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.

Scientists from the Smithsonian Conservation Biology Institute and the Fish and Wildlife Service estimated that U.S. domestic cats -- both the pets 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 rather than introduced pests like the Norway rat.

The estimated kill rates are two to four times higher than previously thought. More birds and mammals die at the mouths of cats, the report said, than from automobile strikes, pesticides and poisons, or other so-called anthropogenic causes.

Peter Marra, of the Smithsonian Conservation Biology Institute and an author of the report, said the mortality figures that emerge from the new model "are shockingly high."

The study appeared last Tuesday in the journal Nature Communications and is likely to fuel the 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.

The new study estimates that free-roaming pets account for about 29 percent of the birds killed by domestic cats each year; the real problem arises with the 80 million or so stray or feral cats that commit the bulk of the slaughter.