What's the Best Way to Manage Free-Roaming Cats?

Researchers look at "trap, neuter, return" for population control.

stray cat outside

Miriam Del Prado / EyeEm / Getty Images

There might be some in your neighborhood: free-roaming cats that look for kindly handouts or feast on wildlife for meals.

Estimates suggest there are more than 600 million cats in the world. It's difficult to count the number of felines that are stray or feral, but some research suggests it’s more than two-thirds of the overall population.

Stray cats are those that were owned by a person but then became lost or were abandoned. Feral cats are domesticated cats that were never around people and are wild.

Scientists estimate that free-roaming domestic cats kill between 1.3 and 4 billion birds and between 6.3 and 22.3 billion mammals each year.

Researchers in Israel recently studied the most effective way to keep their population numbers in check.

“Free-roaming cats are highly abundant in Israel and in many countries, (especially those with warm climates). On one hand, they are lovable pets but on the other hand, they pose a threat to wild animals, they might cause nuisances and public health threats to humans, and additionally, they suffer from poor welfare,” Eyal Klement, lead researcher and professor at the Koret School of Veterinary Medicine at the Hebrew University of Jerusalem, tells Treehugger.

Communities have often controlled free-roaming cat populations with a program called TNR or trap, neuter, return/release. TNR involves trapping roaming cats, spaying and neutering them, and then returning them to their territory. They are typically also vaccinated and "ear-tipped" for identification. That’s when a small part of the top of the ear is removed during surgery. It’s the universally recognized way to tell if a feral cat has been neutered or spayed.

“The issue of controlling their population by TNR was raised several decades ago but despite extensive use of this method it was never tested in a sound scientific methodology (i.e. for a long enough period and with comparison to un-neutered control regions),” Klement says. “The impetus for our research was, therefore, to try and reveal how much this method is indeed effective for controlling cat populations.” 

Pros and Cons of TNR

TNR can be a controversial program. It’s considered more humane than culling, but released cats are still a threat to the wildlife population.

It’s also expensive and laborious, Klement says, pointing out that the program is only effective if at least 70% of cats in an area are spayed or neutered.

“As we showed, things are complicated by the fact that intact cats can migrate between non-neutered areas to the neutered ones and therefore this method should be performed in contiguous areas which are secluded from un-neutered areas,” he says. “Another pitfall is the occurrence of potential compensation mechanisms such as increase in kitten survival.”

Dozen Years of Research

For their study, researchers followed cats for a total of a dozen years so they could analyze the long-term effects of each population-control method. They focused on free-roaming cats in Rishon LeZion, one city in Israel.

They tested three different methods, each for a four-year period. In the first, they did not intervene with the cat population at all for four years. In the second, they organized a program where they altered cats in half of the city’s 50 zones, while the other zones were a control group with no intervention. In the third four-year period, they applied TNR to the city’s entire roaming cat population.

“It was very critical to follow the population for such a long period because even theoretically it takes time in order for TNR to reduce cat numbers,” Klement says.

In many previous studies, the follow-up period was two short (two years or less), he says. Other longer studies didn’t include control groups or control periods.

“Our study is unique as it is controlled twice in time and in space. Just to understand the scale of this study, previous studies included a maximum of four areas of follow up (and those were short-termed), while in the current study we followed 50 regions, half of which were extensively neutered and half of which were not neutered or in which neutering rates were very low,” Klement says.

Researchers kept track of the cat populations by performing professional counts, but they also analyzed citizen reports to the city call center about free-roaming cat births and deaths.

They found that spaying and neutering cats in only half of the city zones didn’t end up reducing the cat population. That was likely because unneutered cats made their way into the areas where other felines had been fixed.

In the final testing period, they found a 7% annual drop in the cat population. But it was accompanied by an increase in the number of kittens and longevity, likely because there was less competition with neutered, less aggressive cats.

“Intact cats are more territorial than their neutered counterparts,” Klement says. “Once they move into a neighborhood with neutered cats, they tend to thrive and take over.”

The results were published in the journal, the Proceedings of the National Academy of Sciences.

No Ideal Answer

Analyzing their results, researchers say there’s still no perfect solution for controlling free-roaming cat populations.

“As we see it, the ideal solution for stray cat control varies from place to place. In natural habitats, culling is probably the best solution. However in the urban environment, TNR can be effective if combined with resource limitation,” Klement says.

He suggests creating cat feeding stations in certain areas while prohibiting it in other public areas and keeping trash cans closed.

“Such stations may help in increasing TNR efficiency as the surveillance in such areas is much easier and feeders can call the veterinarians whenever a new non-neutered cat is entering the feeding stations region,” Klement says.

That still leaves the issue of the harm these cats can cause to birds and other wildlife.

“The argument about the danger that stray cats pose to wild animals is true. It was shown that in the U.S. stray cats kill billions of birds and reptiles each year, with a possible negative impact on the abundance of some species,” Klement says.

Researchers suggest that TNR should be used along with other population-control methods including adopting out some cats and kittens and euthanizing those that are sick.

“There is extensive literature about this. We, therefore, believe that their population should be managed according to the circumstances and that other means of control should be incorporated together with TNR.”

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  6. Eyal Klement, lead researcher and professor at the Koret School of Veterinary Medicine at the Hebrew University of Jerusalem

  7. Gunther, Idit, et al. "Reduction Of Free-Roaming Cat Population Requires High-Intensity Neutering in Spatial Contiguity to Mitigate Compensatory Effects." Proceedings of the National Academy of Sciences, vol. 119, no. 15, 2022, doi:10.1073/pnas.2119000119