Scientists just confirmed the largest bird-killing event in modern history

When Heather Renner and her colleagues began noticing thousands of common murres washing up on Alaskan beaches nearly a decade ago, they knew something was terribly wrong.

It would take years of study to confirm they had witnessed the largest die-off of any bird species ever recorded in the modern era, according to new research published in the journal Science on Thursday.

Back then, the waters of the northern Pacific Ocean where these sleek seabirds spend much of their time were unusually warm, the start of what would become the largest marine heat wave on record. The murres that made landfall were emaciated, showing they had starved to death. The scientists knew then that the die-off was one of the most visible and extreme examples of how climate anomalies in the warming world can throw wildlife populations into turmoil.

But after seven years of monitoring common murre populations across 13 nesting colonies in Alaska, Renner and her colleagues at the U.S. Fish and Wildlife Service now realize they hadn’t fully grasped the scale of what was happening to those birds.

“It was just way worse than we thought,” said Renner, the supervisory wildlife biologist at the Alaska Maritime National Wildlife Refuge.

The research by Renner and her colleagues found that more than half of Alaska’s common murres died — some 4 million birds — in what they described as the largest mortality event of any non-fish vertebrate wildlife species reported during the modern era. The killing was an order of magnitude larger, she said, than the hundreds of thousands of murres that perished in the 1989 Exxon Valdez oil spill in Alaska.

“We’ve had lots of long-term declines that have been observed in wildlife,” she said. “But what’s really different here - that we haven’t seen before - is this really swift catastrophe where in one year we have half the population of this really abundant seabird just wiped out.”

Before the two-year marine heat wave that ended in 2016, Alaska had an estimated 8 million common murres - a quarter of the world’s population - spread across abundant colonies in the Gulf of Alaska and the Eastern Bering Sea. These black-and-white seabirds nest in dense clusters among shoreline cliffs during the summer months and then head to the ocean the rest of the year to feast on schools of small fish such as capelin and sand lance, herring and krill.

Some populations of such forage fish collapsed during the heat wave as temperatures in the north Pacific spiked by 2.5 to 3 degrees Celsius above normal. Many predators that rely on them suffered. The number of Pacific cod in the Gulf of Alaska crashed by 80 percent between 2013 and 2017, the study noted, leading to the closure of the commercial fishery in the Gulf of Alaska, which has since reopened but at a lower level. The number of humpback whales in the north Pacific fell by 20 percent. Other species that are crucial to the economic lifeblood of Alaska - from chum salmon to snow crabs - have also plummeted.

Not everything suffered. A review of a wide range of Gulf of Alaska species from predators to plankton showed about half came out “neutral,” and others did well after the heat wave. That’s part of the enduring mystery of the event. The thick-billed murre, which looks almost identical to the common murre and nests among them in the same areas, did not suffer the same type of precipitous population decline.

Researchers are trying to understand the disparity; whether they went elsewhere in the ocean or were able to switch to a different type of prey.

They know this rattling of the food chain will likely not be an isolated event. With the warming climate, marine heat waves are predicted to become more frequent and intense. The dead murres were a visible part of a broader upheaval under the waves.

“They were one of the first warning signs of a system in distress,” said Megan Williams, a fisheries scientist in the Arctic program for Ocean Conservancy, a nonprofit, who was not involved in the study. “For some other species it took us a while longer to see the effects.”

Brie Drummond saw the carnage unfolding from the beginning. A wildlife biologist who has worked with seabirds for two decades, she spends months at field camps on islands in the Gulf of Alaska monitoring murre colonies. In 2015, she saw bodies of starving common murres washing ashore near Homer, where she lives. Drummond and her colleagues also noticed a dramatic decline in the number birds returning to the cliffs to breed.

A lot of seabirds can skip breeding for a year or two if conditions are bad, Drummond said, so she and her colleagues were unsure if many remained at sea or had died. After years of monitoring these colonies, the answer is now clear.

“They’re not coming back because they died,” said Drummond, of the Alaska Maritime National Wildlife Refuge, who is a co-author of the paper.

The study compared a seven-year period (2008-2014) before the marine heat wave and another seven-year stretch afterward (2016-2022) and found that murre numbers fell 52 to 78 percent at 13 colonies across two large marine ecosystems in the Bering Sea and the Gulf of Alaska. And since that die-off, the common murre population has remained down, showing no signs of recovery.

“This isn’t just some sort of fluke. It’s happened over this huge area, and it’s showing up at all the colonies for which we have reliable data,” Drummond said.

Drummond now oversees field camps on Chowiet and St. Lazaria islands, where technicians use boats and spotting scopes to painstakingly count murres in specific plots and extrapolate their numbers across the wider area. Some sites involved in the study have been monitored going back to the 1970s, the researchers said.

These murre colonies can comprise hundreds of thousands of birds, so to the uninitiated it may seem like they’re still abundant. But for the researchers who have tracked them meticulously for so long, it’s been a shocking decline.

“It’s an absolute gut punch,” said Renner. “I’m not worried that we’re going to lose common murres in the short term. But the ecosystems that Alaskan communities care about so much are going through just really tremendous shifts.”

Williams, of Ocean Conservancy, praised the Science study for harnessing years of data to show that the post-heat-wave ecosystem has fundamentally changed - an important lesson for people who attempt to manage fisheries and conserve this wildlife.

“We can’t expect the Bering Sea, for instance, and the Gulf of Alaska, to stay as productive as they have been for the last 50 to 100 years,” she said. “Some of these species may not be supported by the marine ecosystem anymore. And that’s really tragic.”