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
Slender-billed Gull
Chroicocephalus genei
This species is 37 to 40 cm (14.6 to 15.7 in) long with a 90 to 102 cm (35.4 to 40.2 in) wingspan. It is therefore slightly larger than the black-headed gull, which it resembles, although it does not have a black hood in summer. It has a pale grey body, white head and breast and black tips to the primary wing feathers. The head and dark red bill have an elongated tapering appearance, and this bird also appears long-necked. The legs are dark red, and the iris is yellow. In summer, the breast has a faint pink colouration. This bird takes two years to reach maturity, as is usual in gulls.
ID text from Wikipedia, CC BY-SA 4.0
Where you might meet both
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