The Birds Revisited

How some of the ocean’s smallest creatures triggered a real-life invasion of crazed seabirds, and helped inspire Hitchcock’s famous thriller

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A sooty shearwater flies over the Monterey Bay. P.C. – Alan Schmierer

In the wee hours of the morning on August 18, 1961, the residents of Capitola, California were awakened by a surreal phenomenon: Thousands of crazed sooty shearwaters were flying erratically through the streets, disgorging bits of fish, and crash landing, kamikaze-like into street lamps and roof tops. A few brave souls ventured outside to investigate, but immediately retreated. The birds, upon seeing the light from their flashlights, flew directly toward them.

The story was front-page news in the Santa Cruz Sentinel by morning. “Residents […] were awakened at about 3 a.m. today by a rain of birds, slamming against their homes,” a bewildered reporter wrote. “Dead, and stunned sea birds littered the streets and roads in the foggy, early dawn.”

A few days later, famed filmmaker Alfred Hitchcock phoned the paper to request a copy of the article. He was researching a new thriller, based on a novel by Daphne du Maurier. Within two years, The Birds hit the silver screen, terrifying audiences across the nation with the story of crazed seabirds pelting the residents of a small, coastal town in northern California.

Experts initially suggested that the sooty shearwaters had been disoriented by the dense fog that had descended upon Capitola that fateful morning. But the true answer, reported over fifty years later by scientists from Louisiana State University in the scientific journal Nature Geoscience*, was far more sinister: The birds had been poisoned by tiny marine organisms, called diatoms.

Dangerous Diatoms

Diatoms are tiny, single-celled algae that live in both marine and freshwater environments. The vast majority of diatoms are harmless – in fact, they’re an important source of food for many aquatic animals – but a conspicuous minority produce toxins that can poison birds, marine mammals like seals and sea lions, and humans.

Some diatoms of the genus Pseudo-nitzschia produce a toxin called domoic acid. When plenty of nutrients and light are available, Pseudo-nitzschia can occur at very high densities; these high concentrations of diatoms or other algae are commonly referred to as “algal blooms.” When Pseudo-nitzschia species bloom, shellfish, and small fish like anchovies and sardines, consume large quantities of domoic acid. The toxin does not harm these animals, but it does accumulate in their tissues.

When sea birds, like sooty shearwaters, consume shellfish and anchovies after a Pseudo-nitzschia bloom, the domoic acid poisons them.

The Real Whodunit

In birds, marine mammals and humans, domoic acid acts as a neurotoxin. It interferes with the transmission of nerve signals in the brain, causing lethargy, disorientation, vomiting, seizures, amnesia, brain damage and sometimes death. However, none of this was known during the 1961 attack on Capitola.

It wasn’t until 1991, when similar symptoms affected large numbers of brown pelicans in the same area, that domoic acid was identified as the culprit. Scientists discovered large quantities of domoic acid in the birds’ stomachs, and it was known that they had been eating local anchovies. When scientists looked inside the anchovies’ stomachs, they found large numbers of Pseudo-nitzschia diatoms. The west coast state health departments took immediate action: They closed the shellfish and forage fish fisheries before any humans could suffer the same fate as the birds.

After the 1991 incident, researchers began to suspect that the original 1961 event had also been caused by domoic acid poisoning. However, no one could prove it until 2012, when the Louisiana State research team, led by Sibel Bargu, finally solved the mystery.

Bargu and colleagues examined a number of preserved water samples from 1961, which contained plant-eating zooplankton – tiny ocean predators that partly feed on diatoms like those in the genus Psuedo-nitzschia. They looked in the zooplankton stomachs to see what they had been eating, and found that 79% of the diatoms inside were toxic Pseudo-nitzschia species.

“This […] supports the contention that domoic acid caused the seabird frenzy that eventually led Hitchcock to make his film,” Bargu concluded in her 2012 paper.

The Future of Harmful Algal Blooms

Research on harmful algal blooms – like Pseudo-nitzschia blooms – has accelerated over recent years. However, how and why blooms form continues to be a mystery. Possible culprits include warm water, low wind conditions, and pollution from human homes and farms.

To date, harmful algal blooms have been reported in every coastal state, and the number of these blooms may be on the rise. Just this winter, an exceptionally large and persistent Pseudo-nitzschia bloom occurred off the coast of California, and high levels of domoic acid were found in Dungeness and Rock crabs. This discovery prompted a six-month closure of California’s lucrative commercial crab fishery.

Because of the large economic and health impacts of harmful algal blooms, scientists are working to predict them before they occur. Perhaps in the coming years, the residents of Capitola won’t need a hailstorm of disoriented birds to confirm the presence of a harmful algal bloom.

*PDF available upon request.


Bargu, S. et al. Mystery behind Hitchcock's birds. (2012). Nature Geoscience 5:2-3.

California Department of Public Health. CDPH Issues Warning about Dungeness and Rock Crabs Caught in Waters Along the Central and Northern California Coast. (2015) at

California Ocean Protection Council. Commercial Dungeness Crab Season Opener Delayed and Commercial Rock Crab Season Closed. (2015) at

NOAA - News and Features - What is a harmful algal bloom? (2016) at

Trabing, W. and Santa Cruz Sentinel. Birds "Invade" Santa Cruz, California. (1961) at