Verdict from experts: Can't be Thayers because of black under the wing. This apparently is a Herring with its primary molt in progress - hence the incomplete pigmentation. The color will get there! As an amateur I find this frustrating to say the least... but live and learn!

This bird seemed lighter and a bit smaller than the nearby Herring Gulls. Its gizz was different. When it flew I saw only a hint of black in the wingtips with my naked eye. I got my camera on it and got some good detail of the wingtips. Also the shots show a smaller bill and very bright pink legs. The eye color is inconclusive in the pictures - could be dark but also might not be. Both wings show missing/molted secondaries at the "elbow". So, while this could be a somewhat atypical Herring Gull - multiple things suggest to me that this might be a summer plumage Thayers. These photos are posted with notes under each one for private commentary to rbowes@bowesweb.com