The chemical that provokes the reaction of the birds is called dimethyl sulfide. This releases a scent that seabirds know as a cue to hunt and eat their prey. It is released by hytoplankton as it gets eaten by a predator, like krill, which birds feed in. When the birds catch a whiff of dimethyl sulfide, they know it is time to eat. Coincidentally, plastic in the ocean also releases dimethyl sulfide. It is for this reason that the birds willing consume the plastic.