A new study shows that domestic cats in the US kill up to 3.7 billion birds every year.
According to Agence France-Presse, cats also kill as many as 20.7 billion mice, voles and other small mammals.
A team from the Smithsonian Conservation Biology Institute in Washington led by Scott Loss looked at published research into the predation habits of cats.
More from GlobalPost: New Zealand man wants cats banned to protect birds
"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 said.
The estimates are much higher than earlier thought, with hundreds of millions of annual birth deaths previously being attributed to cats, reported USA Today. The new study was published in Nature Communications.
"I was stunned," said ornithologist Peter Marra of the Smithsonian's Conservation Biology Institute, who conducted the study with Loss, and US Fish and Wildlife Service biologist Tom Will.
Live Science noted that it's difficult to know the exact number of birds living in the US, but the number of deaths could account for as much as 15 percent of the total bird population.
The study is critical of the Trap-Neuter-Return policy advocated by defenders of free-roaming cats, said USA Today. It calls it "potentially harmful to wildlife populations" since it allows so many predators to stay in the wild.