Gannets: Characteristics, Behavior, Nesting and Diving | Sea Life, Islands and Oceania

GANNETS

Northern gannets

Gannets are sea birds famous for their extraordinary diving ability. Feeding alone or in flocks, the dive like missiles from great heights to capture fish and squid. Sometimes gannets will pursue their prey underwater, moving with half open wings and powerful feet.

Gannets and boobies are grouped together in Sulidae bird family comprised of one genus (Sula) and 10 species. Sulids are medium to large birds (.72 to 3.6 kilograms, ; 64-100 centimeters). Gannets are distributed throughout tropical and subtropical oceans. Some sulids are pelagic (open ocean) while others are inshore marine birds. Breeding areas include offshore islands and continental coastlines. Sulids may live for 10-20 or more years. [Source: Laura Howard, Animal Diversity Web (ADW) /=]

In some tropical areas gannets are sometimes hunted for eggs, meat, and feathers. At the turn of the century, the Northern Gannet colony in the Gulf of St. Lawrence, which had over 100,000 pairs, was nearly eradicated by human culling. The colony was reduced to 500 pairs by 1932. Fortunately, by 1984 the colony had recovered to 6,700 pairs. One gannet species is including in the International Union for Conservation of Nature (IUCN) Red List. The Cape Gannet (Sula capensis) is listed as 'Vulnerable'. The main threats include: habitat destruction; human collection of eggs, chicks, and adults; entanglement in fishing gear and overfishing of prey fish by commercial industries. /=\

New Zealand’s gannet population has increased dramatically. One of the theories advanced for why this has occurred is that coastal commercial fishing has reduced the number of large predatory fish, resulting in an increase of the kind small fish the gannets feed on. This abundance of food allows adult gannets to catch more food for their chicks, enabling more of the chicks to survive. Predators tend to go after eggs, chicks and fledglings rather than adults. These include owls, goshawks gulls, sea eagles, frigatebirds, rats, cats, wild pigs, land crabs, sharks, and humans

Websites and Resources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Encyclopedia of Life eol.org; Smithsonian Oceans Portal ocean.si.edu/ocean-life-ecosystems ; MarineBio marinebio.org/oceans/creatures

Gannet History

The evolutionary relationships of gannets and sulids remain unclear. Traditionally sulids have been considered related to other totipalmate birds (tropicbirds, frigatebirds, anhingids, cormorants and shags, pelicans) and taken together form Pelicaniformes. However, a hierarchy based on DNA hybridization includes sulids within a diverse group,Ciconiiformes (stork-like birds such as stroks, herons, ibises and spoonbills). [Source: Laura Howard, Animal Diversity Web (ADW) /=]

Australasian gannet

The earliest fossil sulid (Sula ronzoni), from France, has been dated to the early Oligocene Period (33 million to 23.9 million years ago). The large gannet (Morus magnus) from California dates from the Late Miocene Period (11.6 million to 5.3 million years ago). Other fossil sulids are dated from throughout the Tertiary Period (66 million to 2.6 million years ago), /=\

Jeremy Berlin wrote in National Geographic: Human-gannet relationship stretches back centuries. Beowulf’s liege lord, Hrothgar, called the ocean a “gannet’s bath.” John Daniels, who photographed the Wright brothers’ first flight, said Orville and Wilbur “would watch the gannets and imitate the movements of their wings with their arms and hands.” And melted gannet fat was once used for everything from gout balm to wagon-wheel grease. [Source:Jeremy Berlin, National Geographic, August 2012]

Gannet Species

There are three gannet species worldwide, all with adults similar in appearance. The northern gannet (Morus bassanus) is a North Atlantic bird; the African Cape gannet (Morus capensis), which mainly hangs out around southern Africa. It is recognised by the long black stripe on its throat and a completely black tail. The Australasian gannet (Morus serrator) is found in Australian and New Zealand waters. [Source: Te Ara - the Encyclopedia of New Zealand]

Australasian gannets have golden yellow heads and two-meter wingspan, Adults are about the size of a goose, with black-tipped wings, black central tail feathers and a strong, conical blue-grey beak. Juvenile birds look quite different. In their first year they have speckled brown feathers on their upper body, and white undersides. Each year more white feathers appear on their backs, and the birds acquire their adult appearance by five years of age.

Most adult Australasian gannets do not migrate. The venture once to Australia and after they come back to New Zealand they spend spring and summer at the breeding colony and then disperse around local coastal waters for the winter months. Fresh arrivals from Australia often have a hard time claiming a nest site or a mate in their first couple of years. Once they do find a site and mate they are faithful to both for the rest of their life which may be for 30 years

Gannet Characteristics

Gannets are huge birds. They have 2.5 meter (eight foot) wingspans. Flight feathers are molted almost continuously. 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 centimeters (3 to 3.6 feet) in length, females from 92.5 to 104 centimeters (3 to 3.4 feet). They weigh 2.5 to 3.6 kilograms (5.5 to 8 pounds), with females averaging heavier than males (female averaging three kilograms and males averaging 2.9 kilograms. Wing length is from 48 to 56 centimeters (1.6 yo 1.8 feet). [Source: Tanya Dewey, Animal Diversity Web (ADW) /=]

gannets, terns and frigatebirds in a colony together
According to Animal Diversity Web: Plumage in most species is white on the back with brown-black present on primaries and/or dorsal or head feathers. Juveniles are generally darker than adults. Wings are long and set far back on the body. The stout bill is conical, with serrated edges in some species. They have a desmognathous palate (distinct form of a palate, found in birds in which the maxillopalatine bones are fused) and obsolete nostrils (no exposed external nares). A nasal groove runs along bill. Adults have bare facial and gular skin, which along with the feet, may be black, red or blue in color. The eyes are situated beside the bill giving the birds binocular vision. The irises are often pale in color. Gannets lack brood patches. Legs are short and set far back on the body. Feet are totipalmate (webbed between all four toes) with middle toenail pectinate (narrow projections or divisions set closely in a row). Males and females are roughly the same size. [Source: Laura Howard, Animal Diversity Web (ADW) /=]

Cape gannets are large seabirds, 85 to 94 centimeters (2.8 to 3 feet) in length and weigh about 2.6 kilograms. Like other gannets and boobies, they have a characteristic sleek, but robust body, with strong, webbed feet and a long, robust bill. They have white plumage on most of the body, with yellow on the head, chin, and neck and black primary and secondary wing feathers and tail. About 10 percent of individuals have white feathers in the tail as well. They have a dark gular stripe on their throat, which is longer than those found in other Morus species. Their legs, feet, and webbing are black and their bills are pale yellow with black markings and black skin around the eyes. Juvenile Cape gannets have uniformly brown plumage, gradually becoming white as they mature. They can be confused with masked boobies , which have white heads, northern gannets (, which have white tails and secondary feathers, and Australasian gannets , which have only the central tail feathers black. [Source: Tanya Dewey, Animal Diversity Web (ADW) /=]

Northern gannet 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.

Gannet Behavior

Gannets are considered highly social. Most species breed in dense colonies. Some species forage cooperatively, flying in small groups and diving in unison to capture prey items. Gannets are migratory (make seasonal movements between regions, such as between breeding and wintering grounds). They spend large amounts of time at sea foraging.

Gannets are known for being doting parents but hostile neighbors Complex displays are commonly exhibited in pair bonding and nest-site establishment and defense. Highly colonial , species exhibit some of the most ritualized types of behaviors. Gannet displays are elaborate and stereotyped, including: headshakes, bill menacing, sky pointing, and greeting ceremonies. Gannets are loud and raucous and the sexes' vocalizations are nearly identical. Adults may utter harsh screeches while fishing, rasping when arriving at the nest, or sighs that accompany sky pointing. [Source: Laura Howard, Animal Diversity Web (ADW)]

Australasian gannet in Tasmania

Despite being elegant fliers and superb divers gannets look clumsy walking around on land and can make terrible-looking crash landing. They spend a lot of time on feather care, distributing oil from the preen gland which prevents plumage from becoming waterlogged. Heat-regulation entails several interesting behaviors including exposing the webs on the feet or excreting on them for cooling. Gular fluttering and wing extensions also help to dissipate heat.

Gannet Diving and Feeding

Common gannet prey items include squid and fish such as mackerel, pilchard, anchovies and sandeels. Gannets often regurgitate their stomach contents if disturbed or handled; if the bird has recently fed. Prey can be identified from such events in New Zealand includes small fish such as pilchards, anchovies, saury, jack mackerel and squid.

Gannets are capable of diving at 145 kilometers per hour (90 miles per hour) and can pursue prey to depths of up to 13 meters (40 feet). Describing the spectacular gannet plunge diving. Jeremy Berlin wrote in National Geographic: Fifty feet above the storm-tossed North Sea, a thunderhead of birds has been massing. When the cloudburst comes, it’s quick as lightning. They plunge, a score of white tridents, spearing the waves with a thump and a splash. Moments later they bob to the surface, fish in throat. They shake their heads, rise from the water on six-foot wings, and soar to cliffside homes with a swan’s grace. There they land badly and bicker loudly...Watching these birds plummet headlong to wrest life from the frigid depths, one thinks of Tennyson’s eagle, of crawling seas and falling thunderbolts. One also sees why fishermen have long relied on them as location scouts. [Source: Jeremy Berlin, National Geographic, August 2012]

Gannets fly higher than most seabirds and use their binocular vision to locate prey. When prey is located, they stall from 10-100 meters above the water surface, dive with wings pushed backwards, and penetrate the water as deep as ten meters from the force of the dive alone. Once underwater the birds may use their wings in a swim pursuit of prey to depths of 15-25 meters. Usually prey is captured and ingested as the bird returns to the surface. Some species also surface-dive or forage on foot in shallow water.[Source: Laura Howard, Animal Diversity Web (ADW)]

When searching for food Australasian gannet gannets generally fly parallel to the coast, between one and 20 meters (3 to 66 feet) above the sea, looking for schools of fish or squid. When they spot something, they plunge headlong into the water. Just before they break the surface, they fold in their wings to swoop down beneath their food. They can enter the water at such high speeds because because they have inflatable air sacs around the neck and chest to absorb the shock of impact. Without these they would break their necks. They grab the prey in their beaks and swallow it whole once they have surfaced.

Gannet Reproduction and Nesting

Gannets are least seasonally monogamous. They may mate with one partner and have nest-site fidelity for many successive years. Most gannets breed in colonies usually nest offshore. Nests often located on cliffs, or may be on slopes or on the ground. Ground nests are shallow depressions in accumulations of guano. Gannets breed in mixed colonies with other gannets or sometimes with frigatebirds or cormorants. [Source: Laura Howard, Animal Diversity Web (ADW)]

Breeding can be seasonal (annually or biennially) or varied depending on species and local conditions. Gannets are fiercely territorial nesters. They fortify their nests with seaweed and bird droppings and squawk and scream loudly whenever other birds approach too close. Gannets lay one egg, which is generally white or pale blue. Eggs are incubated for 41-57 days. Both sexes take turns incubating with incubation durations lasting from 12-60 hours. Because gannets lack brood patches, the webs of the feet are used to transfer heat to eggs and chicks. Parents take turns feeding and brooding young continuously for about a month, and thereafter more sporadically. During the post-fledging period adults will feed the flying chicks, for up to nine months in some species.

Jeremy Berlin wrote in National Geographic: A gannet colony is “a riot of squawking, flapping, and jabbing. Landing gannets raise the ire of their neighbors, a common sight in a hectic colony. But beneath the territorial clamoring is a ritualized order. Nests are arranged in a canny geometry of two per ten square feet — just out of jabbing range. The choicest nest sites are in the center, as Once acquired, they’re defended with life and saw-toothed bill. Single birds lurk on the fringes, seeking a partner and a nest of their own. [Source: Jeremy Berlin, National Geographic, August 2012]

To get a site, two males will fight, locking bills and stabbing faces, for up to an hour. When the clash ends, one gannet leaves; the other has a home. “The bird is faithful to the site once it occupies it,” says Stuart Murray, a gruff Scotsman who’s been surveying Britain’s seabirds for four decades. “They attract a mate, she lays an egg, and then they think, Bingo! I’ve done it!”

Gannet Fledglings

Gannet chicks are altricial (born relatively helpless and in need of significant parental care) and remain at nest for about 14-22 weeks. Post-fledging care may last from five to nine months. Gannet fledglings stand on the edge of the cliffs, flapping their wings and strengthening their muscles before they try to fly. [Source: Laura Howard, Animal Diversity Web (ADW)]

Jeremy Berlin wrote in National Geographic: Each season equals one egg, plain and white like a goose’s. Parents take turns incubating it and, after six weeks, feeding what emerges — a shriveled thing, naked and ebony. Over three months it will become a downy, white powder puff, then a slate-plumed juvenile. Two meals a day swell it swiftly; calisthenic wing flapping tones it crucially. When a chick is ready to leave the nest, it splashes into the sea. “At first it just bobs on the waves, bewildered,” says Murray. “But hunger pangs drive it to swim and dive. Then it will learn what to do by watching other gannets.” Growing up will be hard and perilous. Less than half will see a third birthday. [Source: Jeremy Berlin, National Geographic, August 2012]

Australasian gannet chicks are born blind but grow rapidly. In the first week they develop white fluffy down, which is replaced during their second and third months by juvenile plumage. By 14 weeks the chicks weigh 3 kilograms, half a kilogram more than the parents, and they begin flapping and stretching their wings for hours each day. Then suddenly in their fourth month of life they take off, not to return for several years. Their destination was a mystery until the 1950s, when juvenile birds were first ringed and tracked: they turned up in coastal waters around the east coast of Australia. [Source: Te Ara - the Encyclopedia of New Zealand]

In New Zealand the chicks are usually born in October and fed regurgitated squid and mullet by their parents over the next six months. In late summer and early autumn, without any previous flight practice, without having had to seek out their own food, and without any accompanying adults, the fat young gannets fly off and cross the 2,740-kilometer (1,700-mile) -wide Tasman Sea to Australia. The journey takes 8 to 14 days, depending on the weather and wind direction. After two or three years the survivors return to their hatching site in New Zealand in early spring to give birth to their own young and spend the rest of their life in New Zealand. The two-way journey is filled with with dangers, including storms and predators. Many die. In a good year, about a quarter of the birds manage to return safely to New Zealand. Adult plumage and sexual maturity occur at about two to six years.

Image Sources: Wikimedia Commons, NOAA

Text Sources: Animal Diversity Web (ADW) animaldiversity.org; National Oceanic and Atmospheric Administration (NOAA) noaa.gov; Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated May 2023