If you’re ever eating food at the beach, watch out! A hungry gull could be waiting nearby to steal your snack. It turns out that these sneaky thieves decide which treats are best to eat by watching humans.

Scientists from the University of Sussex tested this by placing two unopened chip bags, one green and one blue, on a beach in England. A researcher sat nearby and ate from a third bag, either green or blue. Of the gulls that investigated the unopened bags, about 95 percent of them pecked only at the bag that matched the researcher’s.

This shows how intelligent gulls are, says Kiera Smith, a zoology student who led the experiment. The birds chose what to eat by observing and mimicking another species. So if you’re on vacation enjoying crispy fries, nearby gulls probably notice—and want a taste too!