The toxin is produced by marine algae and accumulates in shrimps, fish, crabs and the seabirds that feed on them. In high enough concentrations, the toxin causes vomiting, severe dementia and extreme aggressiveness.
It was just such behaviour that occurred in 1961 in the small Californian seaside resort of Rio del Mar. A flock of shearwaters, or puffins, caused extensive damage in the town by flying into streetlights and windows and pecking at or vomiting over frightened residents.
Hitchcock, who was living nearby at the time, rang up local newspapers for cuttings of their front-page reports on the incident because he said he was collecting ''research materials''. Two years later he made The Birds, a film based on a Daphne du Maurier short story.
David Garrison, a marine biologist at the University of California at Santa Cruz, began the research into the 1961 incident after a similar case occurred in 1991 near his university.
Groups of pelicans and cormorants that were gathering on the coast seemed ''intoxicated, swimming in circles and squawking pitifully''. More than 100 corpses were collected over the next few weeks.
Dr Garrison and colleagues isolated a chemical, demoic acid, from the corpses and found that it had accumulated in the bodies of the birds after they had gorged on shoals of anchovies that had fed on the algae that had ''bloomed'' in the town's bay.
Realising the algae probably produced the toxin in previous years, the researchers trawled through stored specimens to see if they could find evidence of similar incidents. After analysing preserved samples of marine life dating back to 1977 they found the bay near Santa Cruz had been tinged with the poison at least 12 out of the past 17 years. Earlier incidents were probably equally common, but it was only after Dr Garrison had been to the visitors' centre at nearby Rio del Mar that he saw the actual cuttings of the local newspapers that had inspired Alfred Hitchcock.
According to the 1961 reports the shearwaters bumped into buildings, knocked out car headlights, chased people around the streets and vomited anchovies. ''It was the right time of year for a bloom. The incident could be very well explained by the seabirds feeding off demoic acid-tainted anchovies,'' Dr Garrison said.
The algae appear to produce demoic acid when they run low on vital nutrients in seawater, Dr Garrison said.
The chemical acts on nerves to produce a ''kind of drunkenness''. Hitchcock's birds, he said, were probably high on demoic acid.Reuse content