Distinguishing Characteristics

Largest indigenous seabird in North Atlantic; male slightly larger than female. Total mean length: male 100.1 cm (range 93.5–110.0, n = 66), female 99.2 cm (range 92.5–104.0, n = 66); wing length of male 513.0 mm (range 500.0–535.0, n = 9), female 510.2 mm (range 484.0–522.4, n = 14); body mass of male 2,932 g (range 2,470–3,470, n = 27), female 3,067 g (range 2,570–3,610, n = 27). Adult plumage white with black-tipped wings; juveniles and immatures with a succession of mainly black-brown or brown-and-white plumages for first 3–4 yr; sexes alike, no marked seasonal variation (Palmer 1976, Cramp and Simmons 1977, Nelson 1978a, Harrison 1985).

Adult plumage largely white except for yellowish-buff tinge to crown and nape (most intense in breeding males during spring) and contrasting black primaries (which show as broad black wing-tips), black primary-coverts, and alulae, and black-and-white lesser primary-coverts. Bill stout, pale blue, tending to gray-blue; mandibles serrated, with black nasal grooves on each side of upper mandible; tip of upper mandible downcurved and slightly overhanging lower mandible. Bare skin of face and gular-stripe dark blue-gray, orbital ring of eye cobalt blue, and iris clear, pale blue-gray with a fine, dark outer ring. Legs and fully webbed feet gray-black, with a conspicuous yellowish-green line in males, and bluish green in females, extending from the front of the tarsus down mid-ridge of toes (Palmer 1976, Cramp and Simmons 1977, Nelson 1978a).

Juvenile plumage unlike adult plumage (see Appearance: molts and plumages, below): gray-black, inconspicuously speckled white (speckling largely absent from wing and tail), with a prominent V-shaped white patch on lower rump; bill black, becoming gray; feet and legs black, line on toes and tarsus faintly discernible; eye dull bluish gray. Immatures take 3–4 yr to assume full adult plumage; erratic nature of molt results in a complicated succession of plumages; relatively uniform dark plumage of juvenile increasingly invaded by white, starting on lower body, then head, neck, and breast, and wing-coverts, and last on secondaries and tail; much variation within and between age groups; many fourth-year birds like adults; yellowish-buff tinge of head and nape, bill, face, and eyes as adult by late second and third year (Cramp and Simmons 1977, Nelson 1978a).

In North Atlantic, at close range, adult unlikely to be confused with any other species; from longer distances or in poor light, identification (especially immatures) more difficult, and may be mistaken for one of larger shearwaters (Puffinus spp.). Close attention to structural features of Northern Gannet essential for identification in these situations; note its pointed, broad-based bill; heavy head; long, oval body; and long, pointed tail (giving it an appearance of tapering at both ends), along with its long, pointed, slightly angular wings set across middle of its body (Palmer 1976, Cramp and Simmons 1977).

In southern part of its range may be confused with other Sulidae. In Gulf of Mexico, subadult Northern Gannets with ragged dark secondaries and some dark feathers in tail, may be confused with Masked Booby (Sula dactylatra). Masked Booby smaller, has all-white head, all-black secondary- and tail-feathers, and characteristic “mask” of blue-black facial skin. Immature Northern Gannet and subadult Masked Booby distinguished by yellowish color on head of Northern Gannet that is generally assumed by second year. Immature Northern Gannet may be confused with smaller Brown Booby (Sula leucogaster), but differs in having speckled appearance and, by second year, a mainly white head, and a pronounced white V over rump. Off equatorial w. Africa, where sympatric with Cape Gannet (Morus capensis), adults easily separated by Northern Gannet’s all-white secondaries and tail as well as by more subtle features such as more extensive facial skin and longer gular-stripe that extends posteriorly well beyond posterior end of gape-stripe (Marchant and Higgins 1990, Brinkley et al. 2001); immatures more problematic (see Nelson 1978b), but may be separated by white feathers in secondaries and white outer tail-feathers of Northern Gannet (Cramp and Simmons 1977, Harrison 1985). Australasian Gannet (M. serrator), not known from North Atlantic, similar, but differs in typically having central 2 pairs of rectrices black, entirely black secondaries contrasting with white humerals, and rich, golden-buff head (somewhat faded in nonbreeding birds); potentially confusing rare plumage of some subadult Northern Gannets discussed by Brinkley et al. (2001).