|Vocalizations at a colony on Cape Kidnappers, New Zealand (help·info)|
|Australasian gannet range|
Sula australis Gould, 1841
The Australasian gannet (Morus serrator), also known as Australian gannet and Tākapu, is a large seabird of the gannet family Sulidae. Adults are mostly white, with black flight feathers at the wingtips and lining the trailing edge of the wing. The central tail feathers are also black. The head is yellow, with a pale blue-grey bill edged in black, and blue-rimmed eyes. Young birds have mottled plumage in their first year, dark above and light below. The head is an intermediate mottled grey, with a dark bill. The birds gradually acquire more white in subsequent seasons until they reach maturity after five years.
Their breeding habitat is on islands and the coast of New Zealand, Victoria and Tasmania, with 87% of the adult population in New Zealand. They normally nest in large colonies on coastal islands. In New Zealand there are colonies of over 10,000 breeding pairs each at Three Kings Islands, Whakaari / White Island and Gannet Island. There is a large protected colony on the mainland at Cape Kidnappers (6,500 pairs). There are also mainland colonies at Muriwai and Farewell Spit, as well as numerous other island colonies. These birds are plunge divers and spectacular fishers, plunging into the ocean at high speed. They mainly eat squid and forage fish which school near the surface. It has the same colours and similar appearance to the northern gannet.
Sir Joseph Banks encountered the Australasian gannet in New Zealand waters, shooting three individuals on December 24th 1769 off Three Kings Islands. The birds were cooked in a goose pie, which was enjoyed by the sailors, for Christmas the next day. Daniel Solander wrote a formal description, noting its differences from the familiar northern gannet, initially giving it the name Pelecanus chrysocephalus before crossing it out and changing it to Pelecanus sectator. Parkinson illustrated the bird as P. sectator, which was misread as P. serrator by later authorities.
John Gould described specimens from the Derwent River and Actaeon Island in Tasmania as Sula australis in 1841. However, the binomial name Sula australis had been used by Stephens already for the red-footed booby. English zoologist George Robert Gray wrote of the species in 1843, initially using Gould's name but soon switching to Sula serrator, based on Parkinson's drawing. Although Gould stuck with S. australis, S. serrator became the preferred term over time.
"Australasian gannet" has been designated as the official common name for the species by the International Ornithologists' Union (IOC). It is also known as Australian gannet, diver (from its plunge-diving), booby, or solan goose, in Australia, and Pacific gannet, tākapu, takapu, tākupu or takupu in New Zealand.
The Sulidae, the gannets and boobies, appeared about 30 million years ago. Early Sulidae fossils most resembled the boobies, although they were more aquatic, with 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 splitting 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 the northern gannet (Sula bassanus).
An adult Australasian gannet is 84–91 cm (33–36 in) long, weighs 2.3 kg (5.1 lb), and has a 170–200 cm (67–79 in) wingspan. The two sexes are generally of a similar size and appearance, though a 2015 field study at Pope’s Eye and Point Danger colonies found females to be 3.1% and 7.3% heavier respectively. Females also had a slightly larger ulna and smaller bill. The plumage is white with black flight feathers on the wings, and central retrices of the tail. Some individuals have more extensive black plumage of their tail feathers. There is a sharp demarcation between light and dark plumage. The head and hindneck are tinged buff-yellow. The colour is more pronounced on the head and during breeding season. The eyes have a light grey iris surrounded by a pale blue eye ring, and bare black skin on the face which merges into the bill. In adults, the bill is pearly grey with dark grey or black edges, and a black groove running down the length of the upper mandible. The four-toed feet are dark grey, and joined by a membrane of similar colour. There are light green lines running along the ridges of the toes that continue along up the front of the legs.
This species is distinctive, and only likely to be confused with species that do not generally share its range. The Cape gannet is a rare vagrant to Australasian waters and has an all-black tail, while the masked and red-footed boobies are generally restricted to tropical waters. Although both have mostly white plumage, they lack the buff colouring of the head and have whilte tails. The masked booby has a blue-black face and less black on the wing, while the red-footed booby has red feet.
Distribution and habitat
The Australasian gannet is found from Steep Point in Western Australia, along the southern and eastern Australian coastline to the vicinity of Rockhampton in Queensland, as well as the North and South Islands of New Zealand, Lord Howe and Norfolk Islands. At sea, it is generally restricted to waters over the continental shelf, and may enter harbours, bays and estuaries, particularly in stormy weather.
Numbers of Australasian gannet have been increasing since 1950, although some colonies have disappeared and others have decreased in size.
Breeding colonies are mostly on offshore islands, though several mainland colonies exist in Australia and New Zealand.
Between 1980 and 2000, the population in Australian waters increased from approximately 6,600 to 20,000 breeding pairs.
In Victoria, there are colonies at Pope's Eye and Wedge Light in Port Phillip near Melbourne, and Lawrence Rocks near Portland. Located 5km northeast of Portsea, Pope's eye is a low artificial semicircular stone breakwater.
The colony on Lawrence Rocks increased from 200 pairs in 1873 to around 3100 pairs in 1996-97, by which time all available space on the island had been filled. Gannets began roosting at Point Danger—the closest point on the mainland itself—in 1995, and began nesting the following year after a fox-proof fence was erected around the site. The only nesting locale on mainland Australia itself, the Point Danger colony has increased steadily, reaching 660 pairs in 1999-2000.
Gannets began breeding on manmade structures in Port Phillip in 1966, with three pairs at Wedge Light. By the 1999-2000 season, there were 507 pairs there, and on seven other artificial structures around the bay.
The colony on Black Pyramid grew from 500 pairs in 1961 to 12,300 pairs in 1998.
Eddystone Rock increased from 20 pairs in 1947 to 189 pairs in 1998, and Pedra Branca grew from 1000 pairs in 1939 to 3300 pairs by 1995, but both these sites have little or no room for expansion.
Conversely, the colony at Cat island fell from an estimated 5-10,000 pairs in 1908 to neglible numbers by the turn of the millennium due to predation.
In New Zealand, almost all breeding colonies are on the North Island.
The Australasian gannet is generally solitary when out at sea, though once a bird has found fish to hunt, other gannets may notice and join it. It is gregarious on land, nesting in colonies. Young individuals return to the colonies when they are three years old, and begin breeding between four and seven years of age. Non-breeding gannets often form groups on the outskirts of the colony.
Gannet pairs may remain together over several seasons, until one member dies, though have been known to separate. They perform elaborate greeting rituals at the nest, stretching their bills and necks skywards and gently tapping bills together. The adults mainly stay close to colonies, whilst the younger birds disperse.
These birds are plunge divers and spectacular fishers, plunging from heights of up to 20 m (65 ft) into the ocean at high speed. They may dive from 1-2 metres above the serface at an angle in water less than 3 m (10 ft) deep or in rough weather. They mainly eat forage fish which school near the surface, as well as cephalopods.
The pilchard (Sardinops sagax) is a popular prey item; Australasian gannets switched to anchovy (Engraulis australis) at Farewell Spit, New Zealand in 1996 and barracouta (Thyrsites atun) in Port Phillip Bay in 1998 after pilchard mass mortality events. Other fish species reported eaten include kahawai (Arripis trutta), yellow-eye mullet (Aldrichetta forsteri), western Australian salmon (Arripis truttaceus), cape bonnetmouth (Emmelichthys nitidus), greenback horse mackerel (Trachurus declivis), yellowtail horse mackerel (Trachurus novaezelandiae), striped trumpeter (Latris lineata), New Zealand blueback sprat (Sprattus antipodum) and flyingfish of the genera Cheilopogon and Hirundichthys.
The Australasian gannet is highly territorial when nesting, engaging in agonistic displays to mark their ground against neighbours and interlopers.
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