Loon or Diver

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Appearance

Loons have a long, straight and sharp beak and a sturdy neck.  Their beautiful black and white plumage is thick to give these birds good insulation against the cold weather of arctic and subarctic waters.   In the breeding season, the upper side of most of the 5 species has a black and white chessboard-like apperance.  Only in the Red-throated Loon or Red-throated Diver are there grey-brown colorations:

The Red Throated Loon or  Red Throated Diver (Gavia stellata) is the species with the widest distribution.  This bird has a dark bay colouration on the throat of the adult during Summer.  Despite this ornament, it is the least conspicuous, as it is also the smallest, species of the genus, the back and upper plumage being of a blackish-brown with, a, few insignificant white spots, while the head and sides of the neck are ash-coloured, bounded by a long nuchal band, which lower down advances towards, the breast, of feathers marked with black, grey, and white, to form regular stripes. 

The next bird up in the order of size is the Arctic Loon or Black-throated Diver (Gavia arctica), which has a head of light gray colour, and a purple-black gular pouch.  Above this pouch the Arctic Loon has a white half-collar which has vertical black stripes,  while two patches on the black back, between the shoulders, as well as the scapulars, are conspicuously marked with large sub-quadrangular white spots.

The Pacific Loon or Pacific Diver (Gavia pacifica) is very similar in appearance with the Arctic Loon or Black-throated Diver (Gavia arctica), and it it thought by some ornitholgists to be the same species.  The distinguishing difference is that the Pacific Loon lacks the white flank patch which is present in the Arctic Loon.

Larger still is the Common Loon or Great Northern Diver (Gavia immer) which has a black glossy neck and head, 2 half-collars of white with black vertical stripes, and almost the entire black back and upper wing surface elegantly marked with white spots which vary in size and are arranged in belts.

Very close in appearance to the Common Loon is the Yellow-billed Loon or White Billed Diver (Gavia adamsii).  This bird can be best distiguished from the Common Loon by the Yellow Billed Loon's  ivory white or yellow bill, whereas the bill of the Common Loon is grey or an off-white colour. 

All five species exhibit a near-white plumage on their under-side, and a dark tail.  In the winter the remarkable splendor of the summer plumage is replaced by a simpler and more modest coloration.

Males are somewhat larger and heavier than the females, this being the only visible gender dimorphism.  The plumage of young Divers or Loons throughout the year is of a grayish colour and resembles the adult winter plumage.  Only at the age of approximately three years old do the young birds display the typical breeding plumage of the adults.
 

Next...The Loon or Diver in Flight, on the Water and Diving