Psycho killer? Carlo Allegri/Reuters Humans think of cats as fuzzy and cute. Birds and small mammals have a vastly different perception.

Domestic cats kill between 1.4 and 3.7 billion birds and between 6.9 and 20.7 billion mammals (mostly mice, shrews, rabbits, squirrels, and voles) each year, according to a study published last year in Nature Communications.

The study indicated that both stray and owned cats are responsible for a far greater number of bird and mammal deaths in the contiguous United States than previously estimated, outpacing other threats such as collisions with windows, buildings, communication towers, cars, and poisoning, the report notes.

Free-ranging cats are "likely the single greatest source of anthropogenic (man-made) mortality for US birds and mammals," according to the report.

Un-owned cats (farm cats, feral cats, and stray cats that are fed by humans, for example) are the main perpetrators, but owned cats do their fair share of killing, too.

Researchers guess that a single cat may kill between 100 and 200 mammals annually, meaning an estimated population of between 30 and 80 million un-owned cats would result in the death of 3 and 8 billion mammals. And that's a low-end estimate. The researchers calculated that there are around 84 million owned cats, the majority of which are allowed outdoors.

"The magnitude of our mortality estimates suggest that cats are likely causing population declines for some species and in some regions," the study authors write.

Island cats are thought to be particularly deadly. A study published several years ago in the journal Global Change Biology estimated that island-dwelling feral cats have contributed to at least 14% of the world's bird, mammal, and reptile extinction. That study's authors also estimated that feral island cats were the primary threat to 8% of all critically endangered birds, mammals, and reptiles on earth.

Some experts, including the authors of this study, are critical of current tactics to control feral cat populations, including Trap-Neuter-Return (TNR), a project aimed at capturing and sterilizing feral cats before returning them to the wild, where they can continue to prey on wildlife.

A 2010 letter published in the journal Conservation Biology argued that TNR programs afford wild animals "second-class treatment at the expense of cats." The authors argue that it is conservation biologists' responsibility to advocate for laws making it illegal to maintain cat colonies on public lands. They also call for a more open dialogue between conservationists and veterinarians, animal shelters, and other pet advocates.

Others have been even more abrupt in their calls to action. Last January, a prominent New Zealand economist upset the international cat community when he called for the eradication of cats, citing their threat to the country's unique wildlife.

Animal activists rightly criticized the plan. The author of this post does not condone harm to any animals.