The short-tailed shearwater of Griffiths Island (Puffinus tenuirostiris - Tasmanian Shearwater) is commonly called the 'mutton bird'. Given the name 'mutton bird' by early settlers of Port Fairy because of its fatty flesh, the shearwater was hunted as a food and oil source. Today the shearwater is totally protected in Victoria.
The short-tailed shearwater is the only petrel whose breeding grounds lie solely in Australia. Almost all colonies nest on islands off south-east Australia, concentrating around bass Strait.
The 'Pea Soup' colony at Port Fairy is one of the rare mainland breeding areas.
The shearwater is, perhaps, Australia's most abundant bird. The colony at Griffiths Island totals several tens of thousands.
This mutton bird may not be spectacular in appearance, but it has some remarkable characteristics, including annual migration around the Pacific Ocean, and an uncannily regular lifecyle.
Each year the bulk of the colony (the beeding age birds) return to the nesting grounds on almost the same day. Individuals return to the same nest burrow they occupied the previous year and generally mate with the same partner throughout their breeding life.
For a few weeks after returning to the colony, the birds remain busy digging or clearing out nest burrows. The burrows are tunnels about one metre long dug in soft soil or sand close to the surface (This results in nests being difficult to avoid and easily crushed by walkers). Mating occurs in early November, with the entire population then flying off to sea for about two weeks. Eggs are laid immediately upon return. Each pair has one white oval egg similar in size to a domestic hen egg.
The male and female birds share the duty of incubation, with the male spending the first 12 to 14 days on the egg, followed by the female for 10 to 13 days. This alternating duty continues until the egg hatches about mid January.
Two to three days after hatching, the chick is left during the day while the parents forage at sea for forrd. The food is regurgitated to the youngster at night. Progressively, the period between feeds increases until the chick can wait up to two weeks between meals.
Parent birds can forage up to 1,500 km from the nest during this period. Meanwhile, the chick gain weight rapidly and for a period becomes heavier than the adult birds.
In mid April the adult birds commence their Pacific migration leaving the young behind. Hunger begins to bring the chicks from the nest at night, until they eventually set off after the adults. Somehow they find the migratory route without the guidance of the older birds.
Mortality is high in thefirst year, with only about half of those leaving the nest surviving. The non-breeding young birds follow a slightly later migration timetable. Reproductive maturity is attained at about five years of age.
After departing from the breeding grounds, the birds fly rapidly north to their wintering grounds arounds the Aleutian Islands and Kamchatka peninsula at the most northern extremity of the Pacific.
The journey, of about 15,000 km, passes New Zealand and Japan and is completed in only two months.
The return journey follows the coast of North America to California, then south-west across the Pacific. Prevailing winds aid their flight for most of the journey but on the final leg, from the central Pacific, the birds battle across south-east winds. They return to their nesting grounds exhausted.
CALANDAR OF EVENTS
(Griffiths Island Colony)
from observations of Miss G. Bowker, 1963 to 1973