Morus bassanus

Northern gannets are the largest seabirds in the northern Atlantic. Males are slightly larger than females, although they are similar in plumage. Males are from 93 to 110 cm in length, females from 92.5 to 104 cm. Mass is from 2470 to 3610 grams, with females averaging heavier than males (female average: 3067, male average: 2932). Wing length is from 484 to 535 mm. Adults are white with black-tipped wings and a yellowish crown and nape. The bill is pale blue with black nasal grooves and a black, serrated mandible. The feet and legs are gray-black with a greenish line running down the front of the leg and onto the toes. The line is yellow-green in males and bluish-green in females. The feet have well-developed webbing. The skin of the face is blue-gray and the eye has a bright blue orbital ring with a pale blue-gray iris. The bill is stout and straight and the maxilla overhangs the mandible slightly. The tail is long and tapers to a point. Juveniles are gray, mixed with white feathers. There is a V-shaped white patch on the rump. The bill, legs, and feet are black and they have bluish grey eyes. With each successive molt more white plumage develops, starting with the lower body and belly and moving progressively upwards to include the head, neck, and breast. Molts are erratic and there is no discernible pattern. Young at fledging weigh substantially more than adults, sometimes more than 4 kg. This weight is lost within 7 to 10 days of fledging, at which point they acheive an adult weight. Development of adult plumage takes 3 to 4 years. In the northern part of their range northern gannets are unlikely to be confused with other seabirds, although they may be mistaken for shearwaters at a distance. In the southern part of their range they may be confused with masked boobies ( Sula dactylatra ) or other gannets ( Morus capensis ). There are no described subspecies of Morus bassanus . Basal metabolic rate of northern gannets at a breeding colony was estimated at 0.231 kJ/g/d ± 0.035 SE. Basal metabolic rates are considered high, relative to those of other seabirds, because of the high cost of flapping flight at sea and the high cost of thermoregulation in their cold water environment.

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