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.

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“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.