Monday, 23 December 2013

Shearwater wreck

Once again we are seeing the sad sight of many dead and dying birds on Straddie beaches. They are the short tailed shearwaters also known as mutton birds.

More than 3500 birds were blown up on shore this year and started crashing on October 7, buffeted by strong easterly winds from a cold front out to sea.

The birds die from exhaustion and malnutrition because their store of energy is depleted after a long migration from Siberia.

Short tailed shearwaters are pelagic, that is, they live their life in the open ocean and are a migratory species. There are approximately 23 million in the world, breeding on the Australian islands during the warmer months, mainly in Tasmania and islands in the Bass Straight.

The shearwater is the most abundant Australian seabird. There are about 285 colonies in south-east Australia with 18 million birds arriving in Tasmania each year.

More than 200,000 shearwater chicks are harvested commercially under licence in Tasmania each year, prized for their down and feathers, oil and flesh. The oil is used as an additive for racehorse food, down and feathers for pillows and the preserved meat for human consumption, which apparently resembles mutton.

Related to the petrel, they have adapted to life on the ocean by having webbed feet for swimming, a hooked beak for fishing, long and narrow wings for efficient high speed gliding, and a wingspan of one metre. Weighing about 500g, they are one of the few birds with a well developed sense of smell. They were first recorded by members of Captain Cook’s third expedition on the Discovery sailing through the Arctic and were named for their graceful shearing flight.

Shearwaters start breeding between three and five years of age. In early September/October they meet with their chosen mate and begin to prepare and tidy up old burrows that they reuse year after year, or else excavate new ones.

They mate inside the metre-long burrows and retain their chosen partner for life. They leave the colony to feed, allowing themselves to build up fat. The female then lays a single white egg. An entire colony lays its eggs over a period of two weeks so that the chicks all hatch at the same time, enabling them to start their migration in unison.

Both birds incubate the egg, and the chick hatches after 53 days. Both parents feed the baby, which gorges itself on regurgitated food, and it quickly becomes twice the size of a parent.

In April the adults depart, leaving the downy chick behind. From this time to early May, the chick does not eat, rapidly loses weight and acquires its flight feathers, then moves closer to the shore and starts exercising its wings.

Two to three weeks after the parents leave, the young birds start their amazing migratory flight, unassisted by the experienced birds. These birds have an average life span of 15-19 years but some can live to 38 years.

Between June and August the birds fly north along the western Pacific to the Arctic and Bering Sea to feed. They return south between October and January, along the east coast of Australia, travelling 15,000 km in each direction and, incredibly, some do the whole journey in six weeks.

By the time the birds arrive back in Australia they have expended most of their energy reserves on the journey, losing half their body weight. They may meet with storms and unusual sea surface temperatures, which can affect their main food source of krill, squid and fish. They dive 10 metres to capture this food supply, but when it is unavailable, quickly become exhausted. There is concern being voiced by scientists that seasonal variation due to climate change may have affected the availability of food sources.

Southerly gales make it impossible for the birds to fly on in this weakened state and so they are blown and washed ashore. Sometimes live birds are washed up but are too exhausted to head back out to sea. The elderly and young birds are the most vulnerable.

This was the fate of the many birds found on our beaches.

The Department of Environment and Heritage is urging people not to handle the dead or dying birds as a precaution. The Pelican and Seabird Rescue asks that you call them on 0404 118 301, however the University of Queensland’s Moreton Bay Research Station is of the opinion that it is best to leave them alone as they are notoriously difficult to rehabilitate. If you see that the bird has a numbered metal band around its leg, please contact the Australian Bird and Bat Banding Scheme on (02) 6274 2407.

A special thanks to Mike Hines for his help with information and photos, Emma Lewis at the UQ Research Station, Dunwich and to Robert Ashdown and island photographers for recording the recent events.