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Northern Gannet
Morus bassanus
– Family
Authors: Mowbray, Thomas B.

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Adult Northern Gannets, with chick
Figure 1. Distribution of the Northern Gannet in North America.

This heavyweight among the plunge-divers of the world breeds in often huge, always dense, and extremely raucous colonies on precipitous mainland cliffs, islands, and stacks. As a breeding bird, the Northern Gannet is confined to the continental-shelf waters on both sides of the North Atlantic. In the eastern North Atlantic, it is distributed in 32 colonies from the coast of Brittany in France north to Norway, with its main concentration north and west of Scotland (Nelson 2002). In North America, it is restricted to just 6 well-established colonies: 3 in the Gulf of St. Lawrence, Québec, and 3 in the North Atlantic off the coast of Newfoundland.

This species is monomorphic, with sexes similar in size and appearance. Pairs usually bond monogamously for life, and both mates participate in all aspects of parental care, though sharing of the activities varies over the course of the nesting cycle. The species has a rich repertoire of picturesque sexual and aggressive behaviors for maintaining pair bonds and living in close contact with its neighbors. The female lays a single egg, which is incubated under the highly vascularized webs of the feet (gannets do not have a brood patch). Hatchlings are altricial, with essentially no motor coordination or thermoregulatory capacity, but they develop rapidly during the 13-week nestling period, attaining peak weights at 8–9 weeks of age of up to 50% more than adults. At 13 weeks, chicks depart from nest sites by jumping from their nesting ledges and flying/gliding 400–500 meters from the colony into the water. Unable to rise from the water because of their excessive weight and still without fully developed wing muscles, they begin a southward migration to middle Atlantic regions. Mortality during the first year is high, but those birds that survive have a strong tendency to return during their second or third year to the same colony in which they were hatched.

Like other seabirds, the Northern Gannet takes several years to attain adult plumage, going through various stages of mottled dark-and-white plumages before becoming pure white with black wing-tips at 4–5 years of age. At maturity, the male begins the arduous process of acquiring a breeding site in the colony and attracting a mate. Once acquired, it is likely to be the pair’s nest site for the rest of their lives, where they will raise 1 chick per year. Reproductive success (that is, eggs producing fledging young) at colonies is generally high (>75%), and most colonies today are increasing in population size at a rate of about 3.0–3.5% per year.

The Northern Gannet is a voracious forager, feeding on shoaling fish by plunge-diving. As an opportunistic feeder, it utilizes prey species as they become available through the season, but prefers larger (more than 30.0 cm), more energy-rich (7.0 kJ/g) species over smaller, lower-energy species, and often forages up to several hundred kilometers from its nesting colony to locate such prey. The average annual harvest of squid and mackerel (Scomber scombrus) in Newfoundland by Northern Gannets often exceeds that of the total commercial landings (Montevecchi et al. 1988). Foraging is not without its risks, and mortality of adult gannets is fairly high as a result of fishing accidents, entanglement in nets, and, albeit less today than in the past, persecution by fishermen.

The Northern Gannet has been studied extensively, in both the Old and New Worlds. Excellent intensive, long-term studies of breeding colonies, such as that at Bass Rock, eastern Scotland (Nelson 1978a, 2002), have given us a unique synthesis of the overall biology of this species. In North America, studies at breeding colonies in the Gulf of St. Lawrence and the North Atlantic have contributed to our understanding of this species’ food habits (Kirkham et al. 1985, Montevecchi et al. 1988, Montevecchi et al. 2002), foraging strategies (Montevecchi et al. 1988, Garthe et al. 2000), growth and acquisition of thermoregulation (Kirkham and Montevecchi 1982, Montevecchi et al. 1984, Birt-Friesen et al. 1989, Montevecchi and Vaughan 1989), parental behavior (Montevecchi and Porter 1980), and population regulation (G. Chapdelaine and J.-F. Rail unpubl.). Continuous censusing of colonies (Nettleship 1976, Nettleship and Chapdelaine 1988, Chardine 2000, G. Chapdelaine unpubl.) has allowed us to carefully monitor the population dynamics of the species.