Mediterranean Gull
Ichthyaetus melanocephalus
The Mediterranean gull is slightly larger and bulkier than the black-headed gull with a heavier bill and longer, darker legs. The breeding plumage adult is a distinctive white gull, with a very pale grey mantle and wings with white primary feathers without black tips. The black hood extends down the nape and shows distinct white eye crescents. The sharp-tipped, parallel sided, dark red bill has a black subterminal band. The non breeding adult is similar but the hood is reduced to an extensive dusky "bandit" mask through the eye. This bird takes two years to reach maturity.
ID text from Wikipedia, CC BY-SA 4.0

Pallas's Gull
Ichthyaetus ichthyaetus
This is a very large gull, being easily the world's largest black-headed gull and the third largest species of gull in the world, after the great black-backed gull and the glaucous gull. It measures 55–72 cm (22–28 in) in length with a 142 to 170 cm (56 to 67 in) wingspan. Weight can vary from 0.96–2.1 kg (2.1–4.6 lb), with an average of 1.6 kg (3.5 lb) in males and 1.22 kg (2.7 lb) in females. Among standard measurements, the wing chord is 43.5 to 52 cm (17.1 to 20.5 in), the bill is 4.7 to 7.3 cm (1.9 to 2.9 in) and the tarsus is 6.5 to 8.4 cm (2.6 to 3.3 in).
ID text from Wikipedia, CC BY-SA 4.0
Where you might meet both
Learn this pair in the BeakBrain App
Photo and call practice. Coming soon to iPhone and Android.
Play today's Daily BirdPhotos and recordings are credited to their authors under the licence shown. Full profiles: Mediterranean Gull and Pallas's Gull.