Domestic cats kill many more wild birds in the United States than scientists thought, according to a new analysis. Cats may rank as the biggest immediate danger that living around people brings to wildlife, researchers say.
America’s cats, including housecats that adventure outdoors and feral cats, kill between 1.3 billion and 4.0 billion birds in a year, says Peter Marra of the Smithsonian Conservation Biology Institute in Washington, D.C., who led the team that performed the analysis. Previous estimates of bird kills have varied, he says, but “500 million is a number that has been thrown around a lot.”
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For wild mammals, the annual toll lies between 6.3 billion and 22.3 billion, Marra and his colleagues report along with the bird numbers January 29 in Nature Communications. The majority of these doomed mammals and birds fall into the jaws of cats that live outdoors full-time with or without food supplements from people.
“The results are remarkable, not only for the big number, but also for the proportion of deaths from feral cats,” says Gary M. Langham, chief scientist for the National Audubon Society. The study assigns 861 million to 3.3 billion bird deaths a year to these wild cats. “These numbers really elevate this threat to a new level.”
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To figure out how much wildlife cats catch, Marra and his colleagues combed the scientific literature for the best assessments of how many cats live in the United States and of what cats there and in similar climates hunt. Roughly 114 million cats live in the contiguous United States, 84 million of which share people’s houses. Forty to 70 percent of those household cats do at least some roaming outside. Between half and 80 percent of those outdoor cats hunt.
Marra says scientists have difficulty judging what proportion of total populations the cat catches represent. Comprehensive mammal numbers are a deep mystery, and estimates for U.S. land birds from volunteer counts lie between 10 billion and 20 billion adults.
“Cats are a nonnative species,” he notes, and multiple studies have shown that their hunting often targets natives. In his own research, Marra has shown that hunting cats can transform places that would normally be sources of young birds into sinks that drain birds from neighboring populations.
University of Wisconsin–Madison conservation biologist Stanley Temple says “this huge problem awaits a practical and widely acceptable solution.” The practice of catching free-roaming cats to neuter in hopes of shrinking populations is “simply too difficult, time consuming and expensive,” says Temple, a senior fellow at the Aldo Leopold Foundation. “Even if Herculean efforts made it feasible at very small local scales, from a conservation perspective, [the trapping and neutering approach] maintains free-ranging cat populations that will continue to harm native wildlife.”
An alternative policy of repeatedly rounding up cats and killing them hasn’t worked, says Becky Robinson, president of Alley Cat Allies, based in Bethesda, Md., a national advocacy group for protecting cats and reforming animal control.
“The big message is responsible pet ownership,” Marra says. Even though full-time outdoor cats may be the bigger problem, he says, cats with indoor homes still catch some 1.9 billion wild animals a year.
Cat hunting catches have not gotten the serious conservation attention they deserve, he says, because policy makers often dismiss cats as a minor threat compared with the other mortal dangers that wildlife faces. However, the new estimates outstrip assessments of annual bird deaths from pesticide poisonings or from collisions with windows, communication towers or vehicles.
Marra says he hopes to provide science to encourage dialog, instead of bitter fights, between wildlife conservationists and advocates for cat welfare. “The irony here is that you’ve got people who love animals on both sides,” he says.
Editor’s Note: This story was updated on January 29, 2014, to reflect a correction by Peter Marra and colleagues to their ‘Nature Communications’ paper. The ranges of birds and mammals killed by cats were both updated, as was the range of birds killed by feral cats.