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 âwrecksâ.
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 Tasmaniaâs Institute for Marine and Antarctic Studies, said understanding how many birds had died, and whether it was a normal or mass mortality event was âtrickyâ.
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 peopleâs awareness.
âThereâs a perception that the mortality events are increasing, but itâs very hard to tell whether thatâs actually the case, or just a function of increased awareness.
âIf theyâre right out in the middle of the Tasman Sea, hundreds of kilometres offshore, and thereâs a big mortality event, weâre not going to see that.â
Even a small portion of the population that died closer to the coast could result in tens or hundreds washing up on beaches.
âWhether or not thereâs actually more mortalities than there were in the past, is very difficult to quantify,â she said.
Adrift Lab researcher, Jennifer Lavers, estimated the number of adult seabirds âwashed up, dying on beachesâ was in the âhundreds or thousandsâ this year, based on early analysis of citizen scientist reports.
The mass mortality events were unusual for seabirds with long lifespans, and did not âmake sense from an evolutionary perspectiveâ, 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 werenât on that same scale.
The 2013 event was thought to be associated with an abnormal heat event in the north Pacific Ocean called âthe blobâ.
âWe 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,â 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.
âWe believe that these birds, particularly, didnât have enough food and basically started on their migration with insufficient body reserves,â 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 hadnât 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 shouldnât be alarmed just yet. âIf you find one or two dead ones, I wouldnât worry too much about it, because thatâs natural this time of year.â
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 stateâs marine animal stranding hotline.