'Atlantic' Auks
A somewhat misleading heading, because two of the species treated in this chapter do occur in the Pacific region as well, but the distribution of the species involved is mainly in the Atlantic. They all belong to the same evolutionary lineage. Auks are relatively small winged seabirds that breed on the ledges of rocky cliffs on many coasts of the northern Atlantic and Pacific. There are five species of 'Atlantic' auks. The extinct and flightless Great Auk also belonged to the Atlantic auks.
The skeletal anatomy of the Razorbill and the two species of Murres is very similar, except for the bills. The Razorbill has a vertically flattened, blunt bill with a vertical white line. Both Murres have pointed bills. The Common Murre has a longer bill and the Thick-billed has a shorter stubby bill with Subspecies of all species tend to be gradually larger to the north of their distribution, but differences are slight. In all species of this group the cranium shows a deep depression, the fossa glandula nasalis, above both eye cases for the well developed salt glands. The prominent supraorbital ridge which is developed with age in these species is unique for this group.
Genus Pinguinus
One species, now extinct.
- Great Auk Pinguinus impennis. North Atlantic
At the time of its discovery the Great Auk was locally numerous in the North Atlantic. It became extinct during the second half of the 19th century due to the excessive hunting of this vulnerable flightless bird. It was much larger than it closest relative the Razorbill: total length from 75 to 90 cm (Razorbill 41 to 64 cm). Its bill was also vertically flattened and showed about 6-7 white vertical grooves at the distal end of the upper mandible and 8-9 on the lower. 
Great Auk or Garefowl Pinguinus impennis Replica from the Collection of the California Academy of Sciences
Total: 153.3 mm, unsexed adult
Skeleton
Genus Alca
One species occurring only in the Atlantic. The two recognized subspecies differ in size, torda being the larger.
- Razorbill Alca torda, two subspecies. A third one is not generally recognized.
- A. t. torda, North America, Greenland, Norway and Baltic (Including A. t. pica)
- A. t. islandica, British Isles, Faeroes, Iceland and North Sea coasts.
More about subspecies and ageing
Razorbill Alca torda islandica. Ameland, The Netherlands
Culmen: 33.6 mm; Total: 90.7 mm; unsexed adult
Genus Uria
Two species:
- Common Guillemot or Common Murre Uria aalge. Two Pacific and three to five Atlantic subspecies:
- U. a. aalge, North American Atlantic coast, Iceland, northern Britain and Norway.
- U. a. californica, western US Coast from Washington southward.
- U. a. inornata, from Washington US northward along Pacific rim to Japan.
- U. a. spiloptera, Faeroe Is. (dubious subspecies, probably synonymous to aalge)
- U. a. albionis, England, Ireland, Brittany, Iberia and Heligoland.
- U. a. intermedius, Baltic (dubious subspecies)
- U. a. hyperborea, northern Norway, Bear I., Spitzbergen and Novaya Zemlya.
More about skull development and ageing
Common Guillemot or Common Murre Uria aalge aalge. Netherlands
Culmen: 51.0 mm; total: 115.7 mm; unsexed adult
- Brünnich’s Guillemot or Thick-billed Murre Uria lomvia. Four subspecies of which two dubious.
- U. l. lomvia, northern Atlantic coasts.
- U. l. arra, northern Pacific coasts.
- U. l. eleonorae, northern Siberia and eastern Taimyr Peninsula. (dubious subspecies).
- U. l. heckeri, Wrangel I. and Chukotsk Peninsula (dubious subspecies)
More about the development and ageing of the skull 
Brünnich's Guillemot Uria lomvia lomvia. Iceland
Culmen:36.3 mm; total: 99.4 mm; unsexed adult.
Genus Alle
The only small auk from the Atlantic. A straightforward black and white bird with a short stubby and slightly down-curved bill . It occurs in millions in the North Atlantic and Arctic Sea. It is recently recorded in the Bering and Beaufort Sea area and has bred in Alaska. It may have a circumpolar distribution in the high Arctic during summer. Unlike the other small alcids the skull of this species develops in the same pattern as the large auks with a gradually growing orbital ridge. More about thi
- Dovekie or Little Auk Alle alle. Two subspecies:
- A. a. alle, Greenland, Iceland, Novaya Zemlya, Spitzbergen and Jan Mayen.
- A. a. polaris, Franz Jozef Land.
Dovekie: skull development and ageing
Dovekie or Little Auk Alle alle alle. Texel, The Netherlands
Culmen: 14.4 mm; total: 53.6 mm, unsexed adult