Cats are America’s second most popular pet, behind only fish. There are estimated to be over 85 million pet cats in America, compared to only 78 million dogs. But, there are also an estimated 58 million feral cats in the United States.

These feral cats, as well as millions of pet cats that people let outdoors, have an enormous impact on the environment. Scientists estimate that cats kill 1.3–4.0 billion birds and 6.3–22.3 billion mammals every year in America alone! Most of the birds killed by cats are species native to America, and in suburban and rural areas, most of the mammals killed are also native.

The threat cats pose to animals isn’t limited to America, though. Cats are listed as one of the world’s top 100 worst invasive species, and for good reason: worldwide, cats have contributed to the extinction of 33 (14% of total) bird, mammal, and reptile species in modern times.

The problems caused by cats are made even worse by humans. People provide food and shelter to entire colonies of feral cats, regardless of the consequences for nearby wildlife. For example, at Jones Beach, there is a colony of around 30 feral cats that are fed by people, and they even have shelters people built for them. This colony happens to be very close to an area where Piping Plovers, a bird considered endangered in New York state, nest and raise their young.

Not only would keeping cats indoors save billions of birds and mammals, but it would also be beneficial for cats. Outdoor cats are exposed to dangers such as cars, predators, poison, abuse from people, and diseases. Because of these things, feral cats have an average lifespan of around 2 years, compared to about 15 years for pet cats, with indoor-only cats living on average 2-3 years more than cats that spend a lot of time outdoors.