In 2016, scientists became aware of a die-off of common murres, seabirds resembling flying penguins, that were found washed ashore from Alaska to California. A 2020 study estimated, based on an extrapolation from carcasses found on beaches, that roughly 1 million murres may have died, calling it “unprecedented and astonishing,” even “biblical.”

However, a new study undertaken by some of the same researchers suggests the death toll may have been far greater.

The research estimates that 4 million common murres (Uria aalge) died from 2014 to 2016 during the height of a marine heat wave nicknamed “the blob” in the north Pacific Ocean, with murre deaths seemingly peaking in early 2016. This makes it the largest mortality event not just among birds but of any non-fish vertebrate in the modern era, the researchers say.

“I think what really punched us in the gut was having a really abundant, widespread top predator in the marine ecosystem that, over a really short period of about a year, lost half of its population,” study lead author Heather Renner, a supervisory wildlife biologist with the U.S. Fish and Wildlife Service, told Mongabay.

To determine the scale of the loss, Renner and co-authors compared the common murre populations at 13 breeding colonies in Alaska for seven years before and after the heat wave. Until 2014, there were roughly 8 million murres in Alaska, about one-quarter of the world’s population. But during the heat wave between 2014 and 2016, there was a dramatic population drop-off of 52% to 78% at the 13 colonies, and no recovery thereafter.

The researchers, echoing the conclusion of the 2020 paper, write that the die-off was most likely from starvation due to reduced access to forage fish. Murre typically eat half their weight in food daily, or about 60 to 120 forage fish. The heat wave also affected predatory fish like Pacific cod (Gadus macrocephalus) and humpback whales (Megaptera novaeangliae).

The authors write that their study “is the first to show that climate impacts can be swift (1 year) and intense (eliminating half of the population).”

Tim Birkhead, a murre expert and professor emeritus at the University of Sheffield, U.K., not involved with the research, agreed that the finding was substantial. He noted the study showed that “the marine heatwave has had far reaching, long-lasting effects,” and said this was the latest “tragic” news for seabirds, which have been in substantial decline around the world in recent decades.

“The blob” that hit areas like the Gulf of Alaska in the Pacific Ocean in the mid-2010s has been linked to climate change, and more marine heat waves are expected.

“Common murre populations in the Gulf of Alaska and East Bering Sea will likely fail to recover to pre-heatwave levels before the next extreme warming event occurs,” the authors write.

Banner image: Common murres die-off in the Gulf of Alaska, in 2016. Image by David Irons via U.S. FWS (Public domain).