SHORT-TAILED SHEARWATER – (also called Muttonbird, Short-tailed Petrel)
By Danielle Davis
The name “shearwater” is derived from the fact that they have the ability to cut or shear the water with their sickle-shaped wings which look seemingly motionless when they fly. Their other name “muttonbird” was given to them by early European settlers, who killed these birds for food and found that their flesh tasted like sheep or mutton. (flying sheep). They are closely related to the typical Petrels who were also hunted for their flesh and are a similar seabird but larger. Unfortunately, the Providence Petrel, which inhabited Norfolk Island become extinct following massive harvesting (171,000 in one year) and the introduction of pigs to the island who trampled over their nests and eggs. The Shearwater is also at present Australia’s most abundant seabird with about 23 million Short-tailed Shearwaters breed in about 285 colonies in south eastern Australia from September to April. Early accounts suggest that the population was considerably higher and in 1798 Mathew Flinders estimated that there were at least one hundred million in a single flock sighted in Bass Straight. Although there still appears to be large numbers they are slow breeders (only one egg each year) and face many dangers through migration and harvesting by humans (“muttonbirding” takes 300,000 approx. nestling chicks each year) and without careful consideration their numbers could drop dramatically.
Short-tailed Shearwaters are migratory birds traveling up to 32,000 kms every year and have
been known to fly this remarkable distance in six weeks! After breeding they travel far and wide to places such as Antarctica, Siberia, Japan, Alaska, South America and New Zealand. Recent studies suggest the birds circumnavigate the Pacific flying north along the western part of the ocean to the Arctic region and return southwards through the centre & eastern part of the ocean, then they transverse back across vast areas of open ocean westward towards Australia to breed again, amazingly to the same/almost the same spot with the same partner as the year before. They need an enormous amount of energy to migrate such huge distances and this is stored in the form of body fat but many birds loose up to half their body weight during this long and arduous journey. For optimum breeding grounds and food availability they need to make the voyage but the costs are high and thousands of birds die each year from starvation, bad weather conditions, gillnet fishing lines (150,000-280,000 yearly), ingesting plastics, oil spills and predators. Often exhausted and emaciated birds in great numbers are washed up on the beaches of Japan, the Aleutian Islands, North America and Australia every year. The mortality rate is very high in the first year that the young Shearwaters migrate with up to 50% not making the journey back.
All shearwaters obtain their food from the water and the short-tail is no exception. Traveling in flocks they will land on the water in large groups, known as “rafts” for safety where they swimming, dive or by diving from flight over a metre down “flying” under the water with open wings to catch and feed on krill, squid, fish, crustaceans, molluscs and plankton.
Preferring cool temperate waters, all known breeding colonies are in Australia often on islands from St. Francis Island in S.A. to southern Tasmania (concentrated breeding in Bass Strait & Spencer Gulf) and up the north-east as far as the central coast. There is a breeding colony of Wedge-tail Shearwaters that you can visit on Muttonbird Island near Coffs Harbour. This island is connect to the mainland by a breakwater and visits can be made at night to see this lovely bird walking back to their burrow nests in the ground. All visitors are asked to stay strictly on the walking track as any deviation could lead to a nest or precious egg being destroyed.
Both male and female look the same as do the juveniles. Around 600mm long, brown-black (sooty) plumage all over with silver/grey underwings, tail short and rounded, eye brown, bill olive-grey and web feet purple-grey-black. The chicks are covered in sooty grey thick down with bill and web feet black.
In courtship and greeting, mated birds duet, arc their heads and preen one another. They call to their mate, either to establish and strengthen their bond or to announce a burrow is occupied, is a crooning kooka-rooka-rah repeated rapidly and evenly accented. Calls are given by birds on the ground or from within their burrows at night and rarely when they are in flight. Early in the breeding season there is much noise in the colonies, especially at night and particularly during the evening arrival.
Their birth rate is very low and they only live to 15-19 years approx. with females not mating until 5 years old and males 7 years and even then only one single egg is laid each year. The immature birds either stay in the Antarctic areas for summer or return to the breeding colonies later in the season but the mature birds all arrive back from their long migration late September. Usually shearwaters only visit land to breed, being more adapted to life in the waters and on the wing, and then visiting mainly only at night, where they establish colonies on remote islands, capes or coastal mountains – places where take-offs are assisted by winds and where there are less land predators. A burrow up to 2 metre long is prepared and then lined with dead leaves during nightly visits with the birds almost dragging their keel bone (breast bone) along the ground as their legs and bodies are more suited to and use to being in the water or in the air. Often these breeding ground areas are densely packed with one burrow every square metre.
After mating at the burrow there is an exodus of adults for about 3 weeks to feed up before egg laying. The females return first to lay their single white oval egg, which is done within a remarkably short and constant period from 20th November-2nd December with peak laying occurring between 24-26th November! Males take the first incubation shift of 12 – 14 days while the female go off and feed after which the female comes back and does the same. This goes on back and forth between them and when one of the parents is nesting on the egg they do not eat, drink or leave the burrow until finally after 53 days the heavily down chicks hatch from mid-January onwards. After they hatch the young chicks are left alone in the burrow during the day and then very 2nd/3rd night are feed a diet in fish rich in protein and oil high in vitamin A regurgitated by the both parents.
The chicks remain in the burrow being fed and fattening up for 80 days when they receive their last feed and then both parents leave their chick completely alone (mid April) and start their migration northward once again. After waiting another 14 days, losing some weight and gaining more feathers the chicks venture out of the burrow (late April-early May), where they have been since egg for almost 5 months. They swim out into the oceans and fledge (start to fly) on their own and instinctively know where to head to start their first migration and find their great flocks. If the chick makes it through it’s first 4 years it will then return back with the flocks to Australia.
Approximately 200,000-300,00 chicks are presently harvested from their burrows and killed in Tasmania by commercial operators every year. Many are also taken by private collectors. The little fat nestlings weighing almost one kilo each are prized for their down and feathers for pillows and bedding, their rich oil is extracted from their stomach to be purified for pharmaceutical use and their bodies cleaned and sold as fresh meat or pickled in brine. In places, pigs, cattle and sheep have destroyed whole colonies. Soil erosion after fire can also destroy suitable sites for burrows. Trampling of burrows by humans can also cause their death. Similarly, erosion caused by recreational vehicles can destroy sites suitable for nesting burrows. Gillnet fishing in the north Pacific alone involantarily drowns approximately 50,000 Shearwaters annually. Birds also ingest small plastic particles while at sea which may limit the birds’ ability to maintain condition and contribute to death during migration.
Through the laws of natural selection many of these birds will die naturally before reaching breeding age and maturity but they are now encountering many more dangers which are having a serious impact on the great and important populations.