Females lay a single pale blue egg, the size of a large hen’s egg, any time from mid-September till mid-December. It is laid in a nest prepared from dried seaweed cemented with guano (bird droppings), and incubated by each parent in turn. After 43 days a blind, naked chick hatches, and is fed and cared for by both parents.
The chicks grow rapidly. In the first week they develop white fluffy down, which is replaced during their second and third months by juvenile plumage. By 14 weeks the chicks weigh 3 kilograms, half a kilogram more than the parents, and they begin flapping and stretching their wings for hours each day. Then suddenly in their fourth month of life they take off, not to return for several years. Their destination was a mystery until the 1950s, when juvenile birds were first ringed and tracked: they turned up in coastal waters around the east coast of Australia.
Without any previous flight practice, without having had to seek out their own food, and without any accompanying adults, the fat young birds fly 2,740 kilometres across the Tasman Sea to the east coast of Australia in late summer and early autumn. The journey takes 8 to 14 days, depending on the weather and wind direction. Juvenile birds remain in Australian waters for two or three years, and then return to their hatching site in New Zealand in early spring. The two-way journey is fraught with dangers – including storms and predators – and there is a high mortality rate. In a good year, about a quarter of the birds manage to return safely to New Zealand.
Most adult birds do not migrate. Once back in New Zealand they spend spring and summer at the breeding colony and then disperse around local coastal waters for the winter months. Birds newly arrived from Australia may not be able to claim a nest site or a mate in their first couple of years back at the gannetry, but once successful, they mate for life and may survive for 30 years.
Diving for prey
Westward a gannet dived in a fire-white streak
Straight to the waters, and so was gone like a stone. 1
Although ungainly on land, gannets are magnificent in flight. When searching for food they fly parallel to the coast, between 1 and 20 metres above the sea, looking for schools of fish or squid. They plunge headlong as soon as they spot their prey. Just before they hit the water, they fold in their wings to swoop down beneath their food. They can enter the water at speeds up to 145 kilometres an hour, relying on inflatable air sacs around the neck and chest to absorb the shock of impact. They grab the prey in their beaks and swallow it whole once they have surfaced.
As good as a goose
Gannets make tasty eating, and have not always been protected by law. Māori used to harvest the young, and at Christmas in 1769 the naturalist Joseph Banks, on board the Endeavour with Lieutenant James Cook, made these entries in his diary:
‘24. myself in a boat shooting … killing cheifly several Gannets or Solan Geese … As it was the humour of the ship to keep Christmas in the old fashiond way it was resolvd of them to make a Goose pye for tomorrows dinner.
‘25. Christmas Day: Our Goose pye was eat with great approbation.’ 2
A gannet will often regurgitate its stomach contents if disturbed or handled; if the bird has recently fed, the prey can be identified. Small fish such as pilchards, anchovies, saury, jack mackerel and squid form the basis of the gannet diet. One of the theories advanced for the dramatic increase in New Zealand’s gannet population is that inshore commercial fishing has reduced the number of large predatory fish, resulting in a corresponding increase in small fish. This abundance of food for adult gannets enables more chicks to survive.
Australasian gannets are totally protected by law, and the New Zealand population is in a healthy state. However, gannetries are susceptible to the effects of oil spills, and the birds have been caught in set nets or on the lines of recreational fishermen. Adult and young birds can become entangled in the rope ties from nearby mussel farms, which they incorporate into their nests. Mainland colonies are vulnerable to egg predation by black-backed gulls following human disturbance.