[5] Auks are superficially similar to penguins, having black-and-white colours, upright posture, and some of their habits.

Although not to the extent of penguins, auks have largely sacrificed flight, and also mobility on land, in exchange for swimming ability; their wings are a compromise between the best possible design for diving and the bare minimum needed for flying.

This varies by subfamily, with the Uria guillemots (including the razorbill) and murrelets being the most efficient under the water, whereas the puffins and auklets are better adapted for flying and walking.

Traditionally, the auks were believed to be one of the earliest distinct charadriiform lineages due to their characteristic morphology, but genetic analyses have demonstrated that these peculiarities are the product of strong natural selection, instead; as opposed to, for example, plovers (a much older charadriiform lineage), auks radically changed from a wading shorebird to a diving seabird lifestyle.

Judging from genetic data, their closest living relatives appear to be the skuas, with these two lineages separating about 30 million years ago (Mya).

[10] The flightless subfamily Mancallinae, which was apparently restricted to the Pacific Coast of southern North America and became extinct in the Early Pleistocene, is sometimes included in the family Alcidae under some definitions.

[12] mtDNA cytochrome b sequences, and allozyme studies[6][7] confirm these findings except that the Synthliboramphus murrelets should be split into a distinct tribe, as they appear more closely related to the Alcini; in any case, assumption of a closer relationship between the former and the true guillemots was only weakly supported by earlier studies.

Their ability to spread further south is restricted as their prey hunting method, pursuit diving, becomes less efficient in warmer waters.

Hydotherikornis oregonus (Described by Miller in 1931), the oldest purported alcid from the Eocene of California, is actually a petrel (as reviewed by Chandler in 1990) and is reassigned to the tubenoses (Procellariiformes).

These sediments have been dated through Chandronian NALMA {North American Land Mammal Age}, at an estimate of 34.5 to 35.5 million years on the Eocene time scale for fossil-bearing sediments of the Clinchfield Formation, Gordon, Wilkinson County, Georgia.

Furthermore, the sediments containing this unabraded portion of a left humerus (43.7 mm long) are tropical or subtropical as evidenced by a wealth of warm-water shark teeth, palaeophied snake vertebrae, and turtles.