ONLY half the expected number of short-tailed shearwater birds have arrived at Griffiths Island this year, birdwatchers have estimated.
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Birdlife Warrnambool president Peter Barram said the Port Fairy island could host up to 40,000 birds but this year numbers were around 15,000 to 20,000.
"The whole thing is a very worrying situation," he said.
He said while counting was difficult due to the birds arriving at night after circling the island, burrowing activity also suggested numbers were down.
The Department of Environment, Land, Water and Planning, which relies on birdwatcher counts to monitor numbers, believes it's "unlikely" the island's colony numbers will reach capacity this year.
The migratory species, also known as mutton birds, spends summer breeding in the southern hemisphere and winter feeding in the northern hemisphere.
A department spokeswoman said why the numbers were down was "unknown" and could be connected with reported deaths in the northern hemisphere this year.
"There are several factors that could have impacted the population, such as climate variability and food availability in the northern hemisphere," she said.
The birds usually arrive in Port Fairy around the last week of September every year, after a two-month journey on a 15,000 migration path, which passes over Japan and New Zealand.
Mr Barram said alarm bells first sounded for him in September when the birds arrived later than usual.
"The birds up until this year, except for 2014, have turned up as regular as clockwork," he said.
"The hope is that the population hasn't been completely decimated and the birds will just miss this breeding season."
Tens of thousand of mutton birds reportedly washed up dead on NSW beaches this month, and Mr Barram said the deaths could be due to a lack of food and exhaustion.
He said more research was needed into whether the incidents were related, and the causes.
"It's a global problem we need to look at at. These birds are the canary in the coal mine and a pointer to things that are going wrong with our whole ecosystem," Mr Barram said.
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