Scientists Now Know Why Some Seabirds Eat So Much Plastic

It smells like food.

Southern giant petrel  (NOAA Photo Library)

Gabrielle Nevitt remembers the first time she noticed the smell. It was sometime in the early 1980s and she was an undergraduate, heading out to sea on a research trip.

“Hey, can you smell it?” a fisherman said to her. “It smells like a productive area for fishing.”

She didn’t know what he meant at the time, but the odor has been a familiar part of her life for the last few decades. It’s hard to describe, but it’s unmistakably marine: Nevitt calls it “like oysters” or “kinda seaweed-y.” It’s the scent of dimethyl sulfide, or DMS—a gas that’s been described as a keystone molecule, because it influences everything from the presence of birds to the formation of clouds.

DMS is produced when microscopic animals graze upon the algae that grows near the ocean surface. Since it doesn’t dissolve easily in water, it eventually makes its way into the overlying air. That’s why passing sailors can smell it. Some of the gas rises into the atmosphere, where it seeds the creation of clouds. And other DMS molecules drift through the skies until they find their way into the noses of birds.

Tube-nosed birds—the albatrosses, petrels, and shearwaters—soar across the open oceans, relying on their keen sense of smell to find food. For them, DMS is intensely alluring. This attraction makes sense. DMS gives away the presence of clouds of plankton, and so acts as a loud dinner bell, telling the birds where meals can be found. Nevitt proved this in 1995 by sailing through the southern Atlantic and releasing oil slicks that were scented with either DMS or cod liver oil. Seabirds like white-chinned petrels, prions, and storm petrels flocked to the former over the latter.

But sometimes, that bell rings incorrectly—and with disastrous consequences.

Every year, people dump 8 million tons of plastic garbage into the oceans, which provides hard surfaces on which microbes and algae can grow. “If you work at sea, you know that plastic gets junk growing on it,” says Nevitt. “And it smells like DMS.” Could that explain why an estimated 90 percent of seabirds have swallowed plastic flotsam before, risking their lives for the sake of unpleasant and indigestible mouthfuls?

To find out, Matthew Savoca, one of Nevitt’s graduate students at the University of California, Davis, analyzed data on 25 species of seabirds, which vary considerably in how strongly they respond to DMS. “Right off the bat, we saw a trend: It looked like the DMS responders were consuming more plastics,” says Nevitt. On average, these birds ingest plastic five times more frequently than species that ignore the telltale scent. “I thought: Oh my God, we have to do an experiment!” says Nevitt.

Savoca sailed off the coast of California with bagfuls of beads, made from three common types of plastics. He tied these to buoys and left them for three weeks. After he recovered them, he turned to Susan Ebeler—a chemist who typically studies the smells of wine. She turned her instruments onto the beads and showed that they were giving off DMS—at the same concentrations as plankton, and at levels that seabirds can easily detect.

This discovery might explain why these birds would eat something as clearly weird and visually conspicuous as a scrap of plastic. “We think of these birds as little humans flying around with our senses, and they’re not,” says Nevitt. “Their visual acuity is poorer than ours, and this world of smell is extremely important.” Perhaps plastic acts as a sensory trap, luring them in with the fragrant promise of food and overriding any countervailing information from their other senses.

“It takes us one step closer to understanding why seabirds may be predisposed to plastic ingestion,” says Denise Hardesty from CSIRO in Australia. “At the same time, it reminds us that the entire story can’t be encapsulated by one factor like attraction to DMS,” since some of the species that eat the most plastic aren’t that attracted to DMS. These include the Laysan albatross, which became a poster child for plastic consumption thanks to photographer Chris Jordan’s series of iconic shots.

The plight of the albatross remains a mystery, but Nevitt wants her work on DMS to throw some light onto lesser known species like storm petrels. Since they are drabber and less majestic than albatrosses, they are often ignored. And since they nest underground, they’re often neglected “because you have to stick your arm down a burrow to sample them,” says Nevitt. But they too are suffering from the rubbish that we throw into the oceans.

Other ocean animals, including penguins, sea turtles, sharks, and perhaps large whales, are also known to detect or track DMS. Perhaps we are also inadvertently luring them towards their last suppers.