The stomach of fledgling mutton birds, or Short-tailed Shearwaters, on Victoria’s Phillip Island have been found to contain an average of more than seven pieces of plastic per bird – weighing some 113 mg.
This, says La Trobe University’s Dr Mark Carey, may affect the life cycle of this species and their reproductive success, and cause long-term harm to populations.
Dr Carey says while the plastic load of adult Short-tailed Shearwaters has been well researched and documented in the northern hemisphere, there has been little work on the amount ingested by their offspring on their southern breeding range.
The results of his study – part of a bigger project by into shearwaters – have been published in Emu: Austral Ornithology, the leading scientific journal for ornithological research in the Southern Hemisphere. They were also presented by Dr Carey to an international audience at the Australasian Ornithological Conference in Cairns in September.
Working from a remote Bass Strait island for six years, up to four months at a time, Dr Carey’s research also documents the incredible travels of these birds. He did this by attaching tiny data loggers to the legs of 27 mutton birds before they began their mass migrations to the northern hemisphere. And he managed to recapture an impressive 20 birds the following year to retrieve vital information.
Major conservation concerns
Dr Carey, from the Department of Environmental Management and Ecology on La Trobe’s Albury-Wodonga campus, says pollution of the world’s oceans affects a wide variety of marine organisms and raises major conservation concerns.
‘Ingestion of plastic debris has increased since the 1970s, particularly among birds that roam long distances like petrels and albatrosses, resulting in lethal and sub-lethal side effects.’
The most common type of debris found in the birds, says Dr Carey, is widely used plastics, followed by industrial pellets. The birds also contained a small proportion of other refuse, such as polystyrene and bits of plastic bags.
Seabirds ingest plastic because they confuse it with prey. ‘It can cause physical damage, or perforation, mechanical blockage or impairment of the digestive system, resulting in starvation,’ he says. ‘Some plastics are also a source of toxic pollutants released during digestion.’
Not junk food for dinner again!
Shearwater chicks appear to be at greater risk than adult birds. ‘When plastics are regurgitated by parents to chicks, the physical impact of the plastic and internal ulceration are likely to lower survival. In addition, the chick receives less food, lowering its nutrient intake and increasing its chances of starvation.’
The Short-tailed Shearwater Puffinus tenuirostris breeds on islands and headlands off south-eastern Australia. Eggs hatch in late January, after which parents begin feeding the chicks, and the birds migrate to the northern Pacific in April and May.
Dr Carey’s study was carried out at Cape Woolamai and recorded the stomach contents of newly washed up dead birds. While no clear influence of ingested plastic on body condition could be demonstrated in the young birds, he says there was evidence of physical damage to the gizzard.
Mortality rate unknown
‘Our data suggests that an assessment of the effect of this type of pollution on seabird welfare is urgently required,’ says Dr Carey. ‘The rate of mortality caused by plastic ingestion remains completely unknown for Australian species.’
Dr Carey wrote his PhD thesis on mutton birds. He spent six breeding seasons, each of which lasts about four months, studying the birds on wind-swept Great Dog Island in Bass Strait. The island is in the Furneaux Group, off north-east Tasmania, between Flinders and Cape Barren island. It is the third largest mutton bird rookery in Tasmania.
Shearwaters arrive on the island towards the end of the year on an annual 20,000 kilometres round-trip migration from the northern hemisphere. Females lay one egg each, in burrows among the sandy terrain. Chicks hatch in January. They stay in their burrows for three and half months, avoiding their main predator, tigers snakes.
Most survive, says Dr Carey, and fledge in the late April or early May. By that time they have been abandoned by their parents and have to learn to feed and to fly by themselves, in preparation for the first of their many migrations north.
Amazing migration patterns
Dr Carey’s research has tracked the amazing travels of these birds by attaching the data loggers to 27 mutton birds before they began their migration. The loggers were developed at Cambridge University for the British Antarctic Survey. They fix the location of the birds by recording the times of sunset and sunrise, and also record the time birds spend in the air or resting on water.
Downloading data from the loggers of the 20 birds he recaptured, he found that after leaving their breeding grounds on Great Dog Island, the birds first head south towards Antarctica. Here they feed on krill before their migration north.
That journey, says Dr Carey, takes them from south of New Zealand, up the western Pacific to Japan and the Bering Sea where they spend the northern summer.
800 kilometres a day
His research showed that the small birds, travelling in large flocks, cover some 800 kilometres a day. ‘The whole trip takes them about 12 days, which is an amazing feat.’ Their return journey takes them down the central Pacific, along the Queensland coast, back to their Tasmanian colony.
The mutton bird received its name in 19th century due to its exploitation as a significant source of food and oil. Up to a million young birds a year were once taken, but today that number is down to 300,000.
Dr Carey has also researched the hatching success of Shearwaters. While an estimated 23 million Short-tailed Shearwaters still breed around the coast of Australia, other species of Shearwaters, he says, are in serious decline.
Their breeding habitat is being lost or degraded as a result of human development and the introduction of mammalian predators. Research has shown that they are also threatened by long-line fisheries. (ER)