Every year, a fierce predator kills billions of native birds and mammals, becoming one of the largest conservation threats in inhabited areas.

That predator? The housecat.

Domestic cats (Felis catus) are one of the most abundant carnivores on the planet, numbering over 600 million. The number of domestic cats is so great that it outnumbers the collective number of all other felines in the world.

For a long time, cats have been a point of contention for animal lovers: on one hand, they are beloved by millions of people all around the world. But on the other hand, they can be an absolute plague on local species when allowed to roam free.

It’s been shown for quite a while that cats post a huge ecological threat. They kill billions of birds and mammals every year, and are quite possibly the most threatening invasive species in the world.

In a new study, researchers worked with citizen scientists to track 925 pet cats from six countries. The cats were tracked with a small GPS to see how far the cats go, and the owners were surveyed to see what prey the cats bring home.

“Our citizen science cat-tracking study represents one of the largest studies of animal movement,” the researchers write in the study.

The first results seem quite surprising. Contrary to popular belief, cats don’t seem to have that large of a geographical range. There are exceptions — one British cat called Max walked almost two kilometers back and forth along a road between two neighboring villages, twice — but the average area of house cats 0.036 square kilometers. In total, only 0.3% of animals moved more than 1.1 square kilometers around their house. In other words, most casts never stray more than 100 meters (or yards) away from their home.

But the fact that they had such a small area of activity isn’t necessarily a good thing — instead, the damage they produce is strongly concentrated around individual houses.

Owners reported that their pets killed an average of 3.5 prey items/month, but this could be an underestimation, since it was based on the number of preyed animals cats brought to their house.

Of course, a single pet cat doesn’t kill as many animals as a wild cat does to survive. The house cat doesn’t need to hunt to survive, but housecats still hunt if they are given the chance, and this impact happens in a smaller area.

So in a given area, pet cats kill 2-10 times as many other animals as similarly sized wild predators.

It’s important to note that most cats are still harmless — not because they don’t have the killer instincts, but because they don’t go outside. The majority of cats (about two-thirds to three-fourths) are harmless and seldom set foot outside.

The moral of the story is not to give up on housecats, but to be mindful of the environmental impact they can have — and think twice before you let them roam outside.