Cats may be greater threat to wildlife than first thought

Study estimates free-roaming cats kill billions of birds and mammals in the US annually
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A new study claims free-ranging domestic cats kill substantially more wildlife than previously thought and may be the single greatest source of human-related death of birds and mammals in the United States.

Scientists with the Smithsonian Institution and the U.S. Fish and Wildlife Service estimate that cats kill 2.4 billion birds and 12.3 billion mammals in the contiguous United States every year. The scientists identified feral cats as being responsible for most of the deaths—approximately 69 percent of bird deaths and 89 percent of mammal deaths.

The study goes on to speculate that free-ranging cats could kill 258 million to 822 million reptiles and 95 million to 299 million amphibians annually.

Scott Loss, PhD, lead author of the study published Jan. 29 in the journal Nature Communications, said his team’s research indicates cat predation is an even bigger environmental and ecological threat than anyone realized.

“Our study provides motivation for further research and for incorporating cat impacts into conservation and management efforts,” said Dr. Loss, a fellow with the Smithsonian Conservation Biology Institute.

A cat outdoors crouching under foliage
(Photo by R. Scott Nolen)

Earlier estimates of annual cat-related bird deaths in America were sparse, but a study published in the 2009 Proceedings of the Fourth International Partners in Flight Conference put the number at around 1 billion.

The Nature Communications study is the latest development in a long-running debate over the scope of wildlife death attributable to cat predation and how to reduce the number feral cats, which the American Society for the Prevention of Cruelty to Animals estimates to be in the tens of millions.

“We’ve long known that feral cats can decimate wild animals, but these numbers elevate the threat to a new level,” said Gary Langham, PhD, chief scientist for the National Audubon Society. “The results add new urgency to the feral cat problem and underscore the need for effective solutions to protect wild birds. Audubon strongly supports all efforts using science to better understand the causes and impacts of bird mortality.”

The CATalyst Council, a national initiative comprising animal health and welfare organizations, worries the study and related news reports cast a negative light on cats and might hinder the ability of shelters to place them in adoptive homes.

“We regret the fact that the articles written about the study have maligned cats as a whole, when in fact, the vast majority of the estimated destruction to wildlife was reportedly by feral or stray cats,” Dr. Jane Brunt, CATalyst Council executive director, said. “This works to discourage prospective cat owners from adopting one of the hundreds of thousands of healthy, enjoyable cats that are held in shelters across this nation.”

The International Union for Conser­vation of Nature, an environmental advocacy organization, lists the domestic cat among the world’s 100 worst invasive alien species on account of the danger they pose to native wildlife populations.

The impact of cat predation on wildlife may be grossly underestimated, according to scientists with the Smithsonian Institution and U.S. Fish and Wildlife Service.

Dr. Loss and his colleagues designed a mathematical model based on 21 publications that estimated free-ranging cat predation in the United States and Europe. They took a rigorous and conservative approach, excluding studies that did not distinguish between owned cats and unowned cats and studies based on a small sample size or a short sample collection period.

“When we ran the model, we didn’t know what to expect,” said Pete Marra, PhD, a research scientist with the Smithsonian Institution and the study’s senior author. “We were absolutely stunned by the results.”

Prior to the study’s publication, cats were thought to kill far fewer birds than were other human-related threats, such as building collisions and pesticides, and were unlikely to have a significant effect on mainland vertebrate populations. “Given these results, free-ranging cats are likely having a population-level impact on native species of birds,” Dr. Marra said.

Despite the high wildlife mortality rate, policies to manage free-ranging cat populations are dictated by concerns for the welfare of the cats rather than the ecological impacts they’re having, according to the study. Trap-neuter-return colonies, the study states, are potentially harmful to wildlife populations and are implemented without consideration of the scientific evidence and the environmental review process usually required for actions with harmful environmental consequences.

The Audubon Society’s Dr. Langham said, “We must have the courage to investigate and address all human sources of wild bird mortality.”

Louise Holton believes the study authors are tacitly advocating for lethal control policies to reduce feral cat populations. “They don’t, of course, say ‘killing’ outright, because they know this is a cat-loving society,” said Holton, president and founder of Alley Cat Rescue, which supports nonlethal control of free-ranging cat populations.

Holton said the authors ignored several studies showing cats prey on young, old, and sickly birds and mammals and that any bird population unable to withstand cat predation would’ve been wiped out long ago. Agriculture and habitat loss, she added, are in fact the major causes for declines in wild bird populations.

“Trap-neuter-return definitely works,” Holton said. “Catch-and-kill seems like an attractive, quick way to get rid of cats, but it usually fails, as new cats will move in and start the breeding process all over again.

Smithsonian and FWS scientists plan to further refine the estimates of how many birds, mammals, reptiles, and amphibians are killed by feral cats, including those in TNR colonies. They also hope to determine which wildlife species are most affected by free-ranging cats, the precise numbers of feral cats throughout the country, and where feral cats are more and less abundant.