Declared to be the "worst" invasive species, domestic cats kill 3.7 billion birds and small animals annually in the U.S.

While cats make for cute and adorable pets, they are known to be the worst invasive species and are currently destroying the planet. According to a recent study published Wednesday in Nature, cats reportedly kill 3.7 billion birds and small animals annually in the U.S.

The study was carried out by Scott Loss and his team at the Smithsonian Conservation Biology Institute in Washington. The study found that cats were on the top when it came to threats to birds. Others on the list included buildings, towers, windows, poison and cars. These cats were mostly those that had outdoor access.

Loss found that the U.S. was home to 84 million cats with owners, out of which only a couple of millions didn't have outdoor access. Adding to this number, U.S. was also home to nearly 30 to 80 million cats that had no owners and survived on "goodwill." Authors of the study state that these "unowned" cats were likely the culprits for the depleting number of endangered bird species.

"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. "Unowned cats, as opposed to owned pets, cause the majority of this mortality."

Loss said the number is bigger than what they estimated and cats are likely the "single greatest source of anthropogenic [man-made] mortality for US birds and mammals."