Auks are birds of the family Alcidae in the order Charadriiformes. They 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, but rather are an example of moderate convergent evolution.
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. 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).
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.
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.
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. This varies by subfamily, 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. This reflects the type of prey taken; murres hunt faster schooling fish, whereas auklets take slower moving krill. Time depth recorders on auks have shown that they can dive as deep as 100 m in the case of Uria guillemots, 40 m for the Cepphus guillemots and between 30 m for the auklets.
The earliest unequivocal fossils of auks are from the Miocene (e.g. the genus Miocepphus, 15 mya). Two very fragmentary fossils are often assigned to the Alcidae, although this may not be correct: Hydrotherikornis (Late Eocene, some 35 mya) and Petralca (Late Oligocene). Most extant genera are known to exist since the Late Miocene or Early Pliocene (c. 5 mya). Miocene fossils have been found in both California and Maryland, but the greater diversity of fossils and tribes in the Pacific leads most scientists to conclude that it was there they first evolved, and it is in the Miocene Pacific that the first fossils of extant genera are found. Early movement between the Pacific and the Atlantic probably happened to the south (since there was no northern opening to the Atlantic), later movements across the Arctic Sea. The flightless subfamily Mancallinae which was apparently restricted to the Pacific coast of southern North America became extinct in the Early Pleistocene.
The extant auks (subfamily Alcinae) are broken up into 2 main groups: the usually high-billed puffins (tribe Fraterculini) and auklets (tribe Aethiini), as opposed to the more slender-billed murres and true auks (tribe Alcini), and the murrelets and guillemots (tribes Brachyramphini and Cepphini). The tribal arrangement was originally based on analyses of morphology and ecology. mtDNA cytochrome b sequence and allozyme studies 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.
Compared to other families of seabirds, there are no genera with many species (such as the 47 Larus gulls). This is probably a product of the rather small geographic range of the family (the most limited of any seabird family), and the periods of glacial advance and retreat that have kept the populations on the move in a narrow band of subarctic ocean.
Today, as in the past, the auks are restricted to cooler northern waters. Their ability to spread further south is restricted as their prey hunting method, pursuit diving, becomes less efficient in warmer waters. The speed at which small fish (which along with krill are the auk's principal food items) can swim doubles as the temperature increases from 5°C to 15°C, with no corresponding increase in speed for the bird. The southernmost auks, in California and Mexico, can survive there because of cold upwellings. The current paucity of auks in the Atlantic (6 species), compared to the Pacific (19-20 species) is considered to be because of extinctions to the Atlantic auks; the fossil record shows there were many more species in the Atlantic during the Pliocene. Auks also tend to be restricted to continental shelf waters and breed on few oceanic islands.