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It wasn’t the seabird carcasses that were unusual. Birds that die at sea frequently wash ashore. On regular surveys to tally bodies from Alaska to Central California, for example, volunteer data collectors might find 50 dead birds a day during normal peak periods at some beaches, such as after breeding season when lost fledglings appear or during migration when birds succumb to exhaustion. But over the past decade, a string of major die-offs along the Pacific coast have caused concern among researchers. 

Mass mortality events—when the seabird death toll jumps much higher—occur naturally around once per decade. Recent examples of this phenomenon, however, have been particularly devastating and exceed that frequency. For instance, in 2014, thousands of Cassin’s Auklets began washing up dead along much of the Pacific coast, with tens of thousands more estimated to have died at sea. Then another seabird die-off struck in Alaska in 2015—by the following year, an estimated 1 million Common Murres died. Large seabird tolls continued to hit year after year across the region. 

By 2019 scientists documented five mass mortality events—three of which had estimated death tolls of more than 250,000 birds—over a five year period in the northeast Pacific. “Each individual mortality event was among the largest mortality events that have ever been documented anywhere in the world,” says marine biologist , a researcher at the University of Washington. “And then it just continued year after year.”

Alarm bells blaring, Jones and colleagues decided to sift through nearly 30 years of data, painstakingly collected by volunteers who help monitor Pacific coast seabird deaths as a way to study marine health. The , published in July, revealed that seabird die-offs were more likely to occur during marine heatwaves, which the authors defined as ocean surface temperatures reaching above the 90^th percentile for at least 6 days. If temperatures hovered up to 1 degree Celsius warmer than usual for six months or longer, it wreaked havoc on seabird populations. In those associated mortality events, five times as many birds died.

The new research suggests that, as oceans warm due to climate change, the future likely looks like a world with fewer seabirds. This summer, nearly half of global oceans are experiencing heatwaves. South Florida waters last month, stunning scientists and possibly setting a world record. The North Atlantic has been experiencing an unprecedented marine heatwave since May—its on record since 1850—and a huge swath of hot water moving across the Northeast Pacific Washington and Oregon. And that all started before El Niño arrived, a weather pattern that will amplify warming in the months to come.

Now that marine heatwaves are  across the globe, “we're entering a realm where birds have been dying off faster than their population can recover,” says ���Ϲ���’s Seabird Institute director of conservation science Don Lyons, who wasn’t involved in the study. “That tips this into much more of a serious issue.”

Scientists working on the Pacific coast have observed a relationship between marine warming and negative outcomes for seabirds since . The birds don’t suffer directly but rather from ecosystem cascade effects. Higher temperatures change the composition of the plankton community in the water; without access to their usual prey, birds starve. Warmer waters also foster the ideal conditions for disease outbreaks or algal blooms that choke wildlife. 

That’s why seabird mortality provides a window into the health of the ecosystem, and beach monitoring programs throughout the region arose to monitor both over the long term. Since 1993, more than 7,000 volunteers across four programs have collectively clocked 250,000 hours of surveys. It’s difficult work. “I don't know if you've ever tried to measure a wet, gross, dead cormorant, but it is not a good time,” Jones says. “We owe [the volunteers] everything.”

In the new study, the team used that data to categorize each mortality event by severity. During Category 1 events, for example, an average of 1 to 2 carcasses turned up every kilometer, about twice the typical rate; the most extreme Category 4 events see at least 8 carcasses per kilometer.  The analysis revealed that mortality events were fairly common, and not always associated with elevated sea surface temperatures. But the massive Category 4 events, which only happened a total of eight times over the three decades, were much more likely to happen following a marine heatwave. In other words, the marine heatwaves alone don’t cause mortality events, says study coauthor Julia Parrish, a University of Washington seabird expert. But they’re making them more probable, and far worse. 

When the team looked at a few of these larger heatwave-driven die-offs, they were also surprised to find what looked like a regular three-year pattern: a first wave of dead birds one to six months after ocean temperatures start spiking; a second wave about a year later; then, for the next roughly 16 months, beachcombers would notice a drop in carcass numbers. Only then did a more typical drumbeat of death resume.

For Parrish, who launched the Coastal Observation and Seabird Survey Team (), one of the groups that surveys beaches, the pattern was an “aha” moment that helps explain those quiet periods. “I didn’t realize at first that that was part of the story as well,” she says.

She now believes that those lulls represent a recovery period for seabirds after a die-off. Birds that survive presumably have access to more food and experience less competition and stress. They’d be more likely to successfully raise chicks, eventually allowing populations to recover. However, seabirds lay only one or two eggs a year. That low reproductive rate helps explain why researchers found recoveries happen over 16 months, as the researchers found. “This lull needs to be lengthy for them,” Lyons says. “They don’t breed like rabbits.”

But in a warming world, that reprieve is getting shorter, if it happens at all. More frequent and intense marine heatwaves mean new mortality events every year, and no time for the system to recover. In the short term, climate change may cause more seabird die-offs. But that pattern isn’t likely to hold forever, according to Parrish. Eventually, the ecosystem might simply support lower numbers of seabirds overall. “You wouldn't see continued mass mortality events,” she says. “You'd see a rearrangement of the whole ecosystem to accommodate that new normal.”

Those species that will fare best in a warmer future will likely be those birds that can follow their prey to cooler waters. Many tern species, for example, will adjust their hunting grounds at the drop of a hat, while albatrosses and puffins are more reluctant to change their routine. Some species are extremely loyal to their nesting islands. “Then that colony is kind of chained to a bad situation,” Lyons says.

Once heatwaves are underway, managers can’t always do much to help seabirds. But reducing other threats—such as predators, overfishing, or habitat destruction—can give birds a better chance at surviving such shocks. In Maine, where ���Ϲ��� and partners manage 10 nesting islands, intensive management has helped populations remain stable or even grow. But, Lyons notes, like so much else, the future of seabirds will ultimately depend on reducing carbon emissions.

While the study was based in the northeast Pacific Ocean, its implications worry scientists elsewhere. In Newfoundland, marine biologist of Memorial University has recently observed higher variation in the breeding success and die-offs of the several Northern Gannet colonies he’s monitored for decades. But it’s not all bad news, according to Montevecchi. Despite sea surface temperatures higher than normal last month, it’s been a good summer for the birds thanks to bountiful stocks of capelin this year. “What amazes me,” he says, “is the resilience of these seabirds. And that is the most hopeful thing.”