Cats may kill up to 3.7 billion birds and 20.7 billion mammals in the United States alone each year, a new study has found.
That means predatory felines are likely the leading human-linked cause of death for birds and mammals, surpassing habitat destruction, collisions with structures such as buildings, and pesticide poisoning, reports an article published Tuesday in the journal Nature Communications.
"The magnitude of wildlife mortality caused by cats that we report here far exceeds all prior estimates," said the paper co-authored by three U.S. scientists.
The researchers warned that very large numbers of birds and mammals are likely being killed "in all parts of the world where free-ranging cats occur," not just the United States.
According to the paper, cats were previously thought to be a "negligible" cause of mortality for birds and mammals compared to other human-linked threats, 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.
The study, led by Scott Loss at the Migratory Bird Center of the Smithsonian Conservation Biology Institute at the National Zoological Park in Washington, D.C., combined and analyzed data from as many other studies as the researchers could find about cats preying on birds and mammals in North America.
Strays largely to blame
Although the estimates of the feral cat population and the average kills per cat varied widely among the studies, Loss and his colleagues were able to say that U.S. cats fell 1.4 to 3.7 billion birds a year — a huge number considering that the entire population of North American land birds is estimated to be just 10 to 20 billion.
As for mammals, cats were estimated to kill 6.9 to 20.7 billion per year. For comparison, the human population worldwide is seven billion.
The study found that a large majority of the birds and mammals killed by cats were native species.
Cats without owners are blamed for most of the deaths. There are about 30 million to 80 million feral cats in the U.S., each of which can kill upwards of 200 mammals a year alone, the study reported.
However, pet cats were far from innocent, causing 258 million to 1.5 billion of the bird deaths and 571 million to 2.5 billion of the mammal deaths.
The paper advocated taking measures such as limiting or preventing cats' access to the outdoors.
Two stray kittens pose for a photograph at Battersea Dogs and Cats Home on August 18, 2009 in London, England. Battersea Dogs and Cats Home is seeing a sharp rise in the number of cats requiring a home with 143 of the 145 shelter's pens full. (Photo by Dan Kitwood/Getty Images)
Milly, a 13-week-old kitten waits with her brother Charlie (L) to be re-homed at The Society for Abandoned Animals Sanctuary in Sale, Manchester which is facing an urgent cash crisis and possible closure on July 27, 2010 in Manchester, England. (Photo by Christopher Furlong/Getty Images)
A kitten plays with a squirrel, which was rescued off the streets in Envigado, Antioquia Department, Colombia, on February 16, 2010. (RAUL ARBOLEDA/AFP/Getty Images)
Blondie, a bichon dog, looks at her adopted ten day old kitten, Yako, which was found in a rubbish bin, in Rabat 24 April 2001. Although the dog has had no pups of her own, she has no trouble feeding the kitten. (A DA/AFP/Getty Images)
A kitten is pictured on December 5, 2009 during a cat exibition in Moscow. (NATALIA KOLESNIKOVA/AFP/Getty Images)
A stray kitten is posed for a photograph at Battersea Dogs and Cats Home on August 18, 2009 in London, England. Battersea Dogs and Cats Home is seeing a sharp rise in the number of cats requiring a home with 143 of the 145 shelter's pens full. (Photo by Dan Kitwood/Getty Images)
A cat and her kitten play with a squirrel, which was rescued off the streets, in Envigado, Antioquia Department, Colombia, on February 16, 2010. (RAUL ARBOLEDA/AFP/Getty Images)
Kittens are pictured in a bucket, before the arrival of Britain's Camilla, the Duchess of Cornwall, at Battersea Dogs and Cats Home, in London on October 27, 2010. The Duchess opened the new cattery during her visit to the animal refuge, which is celebrating it's 150th anniversary this year. (Chris Jackson/AFP/Getty Images)
A volunteer displays a newly-born cat delivered by a rescued stray cat at the home of cat lover Duo Zirong on July 10, 2007 in Shanghai, China. (Photo by China Photos/Getty Images)
Two less than a week old kittens of a jungle cat (lebis Chaus) lie inside the forest department office in Mumbai, 11 June 2007. Three abondoned kittens, found in the jungles of Aarey milk colony, on the outskirts of the city, were later handed over to the forest authorities, likely to be released in the Borivali national park in Mumbai. (PAL PILLAI/AFP/Getty Images)
Boggles, a stray kitten, one of the lucky animals at the Barnes Hill RSPCA Animal Rescue Centre, has found a caring home on 4 April 2007, Birmingham, England. (Photo by Christopher Furlong/Getty Images)
Milly, a 13-week-old kitten looks through the glass of her pen as she waits to be re-homed at The Society for Abandoned Animals Sanctuary in Sale, Manchester, which is facing an urgent cash crisis and possible closure on July 27, 2010 in Manchester, England. (Photo by Christopher Furlong/Getty Images)