‘The Birds’ was partly based on a true story.

With apologies to anyone who already found The Birds terrifying while under the impression that it was wholly fictional: Alfred Hitchcock’s avian thriller was partly based on a true story. Said event took place on California’s Monterey Bay in August 1961, when “thousands of crazed seabirds” called sooty shearwaters were seen regurgitating anchovies and flying into objects before dying on the streets. The Master of Suspense happened to live in the area, and called the Santa Cruz Sentinel — which had reported on the strange goings-on in its August 18 edition — for more information. Long after his movie was released two years later, the bizarre event remained shrouded in mystery: What would inspire birds to act this way, and were they as malicious as they seemed in Hitchcock’s movie?

The truth ended up being both straightforward and a little sad. The scientific consensus is now that the birds were poisoned by toxic algae found in a type of plankton called Pseudo-nitzschia. The birds weren’t attacking anyone; they were disoriented and barely in control of their actions. That explanation is absent from Hitchcock’s thriller, which also drew inspiration from Daphne du Maurier’s short story of the same name. (Hitchcock’s Rebecca was also a du Maurier adaptation.) A resounding success, The Birds is widely considered one of Hitchcock’s greatest works, alongside Psycho, Vertigo, Rear Window, and North by Northwest