Cats Kill Billions of Animals Annually, Study Finds Cats are responsible for the deaths of 1.4 to 3.7 billion birds.

Jan. 30, 2013  -- Cats are responsible for the deaths of 1.4 to 3.7 billion birds and 6.9 to 20.7 billion mammals every year, according to research conducted by the Smithsonian Conservation Biology Institute and the U.S. Fish and Wildlife Service.

The study, published on Tuesday in the journal Nature Communications, highlighted the impact that both un-owned cats and owned cats have on wildlife populations in the United States.

Feral, un-owned cats are responsible for the majority of deaths, and the study found previous wildlife mortality estimates to be far too low. It remained to be seen what large-scale impacts the killing sprees have on wildlife populations.

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"It's hard to know," Dr. Peter Marra, research scientist at the Smithsonian Conservation Biology Institute and co-author of the study, told ABC News. "We think there are 15 to 20 billion adult land birds in the U.S. If we are suggesting 2.3 billion are killed annually, that means 1 in 10 birds are taken by cats every year."

The findings were extrapolated from various studies conducted in both the U.S. and Europe to determine the number of cats in the U.S. -- as many as 80 million un-owned and 84 million owned -- and other factors used to approximate how many kitty-caused killings are committed.

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Amy Watts of Athens, Ga., thought she knew her cat, Booker T, but when scientists put a little camera on the kitty, it captured carnage.

"He's the cutest little serial killer you will ever meet." Watts told ABC News' Dan Harris.

In addition to the sheer volume of wildlife deaths occurring annually by the claws and jaws of felines, there are considerable health concerns that cat owners should be aware of.

The two main diseases that cat owners should be wary of are toxoplasmosis and rabies. Cats often become carriers of toxoplasmosis by killing and eating infected prey. Though most cats become immune to the disease, it poses significant health risks to pregnant women and those with a weakened immune system. Currently, more than 60 million Americans carry the disease, according to the Centre for Disease Control.

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Due, in part, to their close contact with wild animals and humans, rabies in cats is on the rise and rabid cats were reported at three times the rate of rabid dogs, according to the CDC.

"When cats are outdoors, they are exposed to animals that are known carriers of rabies," said Marra. "When you have heightened interaction between wildlife and cats, the potential for exposure to rabies increases."

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The study aims to discover the many threats posed by humans and their various activities. The majority of deaths affect native species, raising concerns over the impact to local populations of animals. Unnatural casualty levels can turn areas into demographic sinks, where the population size can only be maintained by migration from other areas.

"We are trying to identify patterns which are reversible with management action," said Marra. "There is something we can still do about this if the mortality is deemed significant enough. We should be asking how many birds we should allow to be taken."

ABC News' Dan Harris contributed to this report.