The Lesser Black-backed Gull is a European gull, smaller than a Herring Gull, known for its dark grey/black back and wings, yellow bill, and bright yellow legs, with adults having white heads (brown-streaked in winter) and black wingtips with white spots.

Immature Lesser Black-backed Gulls (as depicted by above photos) look very different from adults, starting as mottled brown/grey juveniles with dark bills and tail bands, gradually gaining sleeker, checkered brown/white patterns with paler heads, blackish wings, and eventually developing the adult’s dark grey back and yellow legs over 3-4 years, But the non-breeding adult Lesser Black-backed Gull is most distinctly identified by its dark gray back, yellow legs, and dusky brown streaking on the head and neck.

https://datazone.birdlife.org/species/factsheet/lesser-black-backed-gull-larus-fuscus