Shearwaters are one of the world's
most remarkable migratory birds
(Photo by Steve Johnson)
The short-tailed shearwater, or mutton bird as it is often known, is a member of a group of 60 medium to large seabirds in the family Procellaridae. This family includes species such as petrels and prions. All members of the family have tube-like nostrils on the top of their upper beak and are believed to be one of the few bird families with a well-developed sense of smell. Almost all breed in burrows and, like the albatrosses, are truly impressive oceanic fliers.
Adult birds have a wing span of about 1 metre and weigh approximately 500 grams. Shearwaters are good swimmers and have webbed feet. Their legs are placed well back on their body and their wings are long and narrow for efficient high speed gliding. These features suit an oceanic existence so the shearwater has difficulty moving on land or taking flight in windless conditions. Shearwaters are often seen floating in large 'rafts' while feeding off the shores of Tasmania.
The short-tailed shearwater was first formally described by a Dutch ornithologist -- Jacob Temminck in 1835. He named it Puffinus tenuirostris (tenui -- slender, rostrum -- bill). The shearwater was recorded much earlier by members of Captain Cook's Third Expedition in 1778 while sailing in the Arctic Ocean. William Ellis, an artist on the 'Discovery' painted the bird.
The name 'muttonbird' was first used by the early settlers on Norfolk Island, who each year harvested adult providence petrels (Pterodroma solandri) for food. The petrels were similar to but larger than the short-tailed shearwater. An officer in the Royal Marines called them 'the flying sheep'.
Unfortunately the providence petrels became extinct following massive harvesting (171 000 birds one year) and the introduction of pigs to the island. The name 'muttonbird' has been applied to the short-tailed shearwater ever since. The common name, shearwater, is an apt reference to their graceful shearing flight moving from centimetres above the water's surface to high in the sky.
Tasmanian Aborigines have harvested muttonbirds and their eggs for many generations, and a number of families continue this important cultural practice. The muttonbird is one of the few Australian native birds that is commercially harvested. During the muttonbird season, chicks are taken for their feathers, flesh and oil. The industry was established by early European sealers and their Aboriginal families. The recreational harvesting of Short-tailed Shearwaters is limited to the period of the open season that is declared each year. A muttonbird licence must be obtained.
Distribution and migration
The shearwater is the most abundant Australian seabird. Approximately 23 million short-tailed shearwaters breed in about 285 colonies in south-eastern Australia from September to April. Eighteen million of these arrive in Tasmania each year. There are known to be at least 167 colonies in Tasmania and an estimated 11.4 million burrows. The largest colony is on Babel Island which has three million burrows. Their colonies are usually found on headlands and islands covered with tussocks and succulent vegetation such as pigface and iceplant. Headlands allow for easy take off and landing.
Early accounts suggest that the population was once considerably higher. In 1798, Matthew Flinders estimated that there were at least one hundred million birds within a single flock sighted in Bass Strait.
Their migratory path is difficult to define because they don't come to shore during the months of the migration. Exhausted and starved birds are often washed up on beaches of Japan, the Aleution Islands, North America and Australia. Originally this led scientists to believe that the birds flew a figure of eight course across the Pacific Ocean. Recent studies suggest the majority of birds merely fly north along the western part of the Pacific Ocean to the Arctic region and return southwards through the centre of the ocean. Either way the birds travel about 15 000 kilometres in each direction annually. They have been known to fly this remarkable distance in six weeks.
The migration of the shearwater.
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The breeding period occurs between September and April. Each year the length of time spent at the breeding grounds increases until the birds are 5 years old, when they become involved in breeding. As pre-breeders, the birds fly in with the breeding adults in preparation for the following breeding season.
On arrival in late September/early October at the colony the birds meet with their chosen mates and begin tidying up the old burrows or excavating new ones. The burrows are about 1 metre long. Mating takes place inside the burrow.
Each bird generally remains with the same partner throughout their life, although the "divorce rate" does increase to nearly 25% among pairs that fail to produce young.
In early November they leave the colony to spend some time feeding before returning to lay a single white egg in late November. This exodus period is important as it allows the birds to build up fat reserves to see them through the incubation period. There is a distinct peak in egg laying at 27-28 November. Males and females take turns incubating the egg. The male takes the first shift, which lasts for about two weeks, followed by the female. Usually, both sexes have two shifts. During each shift, the "duty" bird does not leave the burrow, nor is it fed by its mate.
The young chicks hatch in the third week of January after an incubation period averaging 53 days. Both parents participate in feeding the chick. The chick quickly puts on weight and before the departure of the parents, is almost twice the weight of an adult. The adults depart from early April leaving behind the young birds still covered in down. From this time until early May the chicks do not eat at all. They rapidly lose weight and acquire their flight feathers. The young spend an increasing amount of time outside the burrow, slowly moving closer to the shore and exercising their wings. Two to three weeks after the parents have left, the young birds begin their migratory flight unassisted by experienced birds.
The period of time spent in the breeding colonies varies with the age of the bird
Food and feeding
Shearwaters feed on krill, squid and fish. Their main methods of feeding are plunging into the water, pursuing underwater, surface seizing, scavenging, hydroplaning and bottom feeding. They are capable swimmers and are able to dive to 10 metres. Their hooked beak allows them to hold on to their prey. During the breeding season the adults generally feed in the locality of the colony. The chicks produce large amounts of oil in their stomach which is high in energy content and sustains them while the parents are away. During migration they feed whenever food is available.
It is possible that krill abundance determines the migration of the species, allowing them to exploit the high concentrations of krill which occur each summer at both polar regions.
Threats and mortality
Although there appears to be a huge number of short-tailed shearwaters, they are still vulnerable to over-harvesting and habitat destruction. In places, pigs, cattle and sheep have destroyed whole colonies. Soil erosion after fire can destroy suitable sites for burrowing.
Gillnet fisheries in the North Pacific accidentally drown up to 50 000 birds annually.
Approximately 200 000 chicks are presently harvested and sold annually in Tasmania by commercial operators. Birds also ingest small plastic particles while at sea which may limit their ability to maintain condition and contribute to deaths during migration.
Feral cats are also a problem, as they find shearwater chicks easy prey.
Trampling of burrows by humans can also cause their death. Similarly, erosion caused by recreational vehicles can destroy suitable sites for burrowing. It is important to keep off colonies.
Natural mortality occurs mainly during the first migration due to exhaustion and starvation. The average lifespan is 15-19 years but birds can live up to 38 years.
Because of the shearwater's international migratory habitats it has become the subject of a joint protection project between Japan and Australia -- the Japan Australia Migratory Bird Treaty. Both countries monitor the shearwater population while the birds are in their area. In Tasmania harvesting limits are imposed to prevent over-harvesting and a number of wildlife sanctuaries protect shearwater colonies. Japanese and other countries are attempting to minimise the number of birds drowned by their fishing operations. It is hoped that these conservation methods will ensure the survival of one of the world's most amazing migratory birds.
Lindsey, T. R. (1986). The Seabirds of Australia. Angus and Robertson.
Marchant, S. and Higgins, P. J. (1990). Handbook of Australian, New Zealand and Antarctic Birds. Vol. 1. Oxford University Press, Melbourne.