A large flock of Mew Gulls (Larus canus) spends the winter at the waste water treatment plant in Forest Grove and at adjacent Fernhill Wetlands (Birding Oregon p. 61).
Adult Mew Gulls are easily recognized in flight by the large white mirrors on the outer two primaries. Ring-billed Gulls have a smaller white mirror on the outermost primary (P10) and a tiny one on P9. You wouldn’t think the size of the white spots on two feathers would make much of a difference in identifying a flying bird, but it really does.
The bird in flight is a second-cycle Mew. He/she has lost the brown plumage of a first-cycle bird, but the tail still shows a broken blackish band, and the primaries (and primary coverts) have more black than those of an adult. Notice the small white mirror on P10 and little or no white on P9.