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In the summer of 2015, dead Common Murres started to wash up along the Alaskan coast. That wasn’t inherently alarming; such die-offs happen occasionally for murres and other seabirds like auklets and shearwaters. But as more and more bodies appeared on beaches from Alaska to California over the next year—roughly 62,000 in total—multiple breeding colonies also failed to produce any chicks. It became clear that something was very, very wrong.

As time went on, the event became regarded as the largest known die-off of a single bird species. Scientists determined the cause of death to be starvation, triggered by a massive two-year marine heatwave dubbed “the blob” that raised ocean temperatures by as much as and devastated the fish stocks that murres depend on. Mortality data suggested that perhaps 1 million Common Murres had died.

Now, nearly a decade after the event, scientists say the true death toll was much higher: 4 million Common Murres, or roughly half of the Alaskan population. It’s believed to be the largest wildlife mortality event documented in modern history, they .

“It was a gut punch to realize how much worse it was than what we had expected,” says Heather Renner, study coauthor and supervisory wildlife biologist for the Alaska Maritime National Wildlife Refuge.

The disastrous die-off shows how quickly the impacts of a changing climate can bring devastation to a wildlife population and offers a glimpse into what the authors say is a new reality of more frequent and intense marine heatwaves.

“Intellectually, we all realized, yes, that was a possibility,” says Don Lyons, director of conservation science at , who was not involved in the study. “But nobody psychologically could wrap our heads around that scale of loss.”

Even more worrying, he says, is that the team continued to monitor colonies after the heatwave and saw no signs of recovery in a state that’s home to around one-quarter of the total Common Murre population. Typically a die-off is followed by a surge in numbers; fewer birds means less competition for food and breeding sites. This time, much to scientists’ surprise and dismay, there’s been no rebound.

“We were looking at those numbers, and we’re thinking, ‘Wait a minute, this can’t be right,’” says study coauthor Julia Parrish, a University of Washington seabird expert. The lack of recovery combined with the death toll, she says, “was a double shocker.”

The researchers reached their findings by poring over decades of monitoring data from breeding colonies in the Gulf of Alaska and eastern Bering Sea. They found that, in the years before the blob emerged, some colonies grew and others shrank—normal fluctuations, according to Renner. Yet every monitored colony lost more than 50 percent of its population during the heatwave. Given Alaska’s massive size, it’s striking that the event was so uniformly catastrophic, Lyons says.

In the years after the heatwave ended, scientists waited for the birds to return. They hoped that colonies were sparsely populated not because the birds were dead, but because they were taking a break from breeding due to stress, Lyons explains. But if that was the case, they’ve had ample time to return to nesting sites, he says. Murre numbers in the colonies have risen from their near-zero low points, but there continues to be far fewer birds than before the heatwave. Current growth rates don’t show any sign of that changing.

Marine heatwaves around the world are becoming due to climate change. A 2022 analysis showed that seabird die-offs are more likely to happen during marine heatwaves, and at five times the scale. Mass mortality events of Pacific seabirds used to happen about once a decade; between 2015 and 2019, there were five including the Common Murre die-off. More frequent heatwaves leave bird populations with less time to recover.

Even without another heatwave, however, Alaska’s Common Murres haven’t bounced back, which suggests that the ocean ecosystem has fundamentally changed in such a way that fewer murres are able to survive, Parrish says. Perhaps their food supply has shrunk, or the huge die-off affected their social interactions, which help them find food and avoid predators. It’s difficult to tease out the exact reason why the birds haven’t recovered, Parrish says. But, she adds, it’s clear and concerning evidence that single climate-related events can cause long-lasting change. 

While the news is “incredibly deflating,” it highlights the importance of the hard work that goes into monitoring these colonies day in, day out, says Lyons. That is exactly what Renner’s team will continue to do.