Sula

Identificazione

An adult northern gannet has a 170–180 cm (67–71 in) wingspan, and is 87–100 cm (34+1⁄2–39+1⁄2 in) long and weighs 2.3–3.6 kg (5–8 lb), making it the largest gannet and the largest seabird native to the western Palearctic. The two sexes are generally of a similar size and appearance. The plumage is white with dark brown to black wing tips; the primary flight feathers, primary coverts and alulae are dark. The head and neck are tinged buff-yellow, becoming much more prominent in the breeding season. Males are more deeply coloured than females. The eyes are surrounded by black bare skin and have a cobalt blue orbital ring, the iris is light blue-grey with a fine dark outer ring. In birds that survive avian influenza, the irises become black.<p>The beak is long, strong and conical with a slight downcurve at the end and a sharp cutting edge. In adults, the beak is blue-grey with dark grey or black edges. There is a black groove running the length of the mandible that merges into the skin around the eyes. A black band of bare skin also separates the pale feathers of the forehead and throat from the bill, which gives the gannet its distinctive face markings. The four-toed feet are joined by a membrane that can vary in colour from dark grey to dark brown. There are coloured lines running along the toes that continue along up the legs. These are typically greenish-yellow in males and bluish in females and probably have a role in mating.Fledglings are dark grey to slate-grey with upperparts and wings finely speckled with white. There is a prominent V-shaped white area under the rump. The wing tips and tail are dark brown-black, partly tipped with white. The bill and iris are dark brown. They can weigh more than 4 kg (9 lb) by the time they leave the nest at about 10 weeks of age. In the second year, the bird's appearance changes depending on the different phases of moulting: they can have adult plumage at the front and continue to be brown at the rear. Gannets gradually acquire more white in subsequent seasons until they reach maturity after five years.<p>Northern gannets are slightly larger and thicker-billed than Cape or Australian gannets. The northern gannet has more white in the wings and an all-white tail, the other species having black tips to their tail feathers. Individuals on the west coast of Africa could be confused with vagrant masked boobies, though the latter is smaller overall, lacks the buff tinge to the head, and has a black tail. From a distance, or in poor visibility, albatrosses can be confused with northern gannets, particularly those with immature plumage that have more black on the wings.<p>Northern gannets have streamlined bodies adapted for plunge-diving at high speed, including powerful neck muscles, and a spongy bone plate at the base of the bill. The nostrils are inside the bill and can be closed to prevent water entry; the eyes are protected by strong nictitating membranes. There are subcutaneous air sacs in the lower body and along the sides. Other air sacs are located between the sternum and the pectoral muscles and between the ribs and the intercostal muscles. These sacs are connected to the lungs and are filled with air when the bird breathes in. The air can be returned to the lungs by muscle contractions.<p>The feathers are waterproof, which allows the birds to spend long periods in water. A water-impermeable secretion produced by a sebaceous gland covers the feathers and the birds spread it across their body using their beak or head. Individuals have a subcutaneous fat layer, dense down feathers and tightly overlapping feathers that help them withstand low temperatures. A reduced blood flow in the webbing on their feet outside of the breeding season also helps to maintain body temperature when the birds swim.<p>The northern gannet is a loud and vocal bird, particularly in the colony. Its typical call is a harsh arrah-arrah or urrah-urrah, which is emitted upon arriving or when challenging other gannets at the colony. The call is shortened to a rah rah when fishing or collecting nesting material, and lengthened to a ooo-ah when taking off. The calls of the sexes are similar. According to Nelson northern gannets can recognize the call of their breeding partner, their chicks and birds in neighbouring nests. Individuals from outside this sphere are treated with more aggression.

Tassonomia

The Swiss naturalist Conrad Gessner gave the northern gannet the name Anser bassanus or scoticus in the 16th century, and noted that the Scots called it a solendguse. The former name was also used by the English naturalist Francis Willughby in the 17th century; the species was known to him from a colony in the Firth of Forth and from a stray bird that was found near Coleshill, Warwickshire. It was one of the many species originally described by the Swedish zoologist Carl Linnaeus in the landmark 1758 10th edition of his Systema Naturae, where it was given the binomial name Pelecanus bassanus. The French biologist Brisson placed it in the genus Sula in 1760, and his compatriot Louis Pierre Vieillot moved the species to his new genus Morus in 1816. Morus is derived from Ancient Greek moros, meaning "foolish", and refers to the lack of fear shown by breeding gannets and boobies, which enables them to be easily killed. The specific name bassanus is from the Bass Rock in the Firth of Forth. The ornithologist Bryan Nelson in 1978 supported the species' inclusion in Sula as he felt the differences in anatomy, behaviour, ecology and morphology between gannets and boobies were not sufficient to warrant separate genera.<p>Charles Lucien Bonaparte described the American populations as Sula americana in 1838, though the basis for distinguishing them from the European species was unclear and the name is now considered to be a synonym.<p>"Northern gannet" has been designated as the official common name for the species by the International Ornithologists' Union (IOC). It is also known as the North Atlantic gannet. Gannet is derived from Old English ganot, meaning "strong or masculine", which is ultimately from the same Old Germanic root as gander. Soland goose and similar old names for the northern gannet such as solan or solan goose derive from a hypothetical Scottish Gaelic sulan, itself borrowed from the Old Norse sula. The literal meaning is "cleft stick", referring to the appearance of the conspicuous crossed black wing tips on a perched northern gannet. Old regional names such as Norfolk's "herring gant" or Yorkshire's "mackerel gant" refer to typical fish prey. Lincolnshire's gaunt, although derived from the same Germanic root, usually applies to the great crested grebe, but the English writer Richard Hakluyt used the term in 1600 to refer to the gannet, "a great White foule". Young birds have been called "spotted booby" or "parliament goose", the former term referring to their plumage. The feeding habits of the gannet have led to its name being used as slang for a gluttonous person, a usage first recorded in 1929.<p>The Sulidae, the gannets and boobies, appeared about 30 million years ago. Early Sulidae fossils resembled the boobies, although they were more aquatic, the gannets splitting off later, about 16 million years ago. The gannets evolved in the northern hemisphere, later colonising the southern oceans. The most ancient extant species may be the Abbott's booby, possibly the sole survivor of an otherwise extinct separate lineage. A 2011 genetic study of nuclear and mitochondrial DNA suggests that the ancestor of the gannets arose around 2.5 million years ago before splitting into northern and southern lineages. The latter then split into the Cape and Australasian gannets around 0.5 million years ago. The three gannets are generally considered to be separate species forming a superspecies, though they have also formerly been classified as subspecies of Sula bassanus.

Nome scientifico

Morus bassanus

Autorità

(Linnaeus, 1758)

Ordine

Suliformi

Famiglia

Sulidae

Genere

Morus

Areale e habitat

The northern gannet's breeding range is on both sides of the North Atlantic on coasts influenced by the Gulf Stream, There are colonies in the Gulf of Saint Lawrence and on the islands off the east coast of Canada. They normally nest in large colonies, on cliffs overlooking the ocean or on small rocky islands. The water needs to be cool enough for Atlantic mackerel and herring, which are the main food source for the northern gannet. These areas also overlie the continental shelf. Northern gannet colonies can be found in the far north in regions that are very cold and stormy, and Nelson has suggested that they can survive in these regions for several reasons, including the combination of body weight and a powerful beak that allows them to capture strong muscular fish, and the ability to dive to great depths and capture prey far from the cliffs. Their fat reserves act as weight when diving and as reserves during extended periods without food.<p>The northern limit of their breeding area depends on the presence of waters that are free of sea ice during the breeding season. Therefore, while Greenland and Svalbard offer suitable breeding sites, the Arctic regions have summers that are too short to allow the northern gannets to lay their eggs and raise a brood, which requires between 26 and 30 weeks. The southern limit of their distribution mainly depends on the presence of sufficient prey. There is fossil evidence of northern gannets breeding on Crete in the Pleistocene.<p>Some northern gannet breeding colonies have been recorded as being located in the same place for hundreds of years. The cliffs containing the colonies appear white when seen from a distance, due to the number of nesting birds present on them. There is a written record of a colony on the island of Lundy from 1274. There were only 70 nests by 1871, and the colony finally disappeared by 1909 at the latest. More than two-thirds of the world population breeds around the coasts of the British Isles. Colonies include:<p>After the breeding season, adult northern gannets disperse over a wide area although they travel no more than 800 to 1,600 km (450 to 850 nmi) from the breeding colony. It is not known if all birds from one colony migrate to the same over-wintering area. Many adults migrate to the west of the Mediterranean, passing over the Strait of Gibraltar and flying over land as little as possible. Other birds follow Africa's Atlantic coastline to arrive in the Gulf of Guinea. Immature northern gannets from colonies in Canada fly to the Gulf of Mexico, much further south than the adults.The immature gannets migrate southwards for great distances and have been recorded as far south as Ecuador. In their second year some birds return to the colony they were born in, where they arrive later than the mature birds. They then migrate south again at the end of the breeding season, but travel shorter distances in this second migration. Gannets from Alderney have been tracked since 2015 to gain better knowledge of their movements. One individual was found to have travelled from its colony in Alderney to Scandinavian waters, a round trip of around 2,700 km (1,500 nmi).<p>The species has been recorded as a vagrant in many central and eastern European countries as far south and west as the Black Sea, and also in Bermuda, Cuba, Cyprus, Egypt, Kazakhstan, Jan Mayen and Syria. In February 2016, one was recorded from Ceará in northeastern Brazil—the first sighting in the Southern Hemisphere.

Areale riproduttivo

AO : islands off subarctic, n temperate coasts of North Atlantic Ocean: Iceland, Faroe Is., British Isles, nw France and n Norway (e North Atlantic); Gulf of St. Lawrence and Atlantic coast of Newfoundland (se Canada, w North Atlantic)

Areale non riproduttivo

to coastal e North Atlantic off sw Europe, w Mediterranean, Macaronesia s through Canary Is. (w of nw Africa) and nw Africa; to coastal w North Atlantic to Florida and n Gulf of Mexico

Migrazione

Migratori completi

Comportamento

Comportamento migratorio: Migratori completi.

Conservazione

Tendenza della popolazione: in aumento.

Tendenza della popolazione

in aumento