Thousands of short-tailed shearwaters are washing up dead on Australian east coast beaches and researchers are uncertain of the cause and scale of these seabird “wrecks”.

Each spring about 20m shearwaters, also called yula or muttonbirds, fly 15,000km back to southern Australia from the northern hemisphere. Since late October, dead shearwaters have been turning up on beaches in south-east Queensland, followed by similar reports in New South Wales and Victoria in recent weeks.

Dr Lauren Roman, who researches shearwaters at University of Tasmania’s Institute for Marine and Antarctic Studies, said understanding how many birds had died, and whether it was a normal or mass mortality event was “tricky”.

In large numbers, these mortality events are called seabird wrecks, she said.

Shearwater wrecks are known to occur during their annual migration, she said, but smartphones and social media had raised people’s awareness.

“There’s a perception that the mortality events are increasing, but it’s very hard to tell whether that’s actually the case, or just a function of increased awareness.

“If they’re right out in the middle of the Tasman Sea, hundreds of kilometres offshore, and there’s a big mortality event, we’re not going to see that.”

Even a small portion of the population that died closer to the coast could result in tens or hundreds washing up on beaches.

“Whether or not there’s actually more mortalities than there were in the past, is very difficult to quantify,” she said.

Adrift Lab researcher, Jennifer Lavers, estimated the number of adult seabirds “washed up, dying on beaches” was in the “hundreds or thousands” this year, based on early analysis of citizen scientist reports.

The mass mortality events were unusual for seabirds with long lifespans, and did not “make sense from an evolutionary perspective”, she said.

The birds that were washing up were emaciated, Lavers said, which indicated the animals were struggling to find enough food.

Roman said there was a significant mass mortality event in 2013 where millions of seabirds perished. Recent reports weren’t on that same scale.

The 2013 event was thought to be associated with an abnormal heat event in the north Pacific Ocean called “the blob”.

“We know that caused a cascade of seabird mortalities in the northern hemisphere as well, and the early stages of that event coincided with when shearwaters were also up there before they started their migration,” Roman said.

Dr Eric Woehler, who has researched seabirds for more than four decades, said shearwater wrecks often occurred in autumn when the youngsters made their first flight north, and occasionally in spring when the adults birds returned. The timing, duration and numbers of birds seen in mortality events varied year to year, he said.

“We believe that these birds, particularly, didn’t have enough food and basically started on their migration with insufficient body reserves,” he said.

Shearwaters live to be more than 40 years old, so the loss of adults probably had a greater impact on the overall population due to the loss of breeding effort, Woehler said. The seabirds only laid one egg per breeding pair, raising one chick each year.

Tasmania and islands in the Bass Strait were a stronghold for the species.

Authorities were also on alert for the highly pathogenic and transmissible H5N1 flu strain, but it hadn’t yet been detected in Australia, or in the shearwaters found on local beaches.

Roman said researchers were working hard to disentangle the factors and implications of wreck events.

These events could be heartbreaking to witness, she said, but people shouldn’t be alarmed just yet. “If you find one or two dead ones, I wouldn’t worry too much about it, because that’s natural this time of year.”

Beachgoers should avoid touching dead birds or letting their pets interact with them.

People could contact wildlife carers if they saw live birds that appeared to be in trouble, and could report larger numbers of dead seabirds to their state’s marine animal stranding hotline.