Pet cats are known for bringing in dead birds and mice, but collectively, our feline friends kill up to 3.7 billion birds and 20.7 billion mice, and other small animals, each year.
Cats are the largest human-induced killer of these animals, beating out deaths from habitat loss, agricultural chemicals and hunting.
The study was published in the journal Nature Communications, and was led by Scott Loss at the Smithsonian Conservation Biology Institute. Loss looked at published research into the killing habits of domestic cats.
Cats who are allowed outside have a high kill-rate, killing between 30 and 47 birds and 177 and 299 mammals a piece in warmer parts of Europe and North America every year.
That means a lot of kills go unnoticed by owners.
In order to come up with the total number killed annually, Loss had to calculate how many cats there are in the United States. He estimated that there are around 84 million cats with owners, and a couple million of those are cats without access to the outdoors.
Loss also added wild or free-ranging cats without owners, of which he estimated to be between 30 and 80 million.
"We estimate that free-ranging domestic cats kill 1.4 to 3.7 billion birds and 6.9 to 20.7 billion mammals annually," the study explains.
"Unowned cats, as opposed to owned pets, cause the majority of this mortality."
The study explains that cats are likely the "single greatest source of anthropogenic [man made] mortality for US birds and mammals. Scientifically sound conservation and policy intervention [are] needed to reduce this impact."
It turns out, cats in certain areas kill in greater numbers. According to "Red List" compiled by the International Union for Conservation of Nature, cats that live on islands caused or contributed to the extinctions of 33 species of birds, mammals and reptiles.
This study gives even more ammo to businessman turned philanthropist Gareth Morgan, who is arguing for cats to be eradicated in New Zealand in order to save the country's unique wildlife.