When the Harry Potter books debuted 20 years ago, they launched a $25 billion industry and an army of wizard-loving muggles. Most of the fun is light-hearted enough: popular sorting hat quizzes, friendly games of quidditch. But that international obsession has an unexpected cost, reports Shaunacy Ferro for Mental Floss: It’s fueling an illegal trade in owls.

The books are filled with owls, from Harry’s BFF Hedwig to Draco Malfoy’s mail-delivering eagle owl. But those fictional owls could be linked with a black market in the real world, reports Ferro.

In a new study in the journal Global Ecology and Conservation, researchers describe what they call the “Harry Potter Effect” in Indonesia. Birds are already popular pets there. But after the release of the Harry Potter books in the early 2000s, owls rose in popularity. The creatures rarely appeared in bird markets before the books were released, the researchers write, only constituting roughly 0.06 of the percent of the black market birds. But by 2008, that number has risen to 0.43 percent.

Majority of owls for sale in the markets were caught in the wild, which is illegal in Indonesia. And the researchers worry that growing demand could deplete owls in the wild.

Expanded internet access and social media in Indonesia during this period could also have played into the owl trade increase. Though this could be a non-Harry Potter related reason for the uptick, the internet could also have paved the way for wider conversation about the books online. But there are other clues to the Harry Potter trade connection: “Whereas in the past owls were collective known as Burung Hantu (“Ghost birds”),” the researchers write in the study, “in the bird markets they are now commonly referred to as Burung Harry Potter ('Harry Potter birds')."

Indonesia isn’t the only place to contend with the world’s newfound love of owls. In 2010, the BBC reported that India has a Harry Potter problem, too. The Indian environment minister claimed that the books were driving an increase in illegal owl trade, but noted that owls are also used for sacrificial purposes. And in the UK, where keeping owls is legal, animal advocates pleaded with the public to keep their pets in large aviaries, not small cages like Hedwig’s.

Not all Harry Potter effects are bad for animals. The books have brought attention to newly discovered species, like the dementor wasp, sorting hat spider and Harryplax severus crab. And the love of owls has also inspired amateur bird watchers to team up with ornithologists to track owls in the wild.

But this latest owl craze, is a reminder that acting out fictional tales can have real-life consequences—and that you should probably think twice before you choose a Harry Potter-themed pet.