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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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