March 22, 2011 -- It's an old adage that news happens when "man bites dog" and not when "dog bites man." But here's a pet-related fact that is generating news: cats kill birds -- a lot of them.

Many cat owners are familiar with the "presents" that feline friends leave on doorsteps after a night on the prowl -- a bird's head or a dead mouse. All those cats have a giant cumulative effect, killing anywhere from 500 million to a billion birds every year, according to estimates from the American Bird Conservancy.

"There are the cats the people let outside, but then you've also got feral cats," said Peter Marra, a research scientist at the Smithsonian Conservation Biology Institute. "The issue is, it adds up."

Marra co-authored a recent study in the Journal of Ornithology that shows the dramatic effects domestic cats can have on bird populations in urban and suburban areas. The study attached radio transmitters to fledgling Gray Catbirds, a common bird species, tracking the factors that led to their mortality or survival.

They found that predators were responsible for nearly 80 percent of the bird deaths, and nearly half of those predators were domestic cats. In areas with fewer cats, the fledglings had a survival rate of roughly 50 percent, while roughly 20 percent survived in areas with more cats.

Cats Threaten Some Bird Species

While cartoons like "Sylvester & Tweety" make it seem as if there's nothing more natural than a cat going after a bird, scientists note that many bird species evolved without having cats as predators. Cats may just be following their instincts, but in some cases, the birds in their mouths are threatened or endangered species.

"Cats are in fact having population-level effects," said Steve Homer, a senior policy advisor with the American Bird Conservancy who said he is longtime cat owner himself. "The big picture is that about a third of the birds in the United States are in decline, and cats have been identified as one of the more significant factors in this decline."

Homer said that habitat loss remains the top threat to birds, but predators rank second. The problem is particularly acute in closed ecosystems like islands, where cats have been blamed for the extinction of 33 bird species, Marra said.

Keep Cats Indoors, Bird Conservancy Argues

For those reasons, the conservancy has for years been urging pet owners simply to keep their cats indoors, arguing that it doesn't only protect birds -- it's good for cats.

"It's not a happy life for these cats to be outdoors like this," Homer said, citing cat deaths from cars and disease. According to some estimates, outdoor cats live half as long as indoor cats.

Marra and his team at the Smithsonian are now attaching video cameras to cats to study their predatory behavior and its impact on birds and small mammals in urban environments. He says it's time for Americans to confront the surprising consequences of allowing cats to roam free.

"I'm not a cat hater. It's really not a personal attack on cats or a personal attack on cat people, although they often take it that way," he said. "It's really about what's good for cats and what's good for wildlife. This is a problem that we need to come to grips with."

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