but with very white head and contrasting darker body, large, rather bulbous and strongly bicolored bill (slightly more black on lower mandible). Primaries appeared dark with pale tips most of the time, but out of shadow looked much browner. Waited a while for it to take flight but didn't see it do so, however I did get to see it again later circling with some TUVUs. Cook Inlet seems possible; usually those I see have more muddy bills but I've only really identified 1st cycles and adults. Also considered Nelson's Gull, given the strongly bicolored and chunky bill, and paleness of the head, but overall appears too dark for that. Closest match I've found is: https://macaulaylibrary.org/asset/612170911, but that bird has a much slimmer bill. Maybe just a large-billed Herring? Appreciate any thoughts on this!