Australasian gannet swimming (New Zealand)
The Australasian gannet (Morus serrator or Sula bassana), also known as Australian gannet and Tākapu, is a large seabird in the family Sulidae. With its 1.8 m wing-span, the Australasian gannet is a conspicuous, predominantly white seabird that is common in New Zealand coastal waters. They can be observed feeding solitarily or in large congregations, especially near the larger colonies. The three gannet species are closely related and are now usually placed in one genus Morus, with subspecies M. serrator for the Australasian gannet, M. bassanus for the northern gannet, and M. capensis for the Cape gannet. Gannets can dive from a height of 30m (98 ft), achieving speeds of 100 km per hour (62 mph) as they strike the water, enabling them to catch fish much deeper than most airborne birds. The gannet's supposed capacity for eating large quantities of fish has led to "gannet" becoming a disapproving description of somebody who eats excessively.
The Australasian gannet is a large, mostly coastal seabird with predominantly white plumage, long, pointed wings, a long neck and slender body shape. The trailing edges of its wings, the flight feathers at the wingtips and a varying proportion of its central tail feathers are black. The wedge-shaped bill is bluish-grey with a lining of black and the skin surrounding the eye is blue. The head plumage is buff-yellow, which extends down the neck. The sexes cannot be reliably distinguished in the field. Juveniles have a dark bill, a mottled dark brown and white plumage in their first year, and an intermediate mottled grey head. The birds gradually acquire more white in subsequent seasons until they reach maturity after five years.
DISTRIBUTION AND HABITAT
Australasian gannets nest in dense breeding colonies on NZ´s mainland, coastal rocks and coastal islands, as well as off south-east Australia and Tasmania. Although gannets can be seen occasionally from most places along the coasts of the NZ main islands, most gannetries are situated off the North Island. Large colonies of over 10,000 breeding pairs each are at Three Kings Islands, Whakaari/ White Island and Gannet Island. The largest mainland gannetry is at Cape Kidnappers, with around 5,000 breeding pairs. Other mainland breeding sites include Muriwai and Farewell Spit.
NZ holds the bulk of the Australasian gannet breeding population, with about 13% of the adult population breeding in Australia. The NZ population was about 46,000 pairs in 1980-81 and continued to increase at about 2% per annum, although some former breeding sites have been abandoned and some colonies have decreased in size. Their population is probably regulated by the availability of suitable prey within easy flying distance of breeding colonies. Southern black-backed gulls take some eggs and young nestlings.
The breeding season extends from July, when birds first return to the gannetries, to fledging in March-April. Males arrive earlier than females, and re-occupy or establish and defend a nest. From the onset of breeding, the male brings nesting material such as brown algae Carpophyllum, which he retrieves from the shallows. Both members of the pair form and maintain the nest mound, particularly when the surrounding ground is soft from rain. The single egg is produced during an asynchronous laying period that starts in August at Hauraki Gulf gannetries, and September at Cape Kidnappers. A replacement egg can be laid within 4 weeks if the first egg is lost. Laying of replacement eggs can extend into January. Rare two-egg nests may be due to two females laying in the same nest. Eggs are incubated while being held between the webbings of the gannets’ feet. The Australasian gannet can only successfully incubate a single egg. Both sexes share the incubation duty, and later brood the chick on the top of their webbed feet. Feeds are delivered by both parents as incomplete regurgitations, which the chick receives by pushing its bill into the parents’ throats.
BEHAVIOUR AND ECOLOGY
Characteristic behaviours at breeding colonies include mutual bill fencing and bowing of mates at the nest, the territorial headshake and bow at the nesting site, and sky-pointing as an indication of the intention to take flight. The high-speed torpedo-like plunge dive of gannets is a spectacular sight, particularly when large foraging flocks form over surface aggregations of fish.
The adults mainly stay close to colonies, whilst the younger birds disperse. Fledglings from NZ fly directly to Australia, and typically do not return to their home colonies until their third year. Some NZ breeders migrate to Australian and Tasmanian waters to winter between breeding seasons. Australasian gannets often breed with the same partner over consecutive seasons. Some birds retain the same mate for the rest of their lives, but divorces do occur.
Australasian gannets - which are plunge divers and spectacular fishers - mostly feed on waters over the continental shelf. They mainly eat squid and forage fish which schools near the surface, particularly pilchards, anchovies, barracouta, garfish, and (horse) mackerel. Other recorded prey include piper, saury, flying fish, yellow-eye mullet, and puffer.
Source: Wikipedia, nzbirdsonline.org.nz