The image of a house cat trotting across the yard with a bird clenched in its teeth is as American as the saga of Sylvester and Tweety Bird.

A new study released today suggests Tweety Bird and his feathered flock are losing that battle, losing it badly.

The study concludes that cats, both house cats and feral cats,

Felis catus

, represent the single largest killer of birds and small mammals in the nation. The toll is impressive.

"We estimate that free-ranging domestic cats kill 1.4–3.7 billion birds

and 6.9–20.7 billion mammals annually," reads the opening of the study. "Un-owned cats, as opposed to owned pets, cause the majority of this mortality."

The study, "

," was conducted by

the Smithsonian Conservation Biology Institute and the U.S. Fish and Wildlife Service, two of the nation's premier conservation organizations.

While numerous studies have suggested that cats were taking a toll on native wildlife, none have been conducted with the scientific rigor of this peer reviewed effort, said the study's authors. Pesticides and collisions with buildings, kill fewer birds and mammals each year, according to the study.

The study concludes that the feral cat population is largely to blame, killing nearly 70 percent of the animals. They include "

farm/barn cats, strays that are fed by humans but not granted access to habitations, cats in subsidized colonies and cats that are completely feral," in that group.

The authors suggest that wild cats need to be dealt with as a threat that is throwing the natural order out of balance. They urge the nation to rethink the widespread practice of catching stray cats, neutering them, and then releasing them into the environment.