Australasian gannet

Australasian gannet
 Vocalizations at a colony on Cape Kidnappers, New Zealand 
Scientific classification
Kingdom:Animalia
Phylum:Chordata
Class:Aves
Order:Suliformes
Family:Sulidae
Genus:Morus
Species: M. serrator
Binomial name
Morus serrator
Gray, 1843
Australasian gannet range
Synonyms[2]

Sula australis Gould, 1841
Sula serrator Gray, 1843

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.[3] 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.

Taxonomy

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.[4] The species name has been translated as "sawyer", from serra "saw", and linked to the serrated bill.[5]

John Gould described specimens from the Derwent River and Actaeon Island in Tasmania as Sula australis in 1841.[6] However, the binomial name Sula australis had been used by Stephens already for the red-footed booby.[4] 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.[7] Although Gould stuck with S. australis, S. serrator became the preferred term over time.[4]

"Australasian gannet" has been designated as the official common name for the species by the International Ornithologists' Union (IOC).[8] It is also known as Australian gannet, diver (from its plunge-diving), booby, or solan goose, in Australia,[5] and Pacific gannet, tākapu, takapu, tākupu or takupu in New Zealand.[9]

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.[10] 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.[11] 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).[12]

Description

adult in flight, showing black markings on wings and tail

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,[13] 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.[14] 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.[15] 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.[13]

Juveniles have spotted brown plumage

Fledglings are brownish-grey speckled with white overall.[15] They have dark brown bills, bare facial skin and eyes, and dark grey legs and feet.[13]

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.[15]

Call

The Australasian gannet is generally silent at sea and loud and vocal at the colony in the day and at times overnight during the breeding season. Its typical call is a harsh arrah-arrah or urrah-urrah, which is emitted upon approaching or arriving at the colony or as a threat. The calls can vary between individuals, and the female's call is lower pitched than the male's.[16]

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.[17] At sea, it is generally restricted to waters over the continental shelf,[15] and may enter harbours, bays and estuaries, particularly in stormy weather.[17]

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.[18] The most recent comprehensive New Zealand census was in 1981, yielding an estimate of 46,600 pairs,[19] estimated to have increased to around 55,000 pairs in 2006.[20]

Breeding colonies

Breeding colony at Muriwai, New Zealand

Breeding colonies are mostly on offshore islands, though several mainland colonies exist in Australia and New Zealand.[21]

In Victoria, there are colonies at Lawrence Rocks near Portland, and Pope's Eye and Wedge Light in Port Phillip near Melbourne.[22] 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.[18] Located 5 km northeast of Portsea, Pope's eye is a low artificial semicircular stone breakwater.[23] 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.[18]

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.[22] The colony on Black Pyramid grew from 500 pairs in 1961 to 12,300 pairs in 1998.[18] 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.[18] 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.[18]

In New Zealand, almost all breeding colonies are on the North Island.[24]

Gannet Island was named by Cook in January 1770 for the gannets seen there.[25]

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.[24]

Whakaari / 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.[24]

The colony at Cape Kidnappers is thought to have been settled around 1850, with 100 pairs reported in 1885.[25]

Muriwai, near Auckland, comprises a mainland colony of Okatamiro Point, estimated at 1385 pairs in 2016, while nearby Motutara (Pillar Rock) had 187 pairs.[24]

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.[26] 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.[27]

Gannets established a colony on Tikitiki Island (Nine Pin) in Northland Region in 2007, which had around 70 pairs by 2017.[24]

On South Island, gannets began breeding at the end of Farewell Spit in 1983, in an area known as Shellbanks—a 2 m (7 ft) high area of shells and driftwood interspersed with low vegetation: marram (Ammophila arenaria), sea rocket (Cakile edentula), velvety nightshade (Solanum chenopodioides) and sowthistle (Sonchus oleraceus). 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.[28] It grew by around 11% per year, reaching an estimated 3,900 pairs in 2011.[29]

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.[30]

Eggs and chicks can fall victim to kelp gulls and Pacific gulls.[17]

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 (to a lesser extent) 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.[31]

Behaviour

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.[32]

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 (1,607 mi) 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.[33]

Breeding

Gannet pairs may remain together over several seasons, until one member dies, though have been known to separate.[32] 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.

Feeding

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.[34] 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.[35]

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.[36][37] 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.[34]

Squid of the genus Nototodarus are among cephalopods eaten.[34]

Displays

The Australasian gannet is highly territorial when nesting, engaging in agonistic displays to mark their ground against neighbours and interlopers.[32] In the bowing display, the male's head and beak points down, and its wings are held up and away from the body, yet folded backwards. It moves his head from side to side before bowing forwards.[38] The male may escalate by lunging at an interloper, either with a closed or open bill, or proceed to locking bills and wrestling for an extended period. Fights can be fatal on occasion.[32] A bird may engage in an appeasement display to calm an attacker by lowering its head and tucking its beill in its chest.[38]

Mated pairs engage in a fencing display when the male arrives back at the next. The two birds stand breast to breast with wings spread and bills extended vertically. They fence and scissor with their bills rapidly, calling loudly at the same time. Fencing is interspersed with bill bowing. The birds generally follow this exchange with allopreening. Mated pairs also engage in sky-pointing, where a bird paces slowly with its neck and bill vertical and its wings partly raised.[38]

Conservation status

The International Union for the Conservation of Nature (IUCN) lists the Australasian gannet as a species of least concern, as the population is large and appears to be growing.[1]

Gannets have been enticed to established breeding colonies by decoys at reserves on Motuora Island,[39] as well as Young Nick's Head.

In culture

The Maori were reported to have harvested young gannets for food, visiting Gannet Island in March.[25]

References

  1. 1 2 BirdLife International (2012). "Morus serrator". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 26 November 2013.
  2. ↑ Australian Biological Resources Study (7 June 2014). "Species Morus serrator (G.R. Gray, 1843)". Australian Faunal Directory. Canberra, Australian Capital Territory: Department of the Environment, Water, Heritage and the Arts, Australian Government. Retrieved 22 June 2018.
  3. ↑ Maggy Wassilieff. Gannets and boobies – Gannets: description and habitat, Te Ara – the Encyclopedia of New Zealand. Wellington: Ministry for Culture and Heritage. Updated 13 July 2012.
  4. 1 2 3 Medway, David G. (1993). "The type specimen of the Australasian Gannet" (PDF). Notornis. 40: 65–70.
  5. 1 2 Gray, Jeannie; Fraser, Ian (2013). Australian Bird Names: A Complete Guide. Collingwood, Victoria: Csiro Publishing. p. 59. ISBN 978-0-643-10471-6.
  6. ↑ Gould, John (1841). "Proceedings of meeting of Zoological Society of London, Dec. 8, 1840". Proceedings of the Zoological Society of London. 8: 168–78 [177].
  7. ↑ Gray, George Robert (1843). Dieffenbach, Ernst, ed. Travels in New Zealand with contributions to the geography, geology, botany, and natural history of that country. London: J. Murray. p. 200.
  8. ↑ Gill, Frank; Donsker, David, eds. (2018). "Hamerkop, Shoebill, pelicans, boobies, cormorants". World Bird List Version 8.1. International Ornithologists' Union. Retrieved 25 June 2018.
  9. ↑ Ismar, S.M.H. "Australasian gannet". New Zealand Birds Online. Miskelly, C.M. Retrieved 26 June 2018.
  10. ↑ del Hoyo, Josep; Elliott, Andrew; Sargatal, Jordi; Christie, David A; de Juana, Eduardo, eds. (2013). "Family Suildae: Gannets and boobies". Handbook of the Birds of the World Alive. Barcelona: Lynx Edicions. Retrieved 19 May 2018. (Subscription required (help)).
  11. ↑ Patterson, S.A.; Morris-pocock, J.A.; Friesen, V.L (2011). "A multilocus phylogeny of the Sulidae (Aves: Pelecaniformes)". Molecular Phylogenetics and Evolution. 58 (2): 181–91. doi:10.1016/j.ympev.2010.11.021. PMID 21144905.
  12. ↑ Nelson 2010, p. 18.
  13. 1 2 3 Marchant & Higgins 1990, p. 761.
  14. ↑ Angel, Lauren P.; Wells, Melanie R.; Rodríguez-Malagón, Marlenne A.; Tew, Emma; Speakman, John R.; Arnould, John P. Y. (2015). "Sexual Size Dimorphism and Body Condition in the Australasian Gannet". PLOS One. doi:10.1371/journal.pone.0142653.
  15. 1 2 3 4 Marchant & Higgins 1990, p. 752.
  16. ↑ Marchant & Higgins 1990, p. 759.
  17. 1 2 3 Marchant & Higgins 1990, p. 753.
  18. 1 2 3 4 5 6 Bunce, A.; Norman, F.; Brothers, N.; Gales, R. (2002). "Long-term trends in the Australasian gannet (Morus serrator) population in Australia: the effect of climate change and commercial fisheries". Marine Biology. 141 (2): 263–69. doi:10.1007/s00227-002-0838-1.
  19. ↑ Wodzicki, K. A.; Robertson, C. J. R.; Thompson, H. R.; Alderton, C. J. T.. (1984). "The distribution and number of gannets (Sula serrator) in New Zealand" (PDF). Notornis. 31 (3): 232–61.
  20. ↑ Stephenson, Brent M. (2007). "Gannets and boobies". In K-J. Wilson. The State of New Zealand’s Birds 2006–Special Report New Zealand’s Seabirds (PDF). Wellington, New Zealand: The Ornithological Society of New Zealand, Inc. pp. 11–12.
  21. ↑ Marchant & Higgins 1990, p. 760.
  22. 1 2 Marchant & Higgins 1990, pp. 754–55.
  23. ↑ Parks Victoria. "Popes Eye". State Government of Victoria. Retrieved 16 July 2018.
  24. 1 2 3 4 5 Frost, Peter G.H. (2017). "Population status and trends of selected seabirds in northern New Zealand" (PDF).
  25. 1 2 3 Fleming, C. A.; Wodzicki, K. A. (1952). "A census of the Gannet ( Sula serrator ) in New Zealand" (PDF). Notornis. 5 (2): 39–78.
  26. ↑ Sawyer, Steve L.; Fogle, Sally R. (2013). "Establishment of a new breeding colony of Australasian gannets (Morus serrator) at Young Nick's Head Peninsula" (PDF). Notornis. 60: 180–82.
  27. ↑ Roy, Eleanor Ainge (2 February 2018). "Nigel the lonely gannet dies as he lived, surrounded by concrete birds". The Guardian. Retrieved 24 July 2018.
  28. ↑ Hawkins, J. M. (1988). "The Farewell Spit gannetry – a new sea level colony" (PDF). Notornis. 35: 249–60.
  29. ↑ Schuckard, Rob; Melville, David S.; Cook, Willie; Machovsky-Capuska, Gabriel E. (2012). "Diet of the Australasian gannet (Morus serrator) at Farewell Spit, New Zealand" (PDF). Notornis. 59 (1&2): 66–70.
  30. ↑ Cooper, Winston; Miskelly, Colin; Morrison, Kim; Peacock, Ron (1986). "Birds of the Solander Islands". Notornis. 33: 77–89.
  31. ↑ Marchant & Higgins 1990, p. 754.
  32. 1 2 3 4 Marchant & Higgins 1990, p. 757.
  33. ↑ Australian Bird & Bat Banding Scheme (ABBBS) (2018). "ABBBS Database Search: Morus serrator (Australasian gannet)". Bird and bat banding database. Australian Government Department of the Environment, Water, Heritage and the Arts. Retrieved 22 July 2018.
  34. 1 2 3 Marchant & Higgins 1990, p. 756.
  35. ↑ Machovsky-Capuska, Gabriel E.; Hauber, Mark E.; Dassis, Mariela; Libby, Eric; Wikelski, Martin C.; Schuckard, Rob; Melville, David S.; Cook, Willie; Houston, Michelle; Raubenheimer, David (2014). "Foraging behaviour and habitat use of chick-rearing Australasian Gannets in New Zealand". Journal of Ornithology. 155 (2): 379–87. doi:10.1007/s10336-013-1018-4.
  36. ↑ Bunce, A.; Norman, F. I. (2000). "Changes in the diet of the Australasian gannet (Morus serrator) in response to the 1998 mortality of pilchards (Sardinops sagax)". Marine and Freshwater Research. 51 (4): 349–53. doi:10.1071/MF99133.
  37. ↑ Schuckard, Rob; Melville, David S.; Cook, Willie; Machovsky-Capuska, Gabriel E. (2012). "Diet of the Australasian gannet (Morus serrator) at Farewell Spit, New Zealand" (PDF). Notornis. 59: 66–70.
  38. 1 2 3 Marchant & Higgins 1990, p. 758.
  39. ↑ Department of Conservation (2013). "Fake gannets entice real gannets to hatch chick". New Zealand Government. Retrieved 21 July 2018.

Cited texts

  • Marchant, S.; Higgins, P.G., eds. (1990). "Sula serrator Australasian Gannet" (PDF). Handbook of Australian, New Zealand & Antarctic Birds. Volume 1: Ratites to ducks; Part B, Australian pelican to ducks. Melbourne, Victoria: Oxford University Press. pp. 752–62. ISBN 978-0-19-553068-1.
  • Nelson, J. Bryan (2010) [1978]. The Gannet. London: A&C Black. ISBN 978-1-4081-3857-1.
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