Every year, during the last week of September, tens of thousands of dark, narrow-winged bodies—Short-tailed Shearwaters—circle overhead before descending on the sandy shores of Griffiths Island off the coast of southeastern Australia. But this year the seabirds broke tradition. When September 21, their typical peak arrival date, rolled around, the skies were empty. It wasn't until two weeks later that they began to show up by the hundreds, and even then, the numbers were low.
Australia’s most abundant seabird with an estimated 30 million individuals, the Short-tailed Shearwater is categorized as a species of least concern by the International Union for Conservation of Nature. But in the past few months, a series of troubling events has raised alarms about the health of this trans-equatorial migrant’s population.
Experts suspect that the shearwaters’ late arrival and reduced numbers at their southern breeding grounds this year is likely tied to a food shortage in the Bering Sea, where the birds spend the austral winter in the Southern Hemisphere. That food shortage resulted in thousands of starved shearwaters washing up dead on Alaskan shores only a couple of months before the birds were supposed to return to the coastlines and islands in the Bass Strait (the sea between Australia and Tasmania), including Griffiths Island.
It is unclear whether this year's situation is a one-off, but considering the fact that mass seabird die-offs have occurred annually in Alaska for the last five years, it is possible that this once common species is already facing a drastic population decline. “One minute there are 30,000 birds flying around in a colony, and the next thing you know, they’re down to 10,000," says Peter Barrand, president of BirdLife Warrnambool. "And if they’re not reproducing, the numbers will continue to plummet."
For the past 30 years, Barrand has observed the birds on Griffiths Island, which can host up to 40,000 Short-tailed Shearwaters each summer, according to the Victoria Department of Environment, Land, Water and Planning. After Barrand was alerted to the birds' delayed arrival, he became increasingly worried when they were still nowhere to be seen a week later. Now, two months after their expected return period, he estimates less than half of the breeding colony is present. “Things aren’t great, and many of them aren’t in condition to breed this year,” he says.
Before they appear in the Bass Strait region, the muttonbirds—the Australian name for Short-tailed Shearwaters—spend the northern summer months foraging in the Bering Sea. They then embark on their 10,000-mile journey south, typically travelling for six weeks before arriving at various breeding grounds around the end of September. In the following months, the adults will nest and feed their young, as well as take multiple foraging trips to Antarctica, says John Arnould, a professor at Deakin University.
In addition to Griffiths Island, several other breeding locations have also shown reduced numbers of shearwaters this year, but Arnould stresses that it's “too early to make a concrete judgement as to what’s going to happen, although it is alarming.” Because there have been no formal counts conducted at sites, and with no reliable long-term monitoring data of the species in Australia, whether there has been a significant decline in breeding population remains speculative, he says. By the end of the austral summer, however, Arould hopes to compare this year’s breeding success to data he's collected over the past three years as part of an acoustic monitoring project tracking occupancy in shearwater burrows.
Both Arnould and Barrand suspect that the lack of Short-tailed Shearwaters this year and the mass die-offs are results of a larger marine ecosystem breakdown in the Northern Hemisphere, specifically in the Bering Sea. Warmer ocean temperatures exacerbated by this year’s Northeast Pacific marine heat wave have likely disrupted the base of the region’s marine food web, and in turn caused migrating seabirds and marine mammals arriving at their seasonal foraging locations to find their aquatic prey missing. This summer, Short-tailed Shearwaters made up more than 95 percent of the total collected seabird carcasses that washed-up on Alaskan coasts. Mass seabird die-offs linked to the ecosystem's food shortage have become an annual occurrance in Alaska since 2015, with this year’s death toll estimated at hundreds of thousands.
“Correlation doesn't prove causality, but it's highly probable that what’s impacting shearwaters in the northern summer is having a carryover effect to numbers down here during the breeding season,” Arnould says.
Birds that did not starve to death likely needed more time to build up energy and fat to start their migration, which would have caused them to arrive at breeding grounds later than usual. Those that were in especially poor condition may have decided to skip breeding completely and head straight to their southern foraging sites in Antarctica, which would have contributed to the reduced numbers on several breeding islands.
“The question is, did a lot of them decide not to breed because of poor conditions, or are there just not that many around?” Arnould says. If it was a one-off year of skipped reproduction, the resilient species can easily recover the next year; but if the population has already declined significantly as a result of the die-offs, the impact is more serious as it limits the number of potential fledglings for years to come.
Although the consequences of the recent die-offs are still uncertain, for Barrand, the years of data from community-science seabird monitoring in Alaska is enough to convince him that the Short-tailed Shearwaters have already taken a hit. “I think the time to start worrying is well past,” he says. “The deaths in the Northern Hemisphere have been well-documented over the past five to six years, and it’s taken this long to suddenly notice an effect here in Australia.”