In November 2007, hundreds of dead and bedraggled seabirds washed up on the shores of Monterey Bay in California. There were no cuts on their bodies and no signs of a struggle. Now it appears that that it was killer foam that sent them to their deaths.

The birds, washed ashore in three distinct incidents, were covered in a slimy, pale yellow-green material. The material, whose pungent smell reminded researchers of linseed oil, dried out to leave a pale yellow crust. Cleaning the feathers of the survivors, feeding them and placing them in warm water helped them recover within 10 days.

The suspicion initially was that the mysterious deaths might be related to the Cosco Busan oil spill in San Francisco Bay the same month, or to controversial aerial insecticide spraying on the Central Coast to control the light brown apple moth.

But Raphael Kudela of the University of California, Santa Cruz, who helped with the rescue, says autopsies of the dead birds turned up no evidence that the oil or insecticide had been involved. Instead, researchers found that they had died from hypothermia - a puzzling cause of death for seabirds.

The timing and location of the three waves of strandings offered a clue to what had happened. Each coincided with a distinct and intense "red tide" - a bloom of microscopic algae suspended at the surface of the ocean. The algae that caused the tides, a species called Akashiwo sanguinea, produces powerful surfactants - the "wetting agents" that are added to soaps and shampoos to make them foam.

"The action is similar to beating egg whites to make meringue," says Kudela. "The proteins in the egg whites form bonds, making them stiff and fluffy. The same thing happens with algal proteins if they are concentrated enough and beaten enough by wave action." This is what produces the yellowish sea foam that can sometimes be seen along shorelines. But this foam is not toxic, so how could it have killed the birds?

Kudela believes that unusual circumstances conspired to make it lethal to birds in Monterey Bay in November 2007. When he and his colleagues analysed samples of the slime they cleaned off the birds' feathers, they found evidence of the algal surfactant. The surfactant disrupted the feather structure, making them less waterproof and exposing the birds to the cold that eventually killed many of them.

"This was a 'perfect storm' scenario," says Kudela. While red tides are not uncommon along the Californian coast, those of 2007 were unusually large and lasted much later into the winter than usual, persisting as winter storms drove large swells and waves into the bay. That brought the algal surfactant into contact with the large population of migratory birds in the bay, many of them moulting and thus already in a weakened state.

No event like this has ever been documented before, but the researchers believe something similar might have happened in 1999, when a large number of mysterious bird deaths also coincided with a large red tide. Kudela says large, late red tides are becoming increasingly common, however, so the killer foam may return in years to come.

Journal reference: PLoS ONE (in press)