Cat lovers may not like this news: The Polish Academy of Sciences in Poland has declared that cats are dangerous, adding them to its list of “invasive alien species.” The institute isn’t alone: In some Icelandic towns, household cats have a curfew.

There is some damning evidence on why cats should stay inside. In the U.S. alone, housecats kill more than 2 billion birds each year, adding to the extinction of 63 different species. It’s this reason that makes cats a problem for the ecosystem, says Laura Helmuth, editor-in-chief at Scientific American.

A number of studies have documented the effect household cats have on outdoor animals.

“A lot of [studies] show that much of the hunting that cats do is just invisible to us, invisible to their owners,” Helmuth says.

In one study, Helmuth put small video cameras on cats and followed what they did over most of a summer. She found the cats killed a lot of species in their neighborhood. Only 20% of the time would they bring their catch back home, leading many owners to believe their cats weren’t causing trouble.

More than 100 years ago, the Lyall’s wren was entirely wiped out by a cat named Tibbles in 1894. Helmuth says cats have that kind of impact because birds and other species have never experienced these predators in their evolutionary history.

A lot of species also make their nests on the ground, making them easy targets for cats to attack and eat chicks or adult birds.

“If you throw a cat a ball of yarn, they pounce,” Helmuth says. “They kill anything that catches their eye. And so it’s able to go through a nesting colony of birds and just kill every single one it sees without slowing down.”

While Poland and Iceland are taking steps to encourage owners to keep their cats indoors, Helmuth says awareness about the issue is growing in the U.S. For cat owners that may say their pet is happier outdoors, Helmuth lists a few reasons why they should stay inside: Outdoor cats die at a much younger age, get hit by cars, fight other cats and can contract diseases.

“I think a lot of it really needs to be kind of a cultural change,” she says, “where people understand the impact of their cat, and that keeping it indoors is something that every cat owner can do as part of their effort to make the environment safer and better for everybody.”

Jorgelina Manna-Rea produced and edited this interview for broadcast with Todd Mundt. Jeannette Muhammad adapted it for the web.

This article was originally published on WBUR.org.

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