A new study shows that cats may kill up to 3.7 billion birds and approximately 20.7 billion mammals in the U.S. every year according to CBS News.

If the report is true, then predatory felines are most likely the top human-linked cause of death for birds and mammals, besting habitat destruction, collisions with structures, and pesticide poisoning according to the article published this week in the journal Nature Communications.

The analysts involved warn people that a large number of mammals and birds are being killed not just in the U.S., but all-around the world as well where cats are allowed to roam free outside.

"The magnitude of wildlife mortality caused by cats that we report here far exceeds all prior estimates," said the paper co-authored by three U.S. scientists.

On average, cats kill approximately 6.9 to 20.7 billion mammals per year. In comparison, the human population is approximately seven billion according to CBS.

A majority of birds and animals killed by felines were native species as well. Cats without owners are blamed for a majority of the deaths reported. There are around 30 million to 80 million cats in the U.S. that kill around 200 mammals in a year alone.

"We estimate that free-ranging domestic cats kill 1.4 to 3.7 billion birds and 6.9 to 20.7 billion mammals annually," said the three scientists in the study according to The New Zealand Herald.

Pet cats are to blame to, as 258 million to 1.5 billion of the bird deaths that occur are caused by them every year. 571 million to 2.5 billion mammal deaths are caused by pet cats as well according to CBS.

The report urges people to take measures with their pets, like limiting or preventing them completely from going outside.

The study was led by Scott Loss who works at the Migratory Bird Center at the Smithsonian Conservation Biology Institute at the National Zoological Park in Washington, D.C. Loss and his team combined and studied data from many previously recorded studies about cats that prey on birds and other mammals around the world.

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