Black-legged Kittiwake vs Audouin's Gull

The most reliable field mark to distinguish a Black-legged Kittiwake from an Audouin's Gull is the leg color: Black-legged Kittiwake has black legs, while Audouin's Gull has grey-green legs. Additionally, the bill of the Black-legged Kittiwake is yellow, whereas Audouin's Gull has a distinctive red bill with a black band near the tip.

Leg color: Black-legged Kittiwake has black legs; Audouin's Gull has grey-green legs. Bill color: Black-legged Kittiwake has a yellow bill; Audouin's Gull has a red bill with a black band. Wingtip color: Black-legged Kittiwake has black wingtips; Audouin's Gull lacks this feature.

Black-legged Kittiwake has a white head and underparts with a grey back and wings, and a black wingtip. Audouin's Gull also has a grey back but is generally larger and has a more robust build.