An auk is a bird of the family Alcidae in the order Charadriiformes. Extant auks range in size from the Least Auklet, at 85 g (3 oz) and 15 cm (6 in), to the Thick-billed Murre, at 1 kg (2¼ lb) and 45 cm (18 in). They are good swimmers and divers, but their walking appears clumsy. Modern auks can fly (except for the recently extinct Great Auk). Due to their short wings, auks have to flap their wings very quickly in order to fly.
Auks are superficially similar to penguins having black-and-white colours, upright posture and some of their habits. Nevertheless they are not closely related to penguins, but rather are believed to be an example of moderate convergent evolution.
Auks live on the open sea and only go ashore for breeding, although some species, like the Common Guillemot, spend a great part of the year defending their nesting spot from others.
Several species have different names in Europe and North America. The guillemots of Europe are murres in North America, if they occur in both continents, and the Little Auk becomes the Dovekie.
Some species, such as the Uria guillemots, nest in large colonies on cliff edges; others, like the Cepphus guillemots, breed in small groups on rocky coasts; and the puffins, auklets and some murrelets nest in burrows. All species except the Brachyramphus murrelets are colonial.