Exe estuary near Topsham

About 10.00 this morning there was a flock of c20 black-headed gulls feeding close to the Exminster side of the estuary opposite the Goat Walk. One of the birds had noticeable pale under-wings apart from a distinct black edge to the outer wing. It looked smaller than the b-h gulls and was in winter plumage (no hood). I suspected that it was a Bonaparte's gull but almost immediately all the gulls vanished from view. They may have landed on the water or mud. As I was viewing from the canal bank on the Exminster marshes I was unable to locate them again. Someone else might have more luck.