Migration and Dispersal of New Zealand Gannets

Abstract

A total of 3,999 Gannet chicks was banded at Plateau station, Cape Kidnappers, Hawke's Bay, and Horuhoru station, Hauraki Gulf, New Zealand, between 1951 and 1957. Up to March 31, 1957, recoveries totalling 207 were made. These recoveries include 26 chicks which had not left the gannetry, 146 birds recovered outside the gannetry, and 35 live sub-adult birds caught on their return to Plateau gannetry. The sources of recoveries and the conditions of birds recovered are described. The percentage of recovery for the whole period was 3·1 per cent (3·3 per cent from Plateau and 2·3 per cent from Horuhoru). The distribution of recoveries throughout the year shows two periods of increased mortality; one affecting first year birds within the first few months of their independent life and associated with the crossing of the Tasman Sea; another peak occurring in late spring and summer and comprising 1 to 2 years-old birds. It was found that full-fledged chicks leave the gannetry at the age of 15½ weeks. The initial flight and its direction are described. The majority of Plateau Gannets take a northern route towards the North Cape; others migrate along the eastern coast of North Island and reach Australia through Cook Strait. Recoveries of Horuhoru Gannets show an almost exclusive use of the northern route. At the age of 24 weeks all but sickly or late chicks cross the Tasman Sea with an average speed of up to 240 miles per day. In Australia young Gannets disperse widely from subtropical waters north of the Tropic of Capricorn to Tasmanian waters in the south and to the Indian Ocean pelagic waters in the west. Horuhoru birds tend to go further south than those banded at Plateau. However, there seems to be no appreciable difference in geographical distribution among various age groups of the sample. Sub-adult Gannets remain in Australian waters until they are two to three years old. From that time an increasing number of recoveries is made in New Zealand where the birds disperse around the coast, and there appears to be some difference in the geographical distribution of recoveries from the two stations. An analysis of live birds recovered at the gannetry shows that young Gannets begin to re-appear at their home-gannetry at the age of 3 years, first as ‘roosting’ or ‘unemployed’ birds; from the age of 4–5 years they commence to breed, but at the age of 6–7 years only about half of the birds are breeding. Finally the similarities and differences in movements between the bird under discussion, considered by us as Sula bassana serrator, and S. b. bassana and S. b.?apensis are discussed. In all three subspecies there is a similar strong urge in juvenile birds to migrate to warmer seas. Also a similar maximum distance of about 4,000 miles, may be flown by the three subspecies, but whereas North Atlantic and Cape Gannets move equator-wards, New Zealand-bred Australasian Gannets migrate westwards.