Cats may kill up to 3.7 billion birds and 20.7 billion mammals in the United States alone each year, a new study has found.

That means predatory felines are likely the leading human-linked cause of death for birds and mammals, surpassing habitat destruction, collisions with structures such as buildings, and pesticide poisoning, reports an article published Tuesday in the journal Nature Communications.

"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.

The researchers warned that very large numbers of birds and mammals are likely being killed "in all parts of the world where free-ranging cats occur," not just the United States.

According to the paper, cats were previously thought to be a "negligible" cause of mortality for birds and mammals compared to other human-linked threats, and that is one of the reasons why policies to deal with stray cats often involve neutering them and then returning them to their hunting grounds.

The study, led by Scott Loss at the Migratory Bird Center of the Smithsonian Conservation Biology Institute at the National Zoological Park in Washington, D.C., combined and analyzed data from as many other studies as the researchers could find about cats preying on birds and mammals.

Strays largely to blame

Although the estimates of the feral cat population and the average kills per cat varied widely among the studies, Loss and his colleagues were able to say that U.S. cats fell 1.4 to 3.7 billion birds a year — a huge number considering that the entire population of North American land birds is estimated to be just 10 to 20 billion.

As for mammals, cats were estimated to kill 6.9 to 20.7 billion per year. For comparison, the human population worldwide is seven billion.

The study found that a large majority of the birds and mammals killed by cats were native species.

There are an estimated 30 million to 80 million feral cats in the U.S. (Gordon King/Yakima Herald-Republic/Associated Press)

Cats without owners are blamed for most of the deaths. There are about 30 million to 80 million feral cats in the U.S., each of which can kill upwards of 200 mammals a year alone, the study reported.

However, pet cats were far from innocent, causing 258 million to 1.5 billion of the bird deaths and 571 million to 2.5 billion of the mammal deaths.

The paper advocated taking measures such as limiting or preventing cats' access to the outdoors.