Australasian gannet

Appearance

An adult Australasian gannet is 84–91 cm long, weighs 2.3 kg , and has a 170–200 cm 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.

Fledglings are brownish-grey speckled with white overall. They have dark brown bills, bare facial skin and eyes, and dark grey legs and feet.

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

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. Between 1980 and 2000, the population in Australian waters increased from approximately 6,600 to 20,000 breeding pairs. The most recent comprehensive New Zealand census was in 1981, yielding an estimate of 46,600 pairs, estimated to have increased to around 55,000 pairs in 2006.

Status

The International Union for the Conservation of Nature lists the Australasian gannet as a species of least concern, as the population is large and appears to be growing.

Gannets have been enticed to established breeding colonies by decoys at reserves on Motuora Island, as well as Young Nick's Head.

Behavior

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.

The maximum age recorded from banding has been 30 years 8.2 months; a bird tagged at Cape Kidnappers in January 1955 was found dead some 2,587 km away at Tangalooma in Moreton Bay, Queensland in September 1985. The longest distance travelled is 8128 km; a bird tagged at Lawrence Rocks was found washed ashore dead on the southeast coast of Mauritius.

Reproduction

Breeding colonies are mostly on offshore islands, though several mainland colonies exist in Australia and New Zealand.

In Victoria, there are colonies at Lawrence Rocks near Portland, and Pope's Eye and Wedge Light in Port Phillip near Melbourne. 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. Located 5 km northeast of Portsea, Pope's eye is a low artificial semicircular stone breakwater. 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.

In Tasmania, there are colonies at Eddystone Rock and Pedra Branca off the south coast, and in Bass Strait at Cat Island off Flinders Island, and Black Pyramid Rock off the northwest coast. 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 negligible numbers by the turn of the millennium due to predation.

In New Zealand, almost all breeding colonies are on the North Island.

Gannet Island was named by Cook in January 1770 for the gannets seen there.

Three Kings Islands contained New Zealand's largest offshore gannet colonies, with 9855 pairs across five smaller colonies in 1981. However, an aerial survey in 2014–15 found that it had shrunk to 6402 pairs.

White Island, another offshore site comprising five smaller colonies, also saw a reduction in numbers, from 6662 pairs in 1980–81 to 5306 pairs in 2014–15.

The colony at Cape Kidnappers is thought to have been settled around 1850, with 100 pairs reported in 1885.

Muriwai, near Auckland, comprises a mainland colony of Okatamiro Point, estimated at 1385 pairs in 2016, while nearby Motutara had 187 pairs.

Gannets established a mainland colony on Young Nick's Head near Gisborne, after decoys of nesting birds and pre-recorded calls were broadcast to passing gannets in September 2008. Successful breeding was recorded at the site from the 2010–11 breeding season onwards. A similar effort to establish a colony on Mana Island led to the arrival of a single gannet, dubbed Nigel "no mates", who lived alone among the 80 decoys for several years until he was found dead in February 2018. However, in summer 2018, three more gannets arrived at the site.

Gannets established a colony on Tikitiki Island in Northland Region in 2007, which had around 70 pairs by 2017.

Gannets began breeding at the end of Farewell Spit in 1983, in an area known as Shellbanks—a 2 m high area of shells and driftwood interspersed with low vegetation: marram , sea rocket , velvety nightshade and sowthistle . Strong winds allow gannets to take off vertically most of the time, and the decline in commercial fishing in nearby Golden and Tasman Bays is thought to have increased food supply. The breeding area is also cut off from the mainland by high tides, but can be badly impacted by storms. It grew by around 11% per year, reaching an estimated 3,900 pairs in 2011.

Little Solander Island in Foveaux Strait hosts the southernmost gannet colony; around 20 pairs recorded on most visits between 1948 and 1986, with one count of 62 pairs in 1984 possibly anomalous.

Eggs and chicks can fall victim to kelp gulls and Pacific gulls.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.

Food

These birds are plunge divers and spectacular fishers, plunging from heights of up to 20 m 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 deep or in rough weather. They mainly eat forage fish which school near the surface, as well as cephalopods. Some local differences have been recorded: Australasian gannets at Farewell Spit mainly forage on coastal fish in water depths of less than 50 m, while those at Cape Kidnappers hunt more oceanic fish at water depths of over 50 m.

The pilchard is a popular prey item; Australasian gannets switched to anchovy at Farewell Spit, New Zealand in 1996 and barracouta in Port Phillip Bay in 1998 after pilchard mass mortality events. Other fish species reported eaten include kahawai , yellow-eye mullet , western Australian salmon , cape bonnetmouth , greenback horse mackerel , yellowtail horse mackerel , striped trumpeter , New Zealand blueback sprat and flyingfish of the genera ''Cheilopogon'' and ''Hirundichthys''.

Squid of the genus ''Nototodarus'' are among cephalopods eaten.

Migration

Over May and June, young Australasian gannets from New Zealand colonies disperse to the north and west, mainly flying north around the North Island and via the Cook Strait. They generally reach as far as southeastern Queensland and Rottnest Island in Western Australia. Far-wandering gannets are occasional visitors to Marion Island and the Crozet Islands. Some immature gannets spend 3 to 4 years in Australian waters before returning to New Zealand, while others remain in New Zealand waters.

Cultural

The Maori were reported to have harvested young gannets for food, visiting Gannet Island in March.