Thousands of dead Alaska seabirds are washing ashore, for a fifth year. Experts call that a climate 'red flag.

For five years in a row, masses of dead birds have been turning up on Alaska shorelines.

The big die-offs involve different species in different places and different seasons of the year, but there are some common threads: The dead or dying birds are emaciated, and the die-offs correlate with a heat-up of the marine environment - at times an extreme heat-up.

Bird die-offs can happen once in a while naturally, but the magnitude of the losses and their persistence over five years is significant, said Kathy Kuletz, of the U.S. Fish and Wildlife Service.

"Every year, it seems like there's been a large die-off event of some kind, so that's unusual," said Kuletz, who heads the seabird section of the Fish and Wildlife Service's Alaska migratory bird management program. "Seabirds are indicators of the ecosystem's functions, if not ecosystem health. These are red flags that we have to pay attention to."

The latest die-off victims are short-tailed shearwaters, birds that spend their summers in the Bering and Chukchi seas after making ultra-long-distance migrations from the southern hemisphere.

Of about 9,200 birds found by mid-September from this year's die-off, about 90% have been short-tailed shearwaters, Kuletz said.

This year's bird die-off reports began in May, according to the U.S. Fish and Wildlife Service and the National Park Service. The reports started with accounts of dead murres and puffins showing up in and just north of the Bering Strait, but shearwaters began to dominate the statistics starting in June.

Initial reports came from the northern Bering and the southern Chukchi area, including the area near Bering Land Bridge National Preserve. Later in the summer, dead shearwaters began piling up in more southern areas, particularly in the Bristol Bay region of southwestern Alaska.

Shearwaters that were not dead were found to be extremely weakened, some of them trying to eat scraps from Bering Sea fishermen's nets, the agencies reported.

There were some reports of dead shearwaters found along Russia's Chukotka Peninsula, and also reports of some oddities - shearwaters gathering in large numbers last month close to shore in parts of the Gulf of Alaska, which is not their usual summer and fall habitat. Those shearwaters were spotted around Kenai Fjords and Glacier Bay National Parks as well as in the Kodiak region, the National Park Service reported.

The count of dead birds is likely only a tiny fraction of the full toll, Kuletz said. Most of the dead birds either sank or died in places where they were never seen by people, she said.

Short-tailed shearwaters breed in the areas off Australia. They come to Alaska to gorge on krill, tiny copepods, fish and a variety of other marine food. They typically dive as deep as 40 feet below the water's surface, and they are usually well offshore, generally far from places where people would see them, Kuletz said.

"They travel 10,000 miles a year and probably spend two-thirds of their time at sea," she said.

The shearwater die-off follows those of other species, and all happening at a time when water and air temperatures are much higher than normal, sometimes hitting record highs.

In 2015 and 2016, so many murres were found dead that the Fish and Wildlife Service classified it as a "wreck." The more than 21,000 bird carcasses discovered represented only a small fraction of those that died, and it is believed to be Alaska's biggest die-off of common murres in recorded history, according to biologists.

The murre die-off occurred mostly in the Gulf of Alaska region, with some events in the Bering Sea - and lingering reproductive failures at known colonies. Warm conditions were blamed for the murres' failure to find food, with warm waters resulting in reduction of prey, redistribution of prey or new competition for prey from other species, scientists said.

In subsequent years, the bird die-offs were centered in more northern regions, from the Bering up to the Chukchi Sea. Birds affected included puffins, northern fulmars, auklets and kittiwakes. Short-tailed shearwaters have been among the dead birds since 2017, but this is the year when they are turning up in massive numbers, according to the agencies.

At the same time, there is a dramatic warmup and ecological transformation in the region that is linked to long-term climate change. Water temperatures have been far warmer than normal - more than 10 degrees Fahrenheit above normal in places - and air temperatures have hit record highs, too. In the Bering, the deepwater "cold pool" that usually serves as a thermal barrier separating northern and southern species has substantially shrunk or disappeared.

Boreal species like pollock and Pacific cod are moving north, eating a lot of the same prey that the seabirds eat, Kuletz noted. Algal blooms have proliferated in the warm conditions, and there is a danger of toxins being released by those blooms, she said.

The bird die-offs coincide with other marine die-offs in Alaska and bordering areas. The National Oceanic and Atmospheric Administration has declared the large numbers of gray whale deaths along the entire Pacific coast and ice seal deaths mostly in the Bering Strait region to be "unusual mortality events" that justify extra resources for study of possible causes.

Also associated with warmer-than-normal temperatures - and lack of sea ice - is this year's record-early gathering of thousands of walruses on a Chukchi Sea beach near the Inupiat village of Point Lay.

While warmth is suspected to be behind the birds' deaths, the mechanism is yet to be determined. So far, there is no sign of disease, but other possibilities are under investigation.

Bird carcasses have been sent for examination at the U.S. Geological Survey's National Wildlife Health Center in Wisconsin, she said. Additionally, agency representatives and residents of Bering Sea communities have been trying to gather enough carcasses to send to the University of Puget Sound, she said. Experts at those laboratories have the ability to search for signs of algal toxins, among other possible proximate causes of death.

Local participation is an important part of the response and investigation, scientists say.

When waves of bird deaths emerge, early notice often comes from the Local Environmental Observer Network maintained by the Alaska Native Tribal Health Consortium.

The observers have been active. At times, they are reporting on simultaneous unusual occurrences, like the marine mammal deaths and bird deaths, said Mike Brubaker, the consortium's director of community environment and health.

"The question is, is this something that's affecting species, or is this something that's affecting ecosystems?" Brubaker said.

When it comes to dead animals, the quicker the reports come in, the better, he said.

"It's so important, especially with a potential culprit being hazardous algae, because you really need to get a fresh carcass to get any kind of meaningful analysis from it," he said.

A bloom that Brubaker spotted recently on satellite imagery that he regularly monitors is probably a mass of coccolithophores, experts determined. Coccolithophores are single-cell organisms that photosynthesize even though they are technically not plants. Coccolithophores do not produce toxins, but they thrive in warm, nutrient-poor conditions - and their blooms are becoming increasingly common in Alaska waters.

Scientists on a NOAA Bering Sea research cruise that will be studying fish stocks in the northern Bering Sea will also be looking for coccolithophores and for toxin-producing harmful algae, Kurletz said.

• This story originally appeared in Arctic Today and is republished here as part of Covering Climate Now, a global journalistic collaboration to strengthen coverage of the climate story.