Debi Shearwater, American Bird Conservancy
Cat with American coot.

Cats kill far more birds and mammals each year than previously thought — as many as 3.7 billion birds and 20.7 billion mammals. More birds die from cat-inflicted injuries than from crashing into windows, buildings and communications towers; building encroachment into habitat; and poisoning from pesticides, according to a new study.

Roaming felines — including those that are wild and house pets allowed outdoors — are the leading "human-linked" cause of death for birds and mammals, especially mice, voles, rabbits, squirrels and shrews, according to the Smithsonian Conservation Biology Institute and the U.S. Fish and Wildlife Service. Their findings were published in the journal Nature Communications.

The study suggested that 69 percent of the bird deaths and 89 percent of the mammal deaths that came from cats involved those that didn't have owners, including farm or barn cats, strays that get fed but not taken indoors, cats in "subsidized colonies" and feral cats. The rest came from pets allowed outdoors.

It's estimated that about one-third of the 800 species of birds found in the United States are endangered or have have severe population declines. A news release from the The American Bird Conservancy hailing the study said that action needs to be taken to preserve bird populations.

For the study, the researchers from the two agencies used 21 American and European studies as the basis for extrapolations that concluded there are roughly 30 to 80 million "unowned" cats and 84 "owned" American cats, how many creatures they kill and more. The range they established was 1.4-3.7 billion birds killed each year and 6.9-20.7 billion mammals.

CBC News reported that cats had been thought a "negligible" cause of bird and mammal death compared to the other human-linked issues, and "that is one of the reasons why policies to deal with stray cats often involve neutering them and then returning them to their hunting grounds."

That trap-neuter-return policy used by many communities that takes felines found roaming and "fixes" them, then returns them outdoors, is criticized in the study because it does nothing to keep the animals from preying on other animals.

Cat lovers and bird lovers are squaring off over the findings. Wrote USA Today's Chuck Raasch: "Cat defenders say that the new estimates won't change their belief that 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, says Becky Robinson, president of Alley Cat Allies, a non-profit that defends outdoor cats. She says the trap-neuter-return policy is growing because people see it as a way to protect birds without killing cats."

"This is not 'Sophie's Choice,' this is not the American people voting to kill one animal over another," Robinson told Raasch.

"To maintain the integrity of our ecosystem, we have to conserve the animals that play integral roles in those ecosystems," said ABC president George Fenwick in his organization's release. "Every time we lose another bird species or suppress their population numbers, we're altering the very ecosystems that we depend on as humans. This issue clearly needs immediate conservation attention."

The small mammals that are killed by the cats are a natural source of food to hawks, owls and eagles, according to the conservancy.

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