Birdwatchers fear for the fate of thousands of short-tailed shearwaters, also known as mutton birds, which failed to arrive in south-west Victoria at the usual time after their annual migration from the northern hemisphere.

Key points: For the past 30 years, short-tailed shearwaters, known as mutton birds, have arrived at an island near Port Fairy, Victoria, in late September

For the past 30 years, short-tailed shearwaters, known as mutton birds, have arrived at an island near Port Fairy, Victoria, in late September So far, only a handful of the migratory birds have turned up, out of a usual colony of 40,000

So far, only a handful of the migratory birds have turned up, out of a usual colony of 40,000 Climate variability or food availability in the northern hemisphere may have delayed the birds' arrival

Each year, hundreds of thousands of short-tailed shearwaters descend on Victoria's coastline to breed following a mammoth journey which takes two months to complete.

The birds spend the northern summer around Alaska, before travelling 15,000 kilometres to Australia where they arrive with precision.

For the past 30 years, the south-west Victorian population has arrived at Griffiths Island, near Port Fairy, a day either side of September 22.

But this year, the date came and went without the usual flurry of activity.

Peter Barrand, president of Birdlife Warrnambool, said he had basically set his watch by the shearwaters' arrival for the past three decades.

A mutton bird flies at sea. ( Supplied: Eric Woehler )

The slow start to the season has him concerned.

"It's extremely worrying — are we looking at an extinction event happening before our very eyes?" Mr Barrand said.

"We couldn't find any at first, but further investigation found there were small numbers coming in.

"For a colony that's something like 40,000 strong — a handful of birds is a significant decline.

Griffiths Island is connected to the seaside town of Port Fairy and sits at the mouth of the Moyne River.

It hosts one of the largest visiting populations of shearwaters in Australia, with the majority of birds nesting on offshore islands where they are safe from predators and human disturbance.

While birds arriving underweight and exhausted isn't uncommon, the scale of this year's delay is what's most concerning to birdwatchers.

"Anecdotal reports indicate that birds haven't turned up anywhere else. This is not just an isolated incident at Griffith Island," Mr Barrand said.

"It's happening right along the coast of Victoria and in Tasmania too.

"You make a significant impact on the population, [then] it'll be a struggle for the birds to recover their numbers, and if this trend continues we're certainly looking at an extinction event."

A spokesperson for Victoria's Environment Department said short-tailed shearwaters typically returned to colonies at Port Fairy and Port Campbell in late September to early October, but so far, only small numbers of birds had been sighted at either location.

It said the possible causes were unknown, but there were several factors that could have delayed the birds' arrival, such as climate variability and food availability in the northern hemisphere.

The department is liaising with agencies in Tasmania to understand whether a similar phenomenon is occurring in other colonies.

The c-word

There have been numerous reports of bird starvation events in the northern hemisphere over the past five years, including several reports in Alaska and even New York.

According to research published in the journal Science, there are now 2.9 billion — or about 29 per cent — fewer birds in North America now than in 1970.

Alaska's government-backed National Park Service has reported massive seabird die-offs, also known as wrecks, for five consecutive years.

A number of dead shearwaters were found at Torquay, Victoria, in 2013. ( ABC News: Margaret Paul )

It's sparked concern in the southern hemisphere.

"Something's obviously gone drastically wrong in the arctic — whatever the shearwaters have been feeding on has failed to appear," Mr Barrand said.

"Autopsies have shown the deaths were all attributed to starvation. And that's the worrying part about it.

"What's gone … in the northern hemisphere to stop these birds feeding as they normally do? Is it the dreaded climate change or some other event that's occurred?"

Patience the key

Philip DuGuesclin, a former biodiversity officer with the Environment Department in Port Fairy, is also taking a conservative approach.

He's spent the past three decades working with and observing the Griffiths Island shearwater population and shares Mr Barrand's concern.

A WIRES volunteer releases a shearwater into the sky. ( Supplied: Kirsty Fikkers )

But Mr DuGuesclin believes it isn't time to panic just yet.

"Reports we've had down the eastern seaboard have been that there's very few to none arriving, but we have also had a report that thousands are streaming past Gabo Island, presumably heading in this direction," Mr DuGuesclin said.

"Are they the seabird equivalent of the canary in the mine if it comes to global warming? I don't know.

"The concern we have is that with all these thousands of birds streaming past Gabo Island, they might just bypass Griffiths Island and head straight down to the Antarctic convergence before coming back to breeding.

"If they're not arriving on the island, it could be because they aren't in breeding condition."

But for Warrnambool's local bird-watching community, that's a wake-up call of the highest order.

"People have really got to sit up and take notice of this — it's a really significant event," Mr Barrand said.

"It's looking like it's an increasing trend over the last five years, but this is a very, very serious situation."