Up to 90 percent of all seabirds eat plastic. In the 1960s that number was only about five percent, but by the 1980s it had risen to a staggering 80 percent. Researchers have found seabirds with all manner of plastics in their digestive tracts—bottle caps, plastic bags, broken-down rice-sized grains of plastic, synthetic clothing fibers and more, according to Laura Parker at National Geographic. It’s one of the factors contributing to a stomach-churning 70 percent drop in seabird numbers since the 1950s.
But bottle caps and Barbie doll heads don’t really look like the small fish and krill many seabirds favor for their meals. So why do so many species of birds actively hunt down these chunks of plastic? A new study in the journal Science Advances suggests that certain chemicals on the plastics mimic the smell of food, tricking the birds into thinking that these colorful bits are lunch, reports Chelsea Harvey at The Washington Post.
Ocean algae produces a chemical called dimethyl sulfide, or DMS—particularly when the algae is being digested by krill, tiny crustaceans that fill much of the worlds oceans. It’s believed that the chemical is part of the mutualistic relationship between birds and algae. The birds smell the DMS, which alerts them that krill are in the area. When they eat the krill, it reduces the number of krill chowing down on the algae.
But when plastic collects in the ocean it tends to also accumulate algae and other tiny bits of organic matter on its surface, writes Harvey, and these emit DMS, attracting the birds. “What we think is going on is that the plastic is emitting a cue that is getting [the birds] into moods to eat,” Gabrielle Nevitt of the University of California Davis, the study’s senior author, tells Harvey.
To arrive at this conclusion, the researchers filled mesh bags with beads of three different types of common plastics, high-density polyethylene, low-density polyethylene, and poly-propylene, according to a press release. They then tied the bags to a buoy and let them soak in the ocean for three weeks, after which they analyzed the plastics at UC Davis’s Robert Mondavi Institute for Wine and Food Science. This analysis showed that these beads were emitting a large amount of DMS. Yet plastic that had not soaked in the ocean did not give off any DMS.
The researchers also teased through 55 studies to figure out which birds are most likely to ingest plastic, reports Hannah Devlin at The Guardian. They found that procellariiform seabirds, which includes albatrosses, petrels and shearwaters, were almost six times as likely to snack on plastic compared to other seabirds—a finding that aligns with the chemistry. Those particular species strongly rely on their sense of smell to find food, which is weaker in other birds, making them more sensitive to DMS.
“This study shows that species that don't receive lot of attention, like petrels and some species of shearwaters, are likely to be impacted by plastic ingestion,” Nevitt says in the press release. “These species nest in underground burrows, which are hard to study, so they are often overlooked. Yet, based on their foraging strategy, this study shows they’re actually consuming a lot of plastic and are particularly vulnerable to marine debris.”
The hope is that materials scientists may be able to produce plastic that accumulates less algae. “[The study] provides a salient mechanism for how this group of birds might be detecting plastic and consuming it,” Nevitt tells Harvey. “And once you have a better idea of how a mechanism might work, you’re in a better position to potentially mediate that.”
But engineering new types of plastic is a big stretch, say the authors. The best and easiest strategy is to keep the plastic out of the oceans in the first place.