Owners of domesticated cats might have received a dead bird offering (or two) at their doorstep as a “gift.” But would you believe the seemingly cuddly felines are actually contributing to the death of billions (yes, billions) of birds and mammals each year.

Turns out pet cats are cold blooded killers. A study published in the journal “Nature Communications” found “free-ranging” domesticated cats kill between 1.4 billion and 3.7 billion birds and between 6.9 and 20.7 billion small mammals each year.

Free Ranging Cats Kill Up to 3.7 Billion Birds and up to 20.7 Billion Mammals Each Year

(Image: Shutterstock.com)

Live Science reported this to be 15 percent of the total bird population in the United States, according to Smithsonian Conservation Biology Institute ecologist and study co-author Pete Marra.

Here’s more from Live Science on the study:

For this broader analysis, the team first looked at all prior studies on bird deaths and estimated that around 84 million owned-cats live in the country, many of which are allowed outdoors.

“A lot of these cats may go outside and go to 10 different houses, but they go back to their house and cuddle up on Mr. Smith’s lap at night,” Marra said.

Based on an analysis of past studies, the researchers estimated that each of those felines killed between four and 18 birds a year, and between eight and 21 small mammals per year.

Granted, its the ownerless, feral cats that cause the most bloodshed, according to the study.

And if you brought a cat into your home to keep mice at bay, the study also found the felines seem to prefer native species like voles and chipmunks, Live Science reported.

Given the effect of cats on native wildlife, one man in New Zealand has recently been advocating a complete cat ban. Mashable reported last week about businessman and philanthropist Gareth Morgan’s #catstogo campaign on Twitter.

Here’s an infographic from Morgan’s campaign:

Infographic Damage cats do in New Zealand

The Smithsonian researchers suggest, first and foremost, keeping domesticated cats inside to help curb the killings. But, as Live Science pointed out, the larger issue are roaming kitties with no owner.

Featured image via Shutterstock.com.