Free-roaming outdoor cats are killing billions of birds and small mammals across the U.S., far more than previously believed, according to a study published Tuesday by scientists from the Smithsonian Conservation Biology Institute and the U.S. Fish and Wildlife Service.
The peer-reviewed study in Nature Communications, an online research journal, found an estimated 2.4 billion birds and 12.7 billion small mammals are killed each year in the U.S. by both feral cats and pets allowed to roam free.
The new study's estimate may be more than the number of birds killed by all other human factors combined -- including collisions with windows, towers, vehicles and buildings.
"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," the authors of the report conclude. "Scientifically sound conservation and policy intervention is needed to reduce this impact."
It's not clear if the cat-caused mortality is one reason some wild bird species are declining across the U.S. More than one-third of the nation's 800 species are in decline, threatened or endangered.
But the study is likely to add new ammunition for bird lovers in their long battle with cat lovers over cats that roam free.
Bird enthusiasts have said for years that domestic cats, which aren't native to the U.S. landscape, should be far more tightly controlled to reduce their impact on wildlife, including more aggressive capturing and euthanizing of outdoor cats. Cat lovers say the cats should simply be neutered and adopted out or released.
The study published Tuesday used data from several other reports to estimate that there are now between 30 million to 80 million feral or un-owned cats in the U.S., which cause most of the bird deaths, and another 84 million pet cats.
"The carnage that outdoor cats inflict is staggering and can no longer be ignored or dismissed. This is a wake-up call for cat owners and communities to get serious about this problem before even more ecological damage occurs," said George Fenwick, president of American Bird Conservancy, in a statement responding to the study.
In Duluth, Carrie Lane, the city's animal control officer, said she believes more pet owners are obeying the city's leash law, which makes it illegal to let any cat roam free -- even in one's own yard -- without supervision.
"The leash law applies to cats just like it does to dogs, and I think public sentiment is really moving now against letting cats roam free," Lane said. "I'm probably seeing fewer pet cats roaming free now than when I started more than 20 years ago."
But Lane said feral or wild cats continue to be a problem in some parts of Duluth, including "colonies of 30, 40, even 50 feral cats in some neighborhoods."
Lane's office captured 623 cats in 2012, most of which, after a five-day hold, are given to Animal Allies Humane Society to be put up for adoption. Cats that can't be adopted out because they are too wild are euthanized, 18 last year, Lane said. She added that some cats are adopted out to rural residents as "barn cats" -- a practice that's also despised by bird lovers because it moves the bird killing from the city to the country.
Lane said some cat supporters have pushed for Duluth to adopt a "trap, neuter and release" program for feral cats so they don't have to be euthanized. But she said releasing cats to roam free in Duluth remains illegal, and bird groups oppose the idea of release because those cats can keep on killing.
"I'm against it for several reasons, including the toll on wildlife. They are going to continue to kill birds," Lane said of cats being released. "But it also doesn't seem humane to me to release cats in a place that gets this cold. Bird groups have vocally opposed trap, neuter and release programs because the cats are set free to continue killing birds and small mammals."
About the study
The peer-reviewed study was based on a review of 90 previous studies and authored by Peter Marra and Scott Loss, research scientists at the Smithsonian Conservation Biology Institute and by Tom Will from the U.S. Fish and Wildlife Service's Division of Migratory Birds.
The study estimated that the median number of birds killed by cats annually is 2.4 billion, while the median number of mammals killed is 12.3 billion. Nearly 70 percent of the bird mortality and 90 percent of the mammal mortality was from feral cats, with the rest from pet cats.
Native species make up the majority of the birds preyed upon by cats. On average, only 33 percent of bird prey items identified to species were non-native species in 10 studies.
Studies of mammals in suburban and rural areas found that 75 to 100 percent of mammalian prey were native mice, shrews, voles, squirrels and rabbits, all of which serve as food sources for birds of prey such as hawks, owls and eagles.
The authors said it's also likely that cats are killing large numbers of frogs and other amphibians, although there have been few studies on that issue.
"This study is part of a continuing campaign to vilify cats," said Becky Robinson, president of Alley Cat Allies, adding that she believes the study was based on false assumptions and avoids bigger issues for birds such as habitat loss.
"Americans should not be fooled by sensational headlines and bad science," Robinson said. "Killing cats will not save birds."
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