In 2006, biologist John Marzluff walked across the University of Washington campus wearing a rubber caveman mask. He and his team netted seven American crows, fitted them with leg bands, and released them unharmed.

The crows did not forgive him.

For years afterward, anyone who strolled through campus in that caveman mask was met with a storm of harsh, scolding caws. It did not matter who was behind the mask. On one walk, 47 of the 53 crows Marzluff passed scolded him. A second mask, a likeness of then Vice President Dick Cheney, drew no reaction at all.

Then the grudge spread. Crows that had never been trapped joined in, including birds that hatched years after 2006. Young crows learned the danger by watching older ones react, and the warning traveled at least three quarters of a mile from the original trapping sites.

The scolding grew stronger for about seven years and peaked around 2013. As the original crows and the birds they had taught grew old and died, it slowly faded.

In September 2023, Marzluff put on the caveman mask once more and walked the campus. Not a single crow scolded him. The grudge had lasted 17 years.