Grey-headed Gull
Chroicocephalus cirrocephalus
The grey-headed gull is slightly larger than the black-headed gull at 42 cm length. The summer adult has a pale grey head, a grey body, slightly darker in tone than in black-headed gull, and red bill and legs. The black tips to the primary wing feathers have conspicuous white "mirrors". The underwing is dark grey with black wingtips. The grey hood is lost in winter, leaving just two dark smudges around and behind the eye. The eye is dark in immatures, but has a white iris in adults. The sexes are similar.
ID text from Wikipedia, CC BY-SA 4.0
Sabine's Gull
Xema sabini
Sabine's gull is a small gull, 27 to 33 cm (10+1⁄2–13 in) in length and weighing 135 to 225 g (4+3⁄4–8 oz). The wings are long, thin and pointed with a span of between 81 and 87 cm (32–34+1⁄2 in). The bill, which is black with a yellow tip, is around 2.5 cm (1 in) long. This species is easy to identify through its striking wing pattern, though at long range it can be confused with immature black-legged kittiwakes. The adult has a pale grey back and wing coverts, four black outer primary flight feathers, and white inner primaries and secondaries. The white tail is slightly forked.
ID text from Wikipedia, CC BY-SA 4.0
Where you might meet both
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