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Stray and feral cats are a greater threat to America's birds than previously thought, researchers say, killing up to 3.7 billion birds annually.

The estimate is much higher than the hundreds of millions of annual bird deaths previously attributed to cats.

"I was stunned," ornithologist Peter Marra of the Smithsonian's Conservation Biology Institute told USA Today.

Marra and colleagues from the Smithsonian Institution and researchers with the U.S. Fish and Wildlife Service conducted the study as part of a USFWS-funded effort to estimate the number of birds killed by predators, chemicals and in collisions with wind generators and windows.

"Our findings suggest that free-ranging cats cause substantially greater wildlife mortality than previously thought and are likely the single-greatest source of anthropogenic mortality for U.S. birds and mammals," Marra and his study co-authors wrote. "Scientifically sound conservation and policy intervention is needed to reduce this impact."

Bird lovers and cat lovers have been at odds over whether outdoor cats should be euthanized or allowed to roam free in managed programs that include neutering.

Cat defenders have responded to the new study by saying it won't change their belief cats are scapegoats for bird habitat loss, chemicals used in fertilizers and insecticides, and collisions with man-made objects.

"Human impact is the real threat" to birds, said Becky Robinson, president of Alley Cat Allies, a non-profit that defends outdoor cats.