The 24 species of Auks (Alcidae) are superficially similar to penguins due to their black-and-white colours, their upright posture and some of their habits. Nevertheless they are not closely related to the penguins - their closest relatives are skuas and jaegers.

In contrast to penguins, the modern auks are able to fly - with the exception of the recently extinct Great Auk. They are good swimmers and divers, but their walking appears clumsy. Due to their short wings auks have to flap their wings very quickly in order to fly.

Auks live on the open sea and only go ashore for breeding. Although not to the extent of penguins, auks have to a large extent sacrificed flight, and also mobility on land, in exchange for swimming; their wings are a compromise between the best possible design for diving and the bare minimum needed for flying. Time depth recorders on auks have shown that some species can dive as deep as 100 m, hunting for fish.