Researchers have untangled the mystery behind a die-off that felled hundreds of thousands of tough seabirds known as Cassin’s auklets in 2014 and early 2015.

It’s not a simple answer: The proximate cause was starvation, but in a study published by Geophysical Research Letters, scientists report that the most likely root cause was an anomaly in Pacific Ocean circulation that came to be known as the Blob.

“This paper is super important for the scientific community because it nails the causality of a major die-off, which is rare,” senior author Julia Parrish, a marine scientist at the University of Washington and executive director of the Coastal Observation and Seabird Survey Team, said today in a news release.

The Blob was a huge region of warm surface water that cropped up in the northeastern Pacific Ocean in late 2013 and persisted through 2014 and 2015. That warmer water had a big impact on the auklets’ main source of food, aquatic zooplankton known as krill and copepods.

Warm-water populations of zooplankton tend to be smaller than the species that favor cold water. Because of the Blob, those less nutritious species came to dominate the auklets’ traditional feeding ground. As a result, the birds’ diet shifted from the usual energy-rich fare to the equivalent of junk food.

An estimated 250,000 to 500,000 auklets died due to starvation in late 2014 and early 2015, and thousands of carcasses washed up on West Coast beaches ranging from California to British Columbia.

Parrish and her colleagues enlisted more than 800 citizen scientists to identify the dead birds and log locations and dates into an online database. State, tribal and federal wildlife experts could use that database to track the die-off in real time, and correlate the deaths with readings for temperature, ocean circulation and the abundance of prey.

“A lot of the evidence points to there being a very tangible link in the warming of the Pacific to changes in ecosystem structure that ultimately led to seabird starvation,” said lead author Timothy Jones, a UW postdoctoral researcher in aquatic and fishery sciences.

Four more bird mortality events have taken place since the auklet die-off — occurring farther north each time, all the way up to the Chukchi Sea in the Arctic. The affected species include murres, puffins, short-tailed shearwaters and northern fulmars.

“When we see these mass mortality events, that’s the ecosystem saying, in big neon letters, that something is wrong. This paper can be used as definitive proof of the impacts of a warming world, and it’s not a pretty picture,” Parrish said.

In addition to Jones and Parrish, the authors of the study in Geophysical Research Letters, titled “Massive Mortality of a Planktivorous Seabird in Response to a Marine Heatwave,” include William Peterson, Eric Bjorkstedt, Nicholas Bond, Lisa Ballance, Victoria Bowes, J. Mark Hipfner, Hillary Burgess, Jane Dolliver, Kirsten Lindquist, Jacqueline Lindsey, Hannahrose Nevins, Roxanne Robertson, Jan Roletto, Laurie Wilson, Trevor Joyce and James Harvey.

The Coastal Observation and Seabird Survey Team, BeachWatch and BeachCOMBERS also passed along acknowledgments to the citizen scientists who aided with data collection.

For more about the impact of warming oceans, check out this 2016 report from National Geographic, plus more recent reports from The Seattle Times and Hakai magazine.