Debunking the Smithsonian’s “killer cat study”

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“Scientists have identified a shocking new truth,” proclaimed New York Times science reporter Natalie Angier in a recent story. “Cats are far deadlier than anyone realized.”


Outdoor cats aren’t the killers rumors claim. Photo: Thinkstock

The Times account was just one of dozens — probably hundreds, including international outlets — describing work published late last month by a team of researchers from the Smithsonian Conservation Biology Institute and U.S. Fish and Wildlife Service. A careful look at the research in question, however, reveals very little that’s actually new. Or, for that matter, true.

Indeed, the authors’ headline-grabbing claim “that free-ranging domestic cats kill 1.4–3.7 billion birds and 6.9–20.7 billion mammals annually” in the U.S. raises serious questions of credibility. Their estimate of bird mortalities, for example (described throughout the paper as “conservative”), represents an astonishing 28.5–75.5% of the estimated 4.7 billion landbirds in all of North America.

Were these figures even remotely accurate, the continent would have been devoid of birds long ago.

A careful examination of the mathematical model developed by researchers Scott Loss, Tom Will, and Peter Marra, reveals one inflated input after another, each one the result of a very selective review of the scientific literature on the subject. Their claim that 40 to 70% of America’s pet cats are indoor-outdoor, for instance, ignores some well-known surveys published in the Journal of the American Veterinary Medical Association (one in 2003, the other in 2008, as well as the American Pet Products Association’s National Pet Owners Survey (which, ironically, the authors actually cite among their sources of information), demonstrating just the opposite: approximately 60% of America’s pet cats are indoor-only, and the trend is upward. And about half of the cats that are allowed outside are outdoors for three hours or less each day.

This error alone exaggerates predation levels for owned cats by a factor of three to six. And this is just the beginning. Garbage in, garbage out, as they say in the computer science field.

Loss, Will, and Marra also fail to acknowledge the fact that predation — even at very high levels — does not necessarily lead to population-level impacts. Like all predators, cats tend to prey on the young, the old, the weak, or unhealthy. Two studies, one published in the journal Oecologia, the other in Ibis (cited by Loss and his co-authors) have investigated this in great detail, and found that birds killed by cats are, on average, significantly less healthy that birds killed through non-predatory events (e.g., collisions with windows or cars).

As the UK’s Royal Society for the Protection of Birds notes on the organization’s website, “Despite the large numbers of birds killed, there is no scientific evidence that predation by cats in gardens is having any impact on bird populations UK-wide… It is likely that most of the birds killed by cats would have died anyway from other causes before the next breeding season, so cats are unlikely to have a major impact on populations.”

Like the folks at Petfinder, I’m a proponent of keeping pet cats indoors — not so much because of any threat they might pose to wildlife; I do it for their own safety. But what about the stray, abandoned, and feral cats who spend the better part of their lives outdoors? Loss, Will, and Marra suggest that these cats are responsible for about 69% of wildlife mortalities “primarily due to predation rates by this group averaging three times greater than rates for owned cats.”

As I’ve already pointed out, though, their analysis doesn’t stand up well to even moderate scrutiny.

And there’s an irony to the authors’ emphasis on “un-owned” cats. Two of the three (Will and Marra) have publicly opposed the trap-neuter-return method of free-roaming cat management. But restrictions or outright bans on TNR would, it’s virtually guaranteed, increase the population of stray, abandoned, and feral cats. Which would, in turn, increase any potential risk to the wildlife about which Loss, Will, and Marra claim to be concerned.

The real story here, contrary to what’s been suggested in the mainstream media, has very little to do with conservation; it’s about how junk science is funded (in this case, by U.S. taxpayers), published, sold to the public, and—as we’re already seeing—used to justify public policy.

Peter J. Wolf has been involved in the world of animal rescue and feral cat management since 2007. He currently lives with seven cats of his own, three foster cats, and helps manage two small feral cat colonies. To learn more about feral cats, visit Peter’s blog, Vox Felina.