For all the adorable images of cats that play the piano, flush the toilet, mew melodiously and find their way back home over hundreds of miles, scientists have identified a shocking new truth: Cats are far deadlier than anyone had realized.

In a report that scaled up local surveys and pilot studies to national dimensions, scientists from the Smithsonian Conservation Biology Institute and the Fish and Wildlife Service estimated that U.S. domestic cats - both the pet Fluffies that spend part of the day outdoors and the unnamed strays and ferals that never leave it - kill a median of 2.4 billion birds and 12.3 billion mammals a year, most of them native mammals like shrews, chipmunks and voles rather than introduced pests like the Norway rat.

The estimated kill rates are two to four times higher than mortality figures previously bandied about, positioning the domestic cat as one of the single greatest human-linked threats to wildlife. More birds and mammals die at the mouths of cats, the report said, than from automobile strikes, pesticides and poisons, collisions with skyscrapers and windmills and other so-called anthropogenic causes.

Peter Marra of the institute, and an author of the report, said the figures that emerge from the new model "are shockingly high."

"When we ran the model, we didn't know what to expect," said Marra, who performed the analysis with his colleague, Scott R. Loss, and Tom Will of the Fish and Wildlife Service. "We were absolutely stunned by the results." The study appeared Tuesday in the journal Nature Communications.

The findings are the first serious estimate of just how much wildlife the country's vast population of free-roaming domestic cats manages to kill each year.

The new report is likely to fuel the sometimes vitriolic debate between environmentalists who see free-roaming domestic cats as an invasive species, and animal welfare advocates who are appalled by the millions of unwanted cats (and dogs) euthanized in animal shelters each year.

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.