I wonder if it's possible the "possible Black-tailed Gull" recently reported
on tweeters could have been a subadult Mew Gull. I see nothing about the
description and sketch that would definitely eliminate that possibility.
Black-tailed Gulls are quite dark-backed, darker than California Gulls, and
if the bird were an adult (as the plumage indicates), a red tip might be
visible on the bill. A subadult would show less white in the wing than an
adult and could show a substantial dark tail bar.

Don't let this stop you from looking for said bird!

Dennis Paulson
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Netta Smith and Dennis Paulson
1724 NE 98 St.
Seattle, WA 98115