A local non-birding friend asked me some questions about the recent STSW
wrecks, some of which I was unable to answer with any confidence (eg. how
long do the birds normally live?) It caused me to head for Google (although
I also have HANZAB and HBW to which I could have gone). I found the
following from the Tasmanian Parks and Wildlife Service website. For those
readers who would like to refresh their knowledge of this interesting, and
currently topical, species it provides quite an interesting overview.
Richard
PS I have left in some colour highlighting I did for my friend - it picks
out some interesting facts (including a reversal of the well-known "figure
of eight" migratory path theory).
Short-tailed Shearwater, Puffinus tenuirostris
Description
<http://www.parks.tas.gov.au/file.aspx?id=5104&mode=standard&sb=.jpg>
Short-tailed shearwater
Shearwaters are one of the world's
most remarkable migratory birds
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
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
<http://www.dpiw.tas.gov.au/inter.nsf/Attachments/JAAR-7ZNSPQ?open>
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 Aleutian 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.
The period of time spent in the breeding colonies varies with the age of the
bird
mut.JPG
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.
International treaty
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.
Further Information
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.
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