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upper and under surfaces
These illustrations are based on small sample sizes, especially for Slaty-backed Gull and the two hybrids. They are probably most accurate for the other three species. Obviously, hybrids will vary. These sketches are meant to show the pattern on the primaries and secondaries primarily, and to some extent the upper secondary greater coverts. The detail on the other upper coverts and on the underwing coverts is not the emphasis of these sketches and less precision went in to those parts. The upperwings are the higher ones, the underwings are below.
On the upperwings, they all follow the same basic pattern of dark outerwebs, paler inner webs, and dark on both webs at the terminal area, and the inner primaries paler than the outer primaries-- it's just a matter of degrees. On the underwings, note the thin dark trailing edge to the outer Ps on Thayer's and Slaty-backed, lacking in Glaucous-winged. Beware that even Herring can look pale-winged from below when backlit-- and birds seen from below are often backlit!