Domestic cats are directly responsible for the extinction of a number of animal species around the world, including 33 bird species. In the U.S., the popular pet is estimated to kill over 1 billion birds and over 6 billion other small animals every year. While the biggest threat are currently posed by feral cats — domesticated breeds that don’t have an owner and aren’t socialized to humans — even common house cats that are well cared for and fed will hunt and kill if let outside.

Cats also transmit diseases. In 2014, of the domestic animals that contracted rabies, which can then be spread to the local wildlife and humans, roughly 60 percent of them were cats.

Like nearly all invasive species, cats also have rapid rates of reproduction. Females can start breeding at just 6 months old and can breed every 4 months, producing up to 12 kittens every year. In just the last 40 years, the number of domestic cats across America has tripled. While it’s difficult to get an accurate count of feral cats, estimates suggest that today there are at least 30 million of them roaming our streets and neighborhoods. An additional 40 million pet cats have regular access to the outdoors.