Slaty-backed Gull
Larus schistisagus
It is tied with the yellow-footed gull for fourth-largest gull species, measuring 55–68.5 cm (21.7–27.0 in) in length, 132–160 cm (52–63 in) in wingspan, and 1.05–1.7 kg (2.3–3.7 lb) in weight. Among standard measurements, the wing chord is 40 to 48 cm (16 to 19 in), the bill is 4.8 to 6.5 cm (1.9 to 2.6 in), and the tarsus is 6 to 7.6 cm (2.4 to 3.0 in). It has a white head, belly, and tail with a dark slaty-gray back and wings with a broad white trailing edge. The wings and back are slightly darker than those of the western gull (Kodak grey scale 9.
ID text from Wikipedia, CC BY-SA 4.0
Yellow-footed Gull
Larus livens
Adults are similar in appearance to the western gull with a white head, dark, slate-colored back and wings, and a thick yellow bill. Its legs are yellow, though first winter birds do display pink legs like those of the western gull. It attains full plumage at three years of age. This species is tied with slaty-backed gull for the world's fourth-largest gull species and is one of the largest gulls in the world, being slightly larger than the western gull. It measures 53 to 72 cm (21 to 28 in) in length and spans 140 to 160 cm (55 to 63 in) across the wings.
ID text from Wikipedia, CC BY-SA 4.0
Where you might meet both
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Play today's Daily BirdPhotos and recordings are credited to their authors under the licence shown. Full profiles: Slaty-backed Gull and Yellow-footed Gull.