Distinguishing Characteristics

Adult Common Loon, Lake Ontario, Canada, July.

Adult Common Loon; Michigan, July.

Adult Common Loon in basic plumage, Monterey Harbor, 30 November.

First-summer Common Loon, Moss Landing, CA, 24 July.

First-winter Common Loon, Barnegat Light, NJ, 11 February.

Juvenile Common Loon, Crosby, MN, 12 August.

Description

A large loon that varies greatly in size based on geography and sex: length 66–91 cm; mass 2.2–7.6 kg; on average male larger and up to 26.6% heavier than female (see Measurements, Appendix I). Sexes similar in plumage. Adult in Definitive Alternate (breeding) plumage: head, neck, back, wings, and sides black, conspicuously marked on scapulars and wing-coverts with relatively large, rectangular white markings; broad patch of vertical white stripes on side of neck; much smaller patch of stripes on upper foreneck; breast and belly white; bill black. Subadults and Basic (winter)–plumaged adults: upperparts gray to gray brown; underparts white; throat grayish white to white. Basic-plumaged adult always has white spots on wing-coverts (usually hidden when wings are folded) and plain grayish-black back; juvenile has broad, pale-fringed feathers on back and scapulars.

Identification

Distinctive in breeding (alternate plumage) when it is unlikely to be confused with any other species. Yellow-billed Loon (Gavia adamsii) shares similar plumage and structure, but has ivory to straw yellow bill (not black). Similar plumage pattern differs in many subtle ways, such as larger and fewer rows of white markings on scapulars, fewer and wider white lines on neck-patches, and differing patterns of gloss on head and neck (see Appearance).

In Basic plumage (including first-summer) Common Loon can be confused with all other loon species (especially Yellow-billed), but note larger size and heavy structure, and blocky headed appearance. All ages in winter show light area in front of and/or above eye (sometimes obscure), and heavily patterned neck with wedge of white extending back toward hindneck and gray half-collar extending forward at base of neck. In subadult and Basic plumage, Yellow-billed Loon most similar, but bill pale, especially at tip, and culmen dark only at base of bill; in Common Loon, culmen dark throughout length of bill, and bill-tip dark (but anterior portion of bill below culmen may be entirely pale during late winter; Jonsson 1992). Yellow-billed Loon also distinguished by overall paler-headed appearance typically with dark 'ear spot', and habit of carrying its bill up-tilted (see Binford and Remsen 1974, Kaufman 1990, North 1994). Arctic (Gavia arctica) and Pacific (G. pacifica) loons also similar in basic plumage, but smaller, with smaller bill and rounded head (especially Pacific), and even, vertical division between gray and white on neck sides; no light areas around eye. Red-throated Loon (G. stellata) also smaller, with more slender bill; neck lacks pale wedge or collar, and crown, hindneck, and back have finer markings. In flight, all species carry their heads slightly lower than their bodies with feet extend to rear, but Common and Yellow-billed loons are larger and more robustly proportioned than the others, with feet projecting farther from body. For additional distinguishing characteristics, see Kaufman 1990 and Jonsson 1992.