At least five subspecies recognized, although the subspecific taxonomy remains complex and poorly resolved:
U. a. aalge
U. a. albionis
U. a. hyperborea
U. a. inorata
U. a. californica

Also known as Common Guillemot or Thin-billed Murre.
 

Identification

The distribution of bridled individuals varies with latitude. As many as 50% of birds may be bridled in the extreme north. Field separation of the different races is poorly understood as is somewhat clinal in nature. Subspecies differ in terms of size, darkness of plumage, prominence of flank streaking. The southern race albionis (southern British Isles and adjacent continental europe) is lighter brown and has weaker flank streaks than more northern races.
 

Where and When

Photographs on the web

Beached adult in alternate plumage Whidbey Island, Washington State. Photo by Jim Nestler.

Immature at sea San Juan Islands, Washington. Photo by Jim Nestler.

Alternate-plumaged adults Photos by Gaylord Mink
 

Ventral view of an adult in alternate plumage. Photos by Arthur Morris and hosted by Don Crockett/The Virtual Birder.

Dorsal views of two adults in alternate plumage. Photos by Arthur Morris and hosted by Don Crockett/The Virtual Birder.
 

Literature

Gaston, A. J. and Jones, I. L. (1998) The Auks. Bird Families of the World 4. Oxford University Press.

Harris, M. P., D. J. Halley, & S. Wanless. (1996) Philopatry in the Common Guillemot Uria aalge. Bird Study 43: 134-137.