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Sea BirdsShort-tailed
Shearwater
Short-tailed Shearwater
Common Name: Short-tailed
Shearwater
Scientific Name: Puffinus tenuirostris
Description
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.
History
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.
Harvesting
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 and today forms an important part
of Aboriginal culture in Tasmania. The chicks are taken under
strict controls and the season is limited to the period 27
March to 30 April.
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.
Breeding
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.
Food and Breeding
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. The are capable swimmers and are able
to dive to 10 meters. Their hooked 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
the birds 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.
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