Balloons..

A new study finds balloons are the deadliest plastic out there for seabirds. Even though balloons represent only 2 percent of all plastics ingested by seabirds, they are responsible for 42 percent of plastic-related deaths. Marine birds are a 32 times as likely to die from eating a little fragment of a popped balloon than they are from a hard plastic like a LEGO brick or straw, the study found. The reason balloons are so lethal is that they’re able to squeeze into stomach cavities and then open up and block them, the researchers said.
A hard piece of plastic has to be the absolute wrong shape and size to block a region in the birds’ gut, whereas soft rubber items can contort to get stuck. Also, balloons look particularly like seafood to the birds.
Link to research: https://www.nature.com/articles/s41598-018-36585-9