Tippi Hedren attacked in “The Birds.” (EVERETT COLLECTION)

The reason for the birds’ erratic behavior has remained a mystery — until now.

Sibel Bargu, one of the authors of a new study published in Nature Geoscience, told USA Today her team believes the birds were poisoned by toxin-producing algae. The researchers looked at the stomach contents of turtles and seabirds gathered in the affected area in 1961 and found toxins that cause nerve damage present in 79 percent of the plankton the creatures had eaten.

Another researcher told USA Today that leaky septic tanks from newly developed homes may have “fed the toxic algae.”

Somehow knowing the probable truth doesn’t make the 1963 masterpiece any less frightening.