Yesterday, we learned that your cat could, in fact, suffer from a mental illness. Today, we learn that, mental illness or not, cats are responsible for killing an insane number of birds and small mammals each year. According to a report from the Smithsonian Conservation Biology Institute and the Fish and Wildlife Service, cats kill an average of 2.4 billion birds and 12.3 billion small mammals a year.

As anyone who's read Jonathan Franzen can tell you, it's no surprise that cats are bird-killing machines. But scientists were shocked by the study's results, which were two to four times higher than previous estimates.

"When we ran the model, we didn't know what to expect," said Dr. Marra, who performed the analysis with a colleague, Scott R. Loss, and Tom Will of the Fish and Wildlife Service. "We were absolutely stunned by the results."

Cat owners who let their pets roam free in the neighborhood shouldn't feel too bad, though; according to the report, domestic cats are only responsible for 29 percent of the birds killed and 11 percent of the small mammals. That said, environmentalists and animal welfare activists (two groups that, apparently, are engaged in "sometimes vitriolic debate") agree that letting your cat outdoors is a bad idea.

All concur that pet cats should not be allowed to prowl around the neighborhood at will, any more than should a pet dog, horse or potbellied pig, and that cat owners who insist their felines "deserve" a bit of freedom are being irresponsible and ultimately not very cat friendly.

Left on their own, cats won't just kill and eat small animals; they'll also run beneath moving cars, fight dogs, and drink antifreeze and sewer sludge. Also: Rabies.

"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."

But what about the feral cats responsible for the vast majority of bird killing? There seem to be schools of thought, split between environmentalists and animal welfare activists. The Humane Society and other welfare groups argue in favor trap-neuter-return programs, which will return the cats to their colony if no home for them can be found. Environmentalists think this just creates "colonies of subsidized predators," which does sound bad, and that more emphasis should be placed on cat adoption and fencing in said cat colonies.

There's a third option being led by an economist (?) in New Zealand, who wants to ban all cats from his country. Good luck with that, guy.

[Image via Shutterstock]