Cats vs. Birds?

A recent report contends that our feline friends are killing wildlife at an alarming rate, but two experts say the issue is more complex than that

Scientists from the Smithsonian Conservation Biology Institute and the U.S. Fish and Wildlife Service recently released a report in the journal Nature Communications that sought to estimate the number of wildlife killed by cats in the United States. The mainstream press seized on the study, which used previous small studies to generate a new estimate that put the U.S. death toll from cats at billions of birds and mammals annually. Within days, the story generated significant public outcry from cat lovers, as well as a handful of news outlets questioning the accuracy of the research itself.

Two experts from the Center for Animals and Public Policy at the Cummings School of Veterinary Medicine—Emily McCobb, V00, M.S.02, the center’s assistant director, and ASPCA senior director of veterinary epidemiology Margaret Slater, who teaches a class on free-roaming cats to students pursuing a master’s degree in animals and public policy—told Tufts Now that the issue is much more complex than stalk, kill and eat.

Tufts Now: How accurate are these new prey estimates?

Margaret Slater: There is no way to know the truth about cats’ effect on wildlife. There’s just not enough data available. The study in question is a meta-analysis of existing research publications. While I haven’t recently reviewed each paper individually, I am familiar with them. They use a number of methodologies to determine the quantity and type of cats’ prey—including owners counting the number of dead animals their cats bring home, scat analysis and stomach-content analysis—and each has its limitations.

“It’s easy to say, Keep your cat indoors. But how many people have actually convinced someone else to do that,” asks Margaret Slater. Illustration: Anthony Lewis/Getty “It’s easy to say, Keep your cat indoors. But how many people have actually convinced someone else to do that,” asks Margaret Slater. Illustration: Anthony Lewis/Getty
For example, it’s pretty common in Australia to find sheep in cats’ stomach contents. It’s not because herds of cats are pulling down sheep, but because cats scavenge sheep carcasses by the side of the road. So when you are looking at what a cat has eaten, you don’t know how it got into the cat’s mouth in the first place, or what was going on with that particular animal at the time the cat found and ate it. Cats are specialists in catching rodents, but they are very flexible in terms of what else they’ll eat. And some data suggest that cats only catch birds that aren’t healthy to begin with.

Emily McCobb: Another issue with the study is that you can trace the research referenced back through several publications only to discover it’s based on one guy’s experiences with his cat in his backyard or that a widely cited study on predation is based on only three pet cats. Using those papers as a basis to extrapolate data for a state or country—or even incorporating them into a series of extrapolations—is problematic.

Are cats the invasive species suggested by this paper?

McCobb: It is true that cats are excellent predators and that they were brought to this continent by human settlers. But how much effect cats have on wildlife really depends on the ecosystem where the cats are living. In Worcester, where the Cummings School works with Spay Worcester to manage feral cat colonies using trap-neuter-release, I don’t think they are having much effect. These cats in urban environments are living off of garbage and rodents that we consider to be pests. But there are other areas, such as protected shoreline, that are not appropriate locations for free-roaming cats, whether they are feral cats or pet cats that are allowed outdoors.  

Slater: Cats aren’t regulated in the wild the same way a fox or a coyote would be. When they are living in urban and suburban areas, cat populations typically don’t go up and down with the population of prey species because people feed them. These cats’ diets are subsidized, which makes it difficult to argue for them being a native species.

It’s too easy, however, for discussions about free-roaming cats to devolve into questions about whether a cat is a native species or an invasive one, or if cats kill more birds or rodents. This leads to circular arguments for which we don’t have good data, and people’s minds are already made up. We don’t get to the important question, which is what should we be doing when it comes to the fact that we have too many cats outside, whether they are free-roaming pets or feral cats? It’s easy to say, Keep your cat indoors. But how many people have actually convinced someone else to do that?

What can cat owners do to keep their pets and wildlife safer?

McCobb: A pet cat should be kept indoors or safely confined on a leash or in a fenced backyard. All cats should wear identification on a collar. They also should get spayed or neutered and be vaccinated, which keeps them from contributing to the population of outdoor cats and decreases their risk of catching a disease if they do get outside.

Slater: Most animal-welfare and rescue organizations try to get people to keep their cats indoors—and the American Bird Conservancy and the Humane Society of the United States had widely publicized programs with that goal—but I’m not aware that any of these educational efforts has ever been evaluated for efficacy. That would be a great thing to do, because if we knew there were successful ways to get people to change their behaviors about their own pets, we could concentrate on those.

A recent cat-predation study by the University of Georgia attached video cameras to the collars of cats. Some of the cats in the study hunted (24 of the 55), although relatively few actually caught anything—only 16 cats. What’s really eye-opening is all the risky things cats are doing. They encounter dogs, get into fights with other cats, dart across busy roads, drink and eat stuff of unknown origin and teeter along rooftops. This might be more compelling to people in terms of understanding why it’s imperative to keep your cat indoors. Everybody thinks, My cat doesn’t go anywhere. But these videos show that’s not true. Ordinary house cats get into an amazing amount of mischief.

Why aren’t cats subject to leash and licensing laws like dogs?

Slater: There are parts of the country—such as Madison and Dane County, Wisconsin, and Bryan, Texas—where cats are subject to the same leash laws as dogs.

McCobb: Some parts of the country, such as New Hampshire, require cats to be licensed, and Concord, Mass., recently was considering an ordinance that would require all cats to be kept indoors, but it did not pass. But the basis for most of the animal-control laws in New England, which focus on preventing dog bites and rabies, harkens back to the turn of the last century, when there were dangerous packs of roaming feral dogs biting people.

Culturally, there’s also a very loose conception of cat ownership and acquisition in the U.S. People make the choice to get a dog. They research a breed before buying one, get a puppy from a friend or adopt a dog from a shelter. But so often cats just seem to show up at the door, where we might be willing to feed them, but maybe not bother with getting them spayed or neutered. There’s a lot of ambivalence toward cat ownership, and for the sake of the cats and the wildlife, we need people to take responsibility for the cats they do come into contact with. Some states, Rhode Island in particular, actually have changed the law so feeding implies ownership.

Genevieve Rajewski can be reached at

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