These four first cycle Ring-billed Gulls
Larus delawarensis were photographed in mid-December,
and feature tail patterns that are very similar to nominate COGU - in fact the first individual also has the typical nominate COGU pattern on the tips of P2 - P4 (the black limited to the outer side of the shaft, at the tip), inviting the notion that it is possibly not a RBGU?:    |