CALIFORNIA GULL Distinctive. Note medium size, long and pointed wings, medium gray upperparts with broad-based black wingtip patch, dark eyes, black bill ring (reduced on breeding birds) with a red gonydeal spot, and greenish to yellow legs.

RING-BILLED GULL (Widespread) smaller with shorter and slimmer bill (that lacks red gonydeal spot), staring pale lemon eyes, and pale gray upperparts (Kodak 4-5) with less contrasting white scapular and tertial crescents. Overhead in flight, Ring-billed shows mostly white underwings, less extensive and narrower—based black wingtip patch, and lacks smoky gray bases to remiges of California. Winter head and neck markings finer and more streaked.

MEW GULL (w. N. America) appreciably smaller with smaller, plain yellow bill and shorter wing projection. Mew has less black in the wingtip and typically larger white tongue-tips on P7-P8. Basic head and neck markings of Mew tend to be more diffuse, or smudgy.

LESSER BLACK-BACKED GULL (widespread, but rare in West) averages larger with slightly stouter bill, slaty gray upperparts (Kodak 9-13), and pale eyes; legs yellow to fleshy yellow, lacking gray-green hues often shown by California; black wingtips contrast less on darker upperwings, and underwings show darker bases to remiges.

AMERICAN HERRING GULL, THAYER’S GULL, and other large gulls with pale gray to medium gray upperparts have pink legs. Occasional Herring Gulls with yellow legs can suggest albertaensis California Gull but average larger and bulkier with shorter wing projection, deeper gonys; Herring also has staring pale eyes, slightly paler upperparts (Kodak 4—5), more tapered black wingtip wedge (often with only a small P10 mirror in w. N. America), underwing lacks smoky gray secondary bases, and bill has less distinct black distal band in winter.

CALIFORNIA GULL is a fairly distinctive but notoriously variable species, and observers in the West should learn it well. Features to note are the relatively long, narrow, and pointed wings, fairly long and parallel-sided bill that is strongly bicolored pink-and-black, pinkish legs with a variable blue-gray hue to the tibia and joints, solidly dark primaries and blackish tail, and frequently extensive PA1 molt that often includes upperwing coverts and tertials.

MEW GULL is smaller and shorter billed, but first-cycle plumages can resemble second-cycle California Gull. Distinguished from second-cycle California by size, pale gray greater coverts (dark brownish on California), contrasting pale gray panel on inner primaries.

AMERICAN HERRING GULL (widespread) averages larger with shorter wing projection, slightly broader and less crooked wings in flight, and less parallel-edged bill that is rarely strongly two-toned in midwinter; its pinker legs lack a bluish hue to the tibia and joints. Herring Gull typically has a contrasting pale panel on the inner primaries, the bases of its greater coverts are less often solidly dark brown, and its tail (especially in the West) often has more-extensive pale barring at the basal corners. PA1 molt averages later (especially in the West) and only exceptionally includes upperwing coverts.

THAYER’S GULL (mainly w. N. America) similar in overall size but often looks relatively shorter legged and has shorter wing projection. Most winter birds told readily by mostly dark bill, which is also relatively shorter with deeper gonys than California. Thayer’s typically retains juvenal plumage through midwinter and has browner, less blackish wingtips; in flight, the pale inner primary panel and barred tail base of Thayer’s are distinctive.

WESTERN GULL (w. N. America) larger and bulkier with stouter, slightly bulbous-tipped, black bill, broader wings (often showing a skirt at rest), and shorter wing projection. Plumage can be very similar to California, but head and neck usually more solidly dark and sootier brown; bill mostly black (develops pinkish at base in midwinter but does not attain sharply two-tone pattern of California). A1 scapulars typically have blackish centers and grayish to pale brownish edgings (a pattern rare on California).

HEERMANN’S GULL (w. N. America) more uniformly dark brown (lacking contrasting whitish tail coverts) with smaller bill (duller flesh basally) and black legs.

LESSER BLACK-BACKED GULL has relatively shorter, all-black bill that starts to show some flesh at base only in late winter; averages colder and grayer overall than California. Upperparts (including tertials) average narrower pale edgings; juvenal plumage often retained into winter, but PA1 molt can include upperwing coverts. Uppertail coverts and tail base contrastingly white with clean-cut, black distal tail band.